Estimation of heterotic effects

Consider a cross between two inbred lines, A and B, with population means of XP1 and XP2, respectively. The phenotypic variability of the F1 is generally less than the variability of either parent. This indicates that the heterozygotes are less subject

to environmental influences than the homozygotes. The heterotic effect resulting from the crossing is roughly estimated as

This equation indicates the average excess in vigor exhibited by F1 hybrids over the midpoint  between the means of the inbred parents. K.R. Lamkey and J.W. Edward coined the term panmictic midparent heterosis to describe the deviation in performance between a population cross and its two parent populations in Hardy–Weinberg equilibrium.

Types of hybrids

As previously discussed, the commercial applications of hybrid breeding started with a cross of two inbred lines and later shifted to the more economic double cross, and then back to a single cross. Other parent combinations in hybrid development have been proposed, including the three-way cross and modified versions of the single

cross, in which closely related crosses showed that the single cross was superior in performance to the other two in terms of average yield. However, it was noted also that the genotype _ environment interaction mean sum of squares for the single cross was more than twice that for the double crosses, the mean sum of squares for the three-way cross being intermediate. This indicated that the single crosses were

more sensitive or responsive to environmental conditions than the other crosses. Whereas high average yield is important to the producer, consistency in performance across years and locations is also important. As R.W Allard and A.D. Bradshaw explained, there are two basic ways in which stability may be achievedin the field. Double and three-way crosses have a more genetically divergent population for achieving buffering. However, a population of single cross genotypes that are less divergent can also achieve stability on the basis of individual buffering, whereby individuals in the population are adapted to a wide range of environments.

 Today, commercial hybrids are predominantly single crosses. Breeders continue to develop superior inbred lines. The key to using these materials in hybrid breeding is identifying pairs of inbreds with outstanding combining ability.

Germplasm procurement and development for hybrid production

As previously indicated, the breeder needs to obtain germplasm from the appropriate heterotic groups, where available. It is critical that the source population has the genes needed in the hybrid. Plant breeders in ongoing breeding programs often have breeding lines in storage or in nurseries from which potential parents could be selected for future programs. These materials should be evaluated for performance

capabilities and, especially, for traits of interest in the proposed breeding program.

Germplasm may be introduced from germplasm banks and other sources. Such material should also be evaluated as done with local materials.

Development and maintenance of inbred lines

An inbred line is a breeding material that is homozygous. It is developed and maintained by repeated selfing of selected plants. In principle, developing inbred lines from cross-pollinated species is not different from developing pure lines in self-pollinated species. About 5–7 generations of selfing and pedigree selection are required for developing an inbred line. As previously indicated, inbreeders tolerate inbreeding, whereas outbreeders experience varying degrees of inbreeding depression. Consequently, the extent of inbreeding in developing inbred lines varies with the species. Species such as alfalfa and red clover that are more intolerant of inbreeding may be selfed only a few times. Alternatively, sib mating may be used to

maintain some level of heterozygosity in these sensitive species. Hybrid breeding, as previously stated, exploits the phenomenon of heterosis. Heterosis will be highest when one allele is fixed in one parent to be used in a cross and the other allele fixed in the other parent.

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Methods for developing heterotic groups

A number of procedures may be used by breeders to establish heterotic groups and patterns. These include pedigree analysis, geographic isolation inference, measurement of heterosis, and combining ability analysis. Some have used diallel analysis to obtain preliminary information on heterotic patterns. The procedure is recommended for use with small populations. The technology of molecular markers may be used to refine existing groups and patterns or for expediting the establishment of new ones, through the determination of genetic distances.

To establish a heterotic group and pattern, breeders make crosses between or within populations. Intergroup hybrids have been shown to be superior over intragroup hybrids in establishing heterotic relationships. In practice, most of the primary heterotic groups were not developed systematically but rather by relating the observed heterosis and hybrid performance with the origin of parents included in the crosses. One of the earliest contributions to knowledge in the areas of developing heterotic patterns was made in 1922. Comparing heterosis for yield in a large number of intervarietal crosses of maize, it was discovered that hybrids between varieties of different endosperm types produced a higher performance than among varieties with the same endosperm type. This discovery, by F.D. Richey, suggested that crosses between geographically or genetically distant parents expressed higher performance and, hence, increased heterosis. This information led to the development of the most widely used heterotic pattern in the US Corn Belt – the Reid Yellow Dent x Lancaster Sure Crop.

Heterotic groups and patterns in crops

Heterotic patterns have been studied in various species. For certain crops, breeders have defined standard patterns that guide in the production of hybrids. As previously indicated in maize, for example, a widely used scheme for hybrid development in temperate maize is the Reid _ Lancaster heterotic pattern. These heterotic populations were discovered from pedigree and geographic analysis of inbred lines used in the Corn Belt of the United States. In Europe, a common pattern for maize is the European flint _ Corn Belt Dent, identified based on endosperm types. In France, F2_F6 heterotic pattern derived from the same open pollinated cultivars was reported. Other patterns include ETO-composite _ Tuxpeno and Suwan 1 _ Tuxpeno in tropical regions. Alternate heterotic patterns continue to be sought.

In rice, some research suggests two heterotic groups within O. indica, one including strains from S.E. China and another containing strains from S.E. Asia. In rye, the two most widely used germplasm three major germplasm pools are available, namely,Minor, Major, and Mediterranean.

Even though various approaches are used for the identification of heterotic patterns, they generally follow certain principles. The first step is to assemble a large number of germplasm sources and then make parent populations of crosses from among which the highest performing hybrids are selected as potential heterotic groups and patterns. If established heterotic patterns already exist, the performance of the putative patterns with the established ones is compared. Where the germplasm accession is too large to permit the practical use of a diallel cross, the germplasm may first be grouped based on genetic similarity. For these groups, representatives are selected for evaluation in a diallel cross. According to Melchinger, the choice of a heterotic group or pattern in a breeding program should be based on the following criteria:

_ High mean performance and genetic variance in the hybrid population.

_ High per se performance and good adaptation of parent population to the target region.

_ Low inbreeding of inbreds.

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Dominance theory

The dominance theory assumes that vigor in plants is conditioned by dominant alleles, recessive alleles being deleterious or neutral in effect. It follows then that a genotype with more dominant alleles will be more vigorous than one with few dominant alleles. Consequently, crossing two parents with complementary dominant alleles will concentrate more favorable alleles in the hybrid than either parent. The dominance theory is the more favored of the two theories by most scientists, even though neither is completely satisfactory. In practice, linkage and a large number of genes prevent the breeder from developing inbred lines that contain all homozygous dominant alleles. If too many deleterious alleles are present it makes it difficult to inbreed to recover sufficient loci with homozygous dominant alleles. Inbreeding depression occurs upon selfing because the deleterious recessive alleles that are protected in the heterozygous condition become homozygous and are expressed. It should be pointed out that highly productive inbred lines have continued to be produced for hybrid production, the reason why single-cross hybrids have returned to dominance in corn hybrid production.To illustrate this theory, assume a quantitative trait is conditioned by four loci. Assume that each dominant genotype contributes two units to the phenotype, while a recessive genotype contributes one unit. A cross between two inbred parents could produce the following outcome With dominance, each locus will contribute two units to the phenotype. The result is that the F1 would be more productive than either parent. D.L. Falconer developed a mathematical expression for the relationship between the parents in a cross that leads to heterosis as follows where HF1 is the the deviation of the hybrid from the

mid-parent value, d is the the degree of dominance, and y is the the difference in gene frequency in the parents of the cross. From the expression, maximum mid-parent heterosis will occur when the values of the two factors are each unity. That is, the populations to be crossed are fixed for opposite alleles and there is complete dominance.

Over dominance theory

The phenomenon of the heterozygote being superior to the homozygote is called overdominance. The overdominance theory assumes thatthe alleles of a gene are contrasting but each has a different favorable effect in the plant. Consequently, a heterozygous locus would have greater positive effect than a homozygous locus and, by extrapolation, a genotype with more heterozygous loci would be more vigorous than one with less heterozygotes.To illustrate this phenomenon, consider a quantitative trait conditioned by four loci. Assume that recessive, heterozygote, and homozygote dominants contribute 1, 2, and 1½ units to the phenotypic value,respectively:

Biometrics of heterosis

Heterosis may be defined in two basic ways:

Better-parent heterosis. This is calculated as the degree by which the F1 mean exceeds the better parent in the cross.

 Mid-parent heterosis. Previously defined as the superiority of the F1 over the mean of the parents.

For breeding purposes, the breeder is most interested to know whether heterosis can be manipulated for crop improvement. To do this, the breeder needs to understand the types of gene action involved in the phenomenon as it operates in the breeding population of interest. As Falconer indicated, in order for heterosis to manifest for the breeder to exploit, some level of dominance gene action must be present, in addition to the presence of relative difference in gene frequency in the two parents. Given two populations, in Hardy–Weinberg equilibrium, with genotypic values and frequencies for one locus with two alleles p and q for population A, and r and s for population B as follows

From the foregoing, if, heterosis. On the other hand, if in population A pผ0 or 1 and by the same token in population B rผ0 or 1 for the same locus, depending on whether the allele is in homozygous recessive or dominant state, there will be a heterotic response. In the first generation, the heterotic response will be due to the loci where pผ1 and rผ0, or vice versa. Consequently, heterosis manifested will depend on the number of loci that have contrasting loci as well as the level of dominance at each locus. The highest heterosis will occur when one allele is fixed in one population and the other allele in the other. If gene action is completely additive, the average response would be equal to the mid-parent, and hence heterosis will be zero. On the other hand, if there is dominance and/or epistasis, heterosis will manifest.

Plant breeders develop cultivars that are homozygous.When there is complete or partial dominance, the best genotypes to develop are homozygotes orheterozygous, where there could be opportunities todiscover transgressive segregates. On the other hand,when non-allelic interaction is significant, the bestgenotype to breed would be a heterozygote. Some recent views on heterosis have been published. Some maize researchers have provided evidence to the effect that the genetic basis of heterosis is partial dominance to complete dominance. A number of research data supporting overdominance suggest that it resulted from pseudo-overdominance arising from dominant alleles in repulsion phase linkage.

Yet, still, some workers in maize research have suggested epistasis between linked loci to explain the terosis.

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Concept of heterotic relationship

Genetic diversity in the germplasm used in a breeding program affects the potential genetic gain that can be achieved through selection. The most costly and time consuming phase in a hybrid program is the identification of parental lines that would produce superior hybrids when crossed. Hybrid production exploits the phenomenon of heterosis, as already indicated. Genetic distance between parents plays a role in

heterosis.In general, heterosis is considered an expression of the genetic divergence among cultivars.When heterosis or some of its components are significant for all traits, it may be concluded that there is genetic divergence among the parental cultivars. Information on the genetic diversity and distance among the breeding lines, and the correlation between genetic distance and hybrid performance, are important for determining breeding strategies, classifying the parental lines, defining heterotic groups, and predicting future hybrid performance.

Definition

A heterotic group may be defined as a group of related or unrelated genotypes from the same or different populations, which display similar combining ability when crossed with genotypes from other germplasm groups. A heterotic pattern, on the other hand, is a specific pair of heterotic groups, which may be populations or lines, which express in their crosses high heterosis and, consequently, high hybrid performance. Knowledge of the heterotic groups and patterns is helpful in plant breeding. It helps breeders to utilize their germplasm in a more efficient and consistent manner through exploitation of complementary lines for maximizing the outcomes of a hybrid breeding program. Breeders may use heterotic group information for cataloging diversity and directing the introgression of traits and creation of new heterotic groups.

The concept of heterotic groups was first developed by maize researchers who observed that inbred lines selected out of certain populations tended to produce superior performing hybrids when hybridized with inbreds from other groups. The existence of heterotic groups has been attributed to the possibility that populations of divergent backgrounds might have unique allelic diversity that could have originated from founder effects, genetic drift, or accumulation of unique diversity by mutation or selection. Interallelic interaction  or repulsion phase linkage among loci showing dominance could explain the observance of significantly greater heterosis following a cross between genetically divergent populations. Experimental evidence supports the

concept of heterotic patterns. Such research has demonstrated that intergroup hybrids significantly out-yielded intragroup hybrids. In maize, one study showed that intergroup hybrids between Reid Yellow Dent x Lancaster Sure Crop out-yielded intragroup hybrids by 21%. D. Melchinger and R.R. Gumber noted that heterotic groups are the backbone of successful hybrid breeding, and hence a decision about them should be made at the beginning of a hybrid crop improvement program. They further commented that once established and improved over a number of selection

cycles, it is extremely difficult to develop new and competitive heterotic groups. This is because, at an advanced stage, the gap in performance between improved breeding materials and unimproved source materials is often too large. However, the

chance to develop new heterotic groups could be enhanced with a change in breeding objectives. Once developed, heterotic groups should be broadened continuously by introgressing unique germplasm in order to sustain medium- and long-term gains from selection.

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Hybrid vigor

Hybrid vigor may be defined as the increase in size, vigor, fertility, and overall productivity of a hybrid plant over the mid-parent value. It is calculated as the difference between the crossbred and inbred means:

The synonymous term, heterosis, was coined by G.H. Shull. It should be pointed out immediately that, as it stands, heterosis is of no value to the breeder, if a hybrid will only exceed the mid-parent in performance. Such advantageous hybrid vigor is observed more frequently when breeders cross parents that are genetically

diverse. The practical definition of heterosis is hybrid vigor that greatly exceeds the better or higher parent in a cross. Heterosis occurs when two inbred lines of outbred species are crossed, as much as when crosses are made between pure lines of inbreeders.

          Heterosis, though widespread in the plant kingdom, is not uniformly manifested in all species and for all traits. It is manifested at a higher intensity in traits that have fitness value, and also more frequentlyamong cross-pollinated species than self-pollinated species. All breeding methods that are preceded by crossing make use of heterosis to some extent. However, it is only in hybrid cultivar breeding and the breeding of clones in which the breeder has opportunity to exploit the phenomenon to full advantage.

Hybrids dramatically increase yields of non-hybrid cultivars. By the early 1930s, maize yield in the United States averaged 1250 kg/ha. By the early 1970s, maize yields had quadrupled to 4850 kg/ha. The contribution of hybrids  to this increase was estimated at about 60%.

Inbreeding depression

Heterosis is opposite to inbreeding depression.In theory, the heterosis observed on crossing is expected to be equal to the depression upon inbreeding, considering a large number of crosses between lines derived from a single base population. In practice, plant breeders are interested in heterosis expressed by specific crosses between selected parents, or between populations that have no known common origin. Reduction in fitness is usually manifested as a reduction in vigor, fertility, and productivity. The effect of inbreeding is more severe in the early generations.Just like heterosis, inbreeding depression is not uniformly manifested in plants. Plants including onions, sunflower, cucurbits, and rye are more tolerant of inbreeding with minimal consequences of inbreeding depression. On the other hand plants such as alfalfa and carrot are highly intolerant of inbreeding.

Genetic basis of heterosis

Two schools of thought have been advanced to explain the genetic basis for why fitness lost on inbreeding tends to be restored upon crossing. The two most commonly known are the dominance theory, first proposed by C.G. Davenport in 1908 and later by I.M. Lerner, and the overdominance theory, first proposed by Shull in 1908 and later by K. Mather and J.L. Jinks. A third theory, the mechanism

of epistasis,has also been proposed. A viable theory should account for both inbreeding depression in cross-pollinated species upon selfing and increased vigor in F1, upon hybridization.

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What is a hybrid cultivar?

A hybrid cultivar, by definition, is the F1 offspring of a planned cross between inbred lines, cultivars, clones, or populations. Depending on the breeding approach,

the hybrid may comprise two or more parents. A critical requirement of hybrid production isthat the parents are not identical. As will be discussed next, it is this divergence that gives hybrids their superior performance. The outstanding yields of certain modern crops, notably corn, owe their success to the exploitation of the phenomenon of heterosis, which is high when parents are divergent. Much of what we know about hybrid breeding came from the discoveries and experiences of scientists engaged in corn hybrid cultivar development. However, commercial hybrids are now available for many crops, including self-pollinating species.

Brief historical perspective

One of the earliest records on hybridization dates back to 1716 when American Cotton Mather observed the effects of cross fertilization in maize, attributing the multicolored kernels to wind-borne inter-mixture of different colored cultivars. However,it was the German T.G. Koelreuter who conducted the first systematic studies on plant hybridization in 1766. Even though previous observations had been

made to the effect that offspring of crosses tended to exhibit superior performance over the parents, it was G.H. Shull who, in 1909, first made clear scientificbased proposals for exploiting heterosis to produce uniform and high yielding cultivars. Unfortunately, the idea was at that time impractical and potentially expensive to commercially exploit. In 1918, D.F. Jones proposed a more practical and cost-effective approach to producing hybrid cultivars by the method of the double-cross. Double-cross hybrids produced significantly more economic yield than the single-cross hybrids originally proposed by Shull. Single-cross hybrid seed was then produced on weak and unproductive inbred parents, whereas doublecross seed was produced on vigorous and productive single-cross plants. The corn production industry was transformed by hybrids, starting in the 1930s.

Other notable advances in the breeding of hybrids were made by researchers, including M.I. Jenkins in 1934 who devised a method  to evaluate the effectiveness of parents in a cross. Through this screening process, breeders were able to select a few lines that were good combiners for use in a hybrid breeding.

 The next significant impact on hybrid production also came in the area of techniques of crossing. Because corn is outcrossed and monoecious, it is necessary to emasculate one of the parents as part of the breeding process. In the early years of corn hybrid breeding, emasculation was accomplished by the labor-intensive method of mechanical detasseling. The discovery and application of cytoplasmic male sterility to corn hybrid programs eliminated the need for emasculation by the late 1960s. Unfortunately, the success of CMS was derailed when the Texas cytoplasm, which was discovered in 1938 and was at that time the dominant form of male sterility used in corn breeding, succumbed to the southern leaf blight epidemic of 1970 and devastated the corn industry. It should be mentioned that mechanized detasselers are used by some major seed companies in hybrid seed production of corn today.

Realizing that the limited number of inbred lines used in hybrid programs did not embody the complete genetic potential of the source population, and the need to develop new inbred lines, scientists embarked on cyclical recombination  to generate new variability and to improve parental lines. Breeders were able to develop outstanding inbred lines to make single-cross hybrids economical enough to replace double-cross hybrids by the 1970s. By this time, corn hybrid production programs had developed a set of standard practices consisting of the following, as observed by N.W. Simmonds:

_ Maintenance and improvement of source population by open pollinated methods.

_ Isolation of new inbreds and improvement of old Ones.

_ Successive improvement of single-cross hybrids by parental improvement.

_ CMS-based seed production.

The application of hybrid methodology in breeding has socioeconomic implications. The commercial seed industry has rights to its inventions that generate royalties. More importantly, because heterosis is maximized in the F1, farmers are generally prohibited from saving seed from the current season’s crop to plant the next year’s crop. They must purchase seed from the seed suppliers each season. Unfortunately, poor producers in developing countries cannot afford annual seed purchase. Consequently, local and international  efforts continue to be largely devoted to producing propagable improved open-pollinated cultivars for developing countries.

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Factors affecting performance of synthetic cultivars

Three factors are key in determining the performance of a synthetic cultivar.

Number of parental lines used. Synthetic cultivars are maintained by open pollination. Consequently, the F2 yield should be high to make it a successful cultivar. The Hardy– Weinberg equilibrium is reached in syn-2 for each individual locus and, hence, should remain unchanged in subsequent generations. It follows then that the F3 should produce as well as syn-2. Some researchers have even shown that F3 and F4 generations yielded as much or slightly better than F2 generations, provided the number of lines included in the synthetic cultivar is not small. With nผ2, the reduction in performance will be equal to 50% of the heterosis. Consequently, n has to be increased to an optimum number without sacrificing high GCA. When n is small, the yields of syn-1 are high, but so are the decline in syn-2 yields. On the other hand increasing n decreases syn-1 yields and syn-2 decline. A balance needs to be struck between the two effects. Some researchers estimate the

optimum number of lines to include in a synthetic to be five or six.

Mean performance of the parental lines. The lines used in synthetics should have high performance. A high value of parental lines reduces the reduction in performance of syn-2 over syn-1. Preferably, the parents should be non-inbreds or have minimum inbreeding.

Mean syn-1. In theory, the highest value of syn-1 is produced by a single cross. However, alone it will suffer from a higher reduction in performance. It is important for the mean F1 yield of all the component F1 crosses to be high enough such that the syn-2 yield would remain high in spite of some decline.

Advantages and limitations of development of synthetics

Advantages

_ The method is relatively easy to implement.

_ It can be used to produce variability for hybrid breeding programs.

_ Advanced genotypes of synthetics show little yield reduction from syn-1, making it possible for farmers to save and use seed from the current season to plant the next season.

Disadvantages

_ Because inadequate seed is often produced in syn-1, the method fails to exploit to the maximum the effects of heterosis, as is the case in conventional F1 hybrid breeding. The method of synthetics is thus a compromise to the conventional means of exploiting heterosis. 352 CHAPTER 17

_ Natural selection changes the genotypic composition of synthetics, which may be undesirable.

Backcross breeding

The key concern in the application of the backcross technique to cross-pollinated species is the issue of inbreeding. Selfing cross-pollinated species leads to inbreeding depression. The use of a recurrent parent in a backcross with crosspollinated species is tantamount to inbreeding. To minimize the loss of vigor, large populations should be used to enable the breeder sample and maintain the diversity of the cultivar and to insure against the harmful effects of inbreeding. Just like self-pollinated species, it is relatively straight forward to improve a qualitative trait conditioned by a single dominant gene. The breeder simply selects and advances individuals expressing the trait. Where a recessive gene is being transferred, each backcross should be followed by one round of intercrossing to identify the recessive phenotype. Improving inbred lines is equivalent to improving self-pollinated species. The key to success is for the breeder to maintain a broad gene base by using adequate number of backcrosses and a large segregating population.

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Highest yield performance is obtained in the syn-1 generation

The highest yield performance is obtained in the syn-1 generation, hybrid vigor declining with subsequent generations. It is generally estimated that a synthetic forage cultivar of cross-fertilized diploid or polyploidy species will experience a maximum yield decline of 10–12% from syn-1 to syn-2 generation, as previously stated. The yield decline is less in subsequent generations. Sewall Wright proposed a formula to predict the F2 yield of a group of inbred lines as follows:

where F2 is the expected performance of the F2, F1 is the mean F1 hybrid performance from combinations of inbred lines, P is the average performance of inbred lines, and n is the number of inbred lines. That is, one can increase F2 yield by increasing the average F1 yield, increasing yield of parental lines, or increasing the number of lines used to create the synthetic. This formula assumes that the species has diploid reproduction and that the parents are inbred. Hence, even though shown to be accurate for maize, it is not applicable to polyploid species and those that are obligate outcrossers.

The formula may also be written as follows:

Studies involving inbred lines and diploid species have indicated that as the number of parental lines increase the F1 performance is increased. Parental lines with high combining ability will have high F1 performance. In practice, it is a difficult task to find a large number of parents with very high combining ability. Furthermore, predicting yield performance of synthetic cultivars of cross-fertilized diploid and polyploidy forage species is more complicated than is described by their relationship in the equation. Given a set of n inbred lines, the total number of synthetics, N, of size ranging from 2 to n is given by:

As inbred lines increase, the number of possible synthetics increases rapidly, making it impractical to synthesize and evaluate all the possible synthetic cultivars. The theoretical optimum number of parents to include in a synthetic is believed to be about 4–6. However, many breeders favoring yield stability over yield ability tend to use large numbers of parents ranging from about 10–100 or more. Large numbers are especially advantageous when selecting for traits with low heritability.

Synthetics of autotetraploid species  are known to experience severe and widespread decline in vigor between syn-1, and syn-2, which has been partly attributed to a reduction in triallelic and tetrallelic loci. Higher numbers of tetrallelic loci have been shown to be associated with higher agronomic performance of alfalfa. The number of selfed generations is limited to one. Selfed seed from selected S0 plants are intermated to produce the synthetic population. The rationale is that S0 plants with high combining ability would contain many favorable genes and gene combinations. Selecting specific individuals from the segregating population to self could jeopardize these desirable combinations.

Additive gene action is considered more important than dominance genotypic variance for optimum performance of synthetic cultivars. In autotetraploids in which intralocus and allelic interactions occur, high performing synthetic cultivars should include parents that have a high capacity to transfer their desirable performance to their offspring. Such high additive gene action coupled with a high capacity for intralocus or allelic interactions will likely result in higherperforming synthetics.

Synthetic cultivars exploit the benefits of both heterozygosity and heterosis. J.W. Dudley demonstrated that yield was a function of heterozygosis by observing that, in alfalfa crosses, the F1 yields reduced as generations advanced. Further, he

observed that allele distribution among parents used in a cross impacted heterozygosity. For example, a cross of duplex _ nulliplex always had higher degree of heterozygosity than say a cross of simplex _ simplex regardless of the clonal generation used to make the cross. Natural selection changes the genotypic composition of synthetics. The effect can have significant consequences when the cultivar is developed in one environment and used for production in a distinctly different environment. There can be noticeable shifts in physiological adaptation as

well as morphological traits. For example, growing alfalfa seed in the western states

for use in the Midwest, the cultivars may lose some degree of winter hardiness, a trait desired in the production region of the Midwest. A way to reduce this adverse impact is to grow seed crop in the west using foundation seed from the Midwest.

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Primary steps in the development of a synthetic

There are three primary steps in the development of a synthetic:

Assembly of parents.

Assessment of GCA.

Random mating to produce synthetic cultivars.

The parents used in synthetics may be clones or inbred lines. Whereas forages can be increased indefinitely by clonal propagation, inbred lines are needed to perpetuate the genotypes used in hybrid production. The parental materials are reproducible and may be substituted with new genotypes as they become available, for some improvement in the synthetic cultivar. The parents are selected after progeny testing or general combining ability analysis using a test cross or topcross, but most frequently a polycross, for evaluation.

1 Test for GCA

Testers

_ Polycross. A polycross test is generally preferred because it is simple and convenient to conduct and also, by nature, provides an efficient estimate of GCA, a desired attribute in synthetic production. Furthermore, it allows an adequate amount of seed to be obtained for more comprehensive testing using commercial standards. It provides a greater insurance to cultivars against genetic shifts that could arise during seed increase. However, any significant amount of selfing or non-random cross-pollination could result in bias. The component clones may vary in self-fertility and other biological characteristics that impact fertilization. To minimize such deviations from a perfect polycross, the Latin square design may be used to establish the polycross nursery. In theory, the polycross allows each clone in the nursery to be pollinated by about the same pollen sources as a result of random pollination from all the entries in the same plot.

_ Topcross. Selected clones are grown in alternative rows with an open-pollinated cultivar as tester. The test cross seed includes both selfs and intercrosses among the clones being evaluated.

_ Diallel cross. A diallel cross entails achieving all possible single crosses involving all the parents. This is laborious to conduct. It requires that each parent

be grown in isolation. It provides information on both GCA and SCA.

Procedure

A procedure for crops in which selections are clonally propagable is as follows:

_ Year 1: The source nursery. The source population consists of clones. The source nursery is established by planting several thousands of plants assembled from many sources to provide a broad genetic base of the clonal lines for selection.

The germplasm in the nursery is screened and evaluated to identify superior individuals according to the breeding objectives.

_ Year 2: Clonal lines. The breeder first selects 100 to 200 superior plants on phenotypic basis to multiply clonally to produce clonal lines. A clonal line nursery is established, each line consisting of about 20–25 plants derived from the same parental line.The breeder may impose various biotic and abiotic selective pressure

to aid in identifying about 25–50 most desirable clones.

_ Year 3: Polycross nursery. The selected clonal lines are planted in a polycross nursery to generate seed for progeny testing. Ideally, the layout of the polycross in the field should allow each clone to be pollinated by a random sample of pollen from all the other entries. A method of layout to achieve this objective is a square plot in which every clone occurs once in every row. Covering with a fine mesh tent or separating the plots by adequate distance isolates each square plot. The mesh is removed once the pollination period is over. A large number of replications of the single randomized clones is suggested for achieving a highly mixed pollination. Seed from each clone is harvested separately. The polycross test is valid if the layout ensures random interpollination. Alternative methods of evaluating clones for quantitatively inherited traits are available. Self-fertilization may

be used but it often yields a little amount of seed. A diallel cross is cumbersome to conduct, especially for large entries. A topcross evaluates SCA. The polycross is used because it evaluates GCA.

_ Year 4: Polycross progeny test. Seed is harvested from the replicated clones and bulked for planting progeny rows for performance evaluation. The progeny test evaluates yield and other traits, according to the breeding objective. The top performing 5–10 clones are selected for inclusion in the synthetic cultivar.

_ Year 5: Syn-0 generation. The selected clones are vegetatively propagated and randomly transplanted into an isolated field for cross-fertilization to produce syn-0 seed. Leguminous species may be isolated in an insect-proof cage and cross fertilized by using insects.

_ Year 6: Syn-1 generation. The syn-0 seed is increased by planting in isolation. Equal amount of seed is obtained from each parent and mixed to ensure random mating in the field. Bulk seed is harvested from seed increased in syn-1 generation that may be released as a commercial cultivar provided sufficient seed is produced.

_ Year 7: Subsequent syn generations. Frequently, the syn-1 seed is not sufficient to release to farmers. Consequently, a more practical synthetic breeding

scheme is to produce syn-2 generation by openpollinated increase of seed from syn-1. The syn-2 seed may be likened to a breeder seed. It is further increased to produce syn-3 and syn-4. Commercial seed classes are discussed in detail in Chapter 27. The pattern of loss in vigor, progressively with advancement of generations from syn-1, syn-2, and syn-n, is similar to what occurs when hybrids are progressively selfed from F1, F2 . . . Fn generations. It is important to maintain the original clones so that the synthetic can be reconstituted as needed. The steps described are only generalized and can be adapted and modified according to the species and the objectives of the breeder.

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Development of synthetic cultivars

1. Synthetic cultivar versus germless composites

There are two basic types of open-pollinated populations of crops – those produced by population improvement, and synthetics. As previously discussed, population improvement methods can be categorized into two – those that depend on purely phenotypic selection and those that involve selection with progeny testing. A synthetic cultivar may be defined as an advanced generation of crossfertilized seed

mixture of parents that may be strains, clones, or hybrids. The parents are selected based on GCA. The primary distinction between these basic types of populations mentioned in this section is that population improvement cultivars can be propagated indefinitely as such. However, a synthetic cultivar is propagated for only a limited number of generations and then must be reconstituted from the parental stock. A synthetic population differs from a natural population by consisting of breeder-selected parental stocks. Germplasm composites is a broad term used to refer to the mixing together of breeding materials on the basis of some agronomic trait, followed by random mating. There are many ways to put a composite together. Germplasm composites are by nature genetically broad based and very complex. They can be

used as for commercial cultivation over a broad range of agroecological environments. However, they can also be used as reservoirs of useful genes for use in breeding programs.

2 Desirable features of a synthetic cultivar

K.J. Frey summarized three major desirable features of synthetic cultivars as:

Yield reduction in advanced generations is less than with a single or double cross. For example,in maize an estimated 15–30% reduction occurs between F1 and F2, as compared to only a reduction of 5–15% from syn-1 to syn-2. This slow rate of reduction in yield makes it unnecessary for producers to obtain new seed of the cultivar for planting in each season.

A synthetic cultivar may become better adapted to the local production environment over time, as it is produced in successive generations in the region.

A synthetic cultivar is genetically heterogeneous, a population structure that makes it perform stably over changing environmental conditions. Further, because of this heterogeneity, both natural and artificial selection can modify the genotypic structure of synthetic cultivars. That is, a breeder may achieve gain in performance by practicing selection in syn-2 and subsequent generations.

3 Application

The synthetic method of breeding is suitable for improving cross-fertilized crops. It is widely used to breed forage species. Successful synthetic cultivars have been bred for corn, sugar beets, and other species. The suitability of forage species for this method of breeding stems from several biological factors. Forages have perfect flowers, making it difficult to produce hybrid seed for commercial use. The use of male sterility may facilitate controlled cross-pollination, which is difficult to achieve in most forage species. To test individual plants for use in producing the commercial seed, it is essential to obtain sufficient seed from these plants. The amount of seed obtained from single plants of these species is often inadequate for a progeny test. Furthermore, forage species often exhibit self-incompatibility, a condition that inhibits the production of selfed seed. Synthetic cultivars are also used as gene pools in breeding progeny. Synthetic cultivars are advantageous in agricultural production systems where farmers routinely save seed for planting. One of the well-known and widely used synthetic is the Iowa stiff-stalk synthetic of maize.

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2 Full-sib reciprocal recurrent selection

Developed by Hallauer and Eberhart as modifications of the method by Comstock and colleagues, the fullsib method requires at least one of the populations to be prolific. The recombination units are half-sibs. Developed for maize, full-sib families are produced by pairing plants from two populations, A and B. The top ear of a plant from population A is crossed with a plant from populations B. The lower ear is selfed to be saved as remnant seed. The same is done for the reciprocal plant from population B, if they have two ears, otherwise, they are selfed.

_ Season 1. Plant population A as females in an isolated block and population B as males in field 1. Plant population B as females and population A as males in field 2. The upper ears in each field are open pollinated, while the lower ears are protected and pollinated manually. The result is that the upper ear is an interpopulation half-sib family while the lower ear is an intrapopulation half-sib family.

_ Season 2. Evaluate 100–200 A _ B and B _ A halfsibs in replicated trials. Select best half-sibs from both sets of crosses.

_ Season 3. Plant the remnant seed of lower ears selfed by hand pollination that corresponds to the best A _ B half-sibs in ear-to-row as females.The males are the bulk remnant half-sib seed from population B corresponding to the best B _ A

crosses. They are randomly mated. The open pollinated seed in populations A and B are harvested to initiate the next cycle.

Advantages

_ As compared to the half-sib method, one half of the families are evaluated in each cycle because the evaluation of each full-sib reflects the worth of two parental plants, one from each population.

_ Superior S0

_ S0 crosses may be advanced in further generations and evaluated as S1 _ S1, S2_ S2,. . . . . . ., Sn_ Sn to allow the breeder to simultaneously develop hybrids while improving the populations.

Genetic issues

Another advantage of this method is that additive genetic variance of full-sib families is twice that of the half-sib families. The expected genetic gain is given by:

where sPFS is the phenotypic standard deviation of the full-sib families.

Application

The scheme has been used in crops such as maize and sunflower with reported genetic gains of the magnitudes of 2.17% for population per se and 4.90% for the population hybrid.

Optimizing gain from selection in population improvement

The goal of the breeder is to make systematic progress in the mean expression of the trait of interest from one cycle to the next. Achieving progressive gains in yield depends on several factors.

_ Genetic variance. As previously indicated, additive genetic variance is critical to increase in gains per cycle. Additive genetic variance can be increased

through increasing diversity in the entries used in population improvement.

_ Selection intensity. The rate of gain with selection is increased when selection intensity is increased. The number of individuals selected for recombination in each cycle should be limited to the best performers.

_ Generations per cycle. Breeder’s choice of the breeding system to use in a breeding project is influenced by how rapidly each cycle of selection can be completed. When possible, using 2–3 generations per year can increase yield gains. Multiple generations per year is achieved by using off-season nurseries, or planting in the dry season using irrigation.

_ Field plot technique. Breeders select in the field, often handling large numbers of plants. Heterozygosity in the field should be managed by using proper experimental designs to reduce random variation. Whenever possible, uniform fields should be selected for field evaluations. The cultural conditions 348 CHAPTER 17 should be optimized as much as possible.This practice will reduce variation between replications. Other factors to consider are plot sizes, number of plants per plot, number of replications per trial, and number of locations. Implemented properly, these factors reduce random variations that complicate experimental results.

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2 Half-sib selection with test cross

Another way of evaluating genotypes to be composited is by conducting a test cross.

This variation of half-sib selection allows the breeder to more precisely evaluate the genotype of the selected plant by choosing the most suitable test cross parent. The half-sib lines to be composited are selected based on a test cross evaluation not progeny performance.The tester may be an inbred, in which case all the

progeny lines will have a common parental gamete.

Like half-sib selection with progeny test, this procedure is applicable to cross-pollinated species in which sufficient seed can be produced by crossing to grow a

replicated testcross progeny trial. However, in procedures in which self-pollination is required, the method cannot be applied to species with selfincompatibility.

In season 1, the breeder selects 50–100 plants from the source population. A tester parent is pollinated with pollen from each of the selected plants.The crossed seed from the tester as well as the open-pollinated selected plants are harvested separately. In season 2, the test cross progenies are grown in replicated plots. In season 3, an equal amount of open-pollinated seed from 5–10 superior plants is composited and grown in isolation for openpollination to occur.

Pollen from each selected plant may be used to pollinate a tester plant and self-pollinate the selected plant. Also, in season 3, equal quantities of selfed seed may

be composited and planted in isolation. 3 Interpopulation improvement methods The purpose of this group of recurrent selection schemes is to improve the performance of a cross between two populations. To achieve this, interpopulation heterosis is exploited. The procedures are appropriate when the breeder’s goal is hybrid production. Developed by P.E. Comstock and his colleagues, the procedures allow the breeder to improve two genetically different populations for GCA and SCA, thereby improving their crossbred mean.

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1 Half-sib selection with progeny test

Half-sib or half-sib family selection is so-called because only one parent in the cross is known. In 1899, C.G. Hopkins first used this procedure to alter the chemical composition of corn by growing progeny rows from corn ears picked from desirable plants. Superior rows were harvested and increased as a new cultivar. The method as applied to corn is called ear - to- row breeding.

There are various half-sib progeny tests, such as, topcross progeny test, open-pollinated progeny test, and polycross progeny test. A half-sib is a plant  with a common parent or pollen source. Individuals in a half-sib selection are evaluated based on their half-sib progeny. Unlike mass selection in which individuals are selected solely on phenotypic basis, the half-sibs are selected based on the performance of their progenies. The specific identity of the pollen sources is not known.

Recurrent half-sib breeding has been used to improve agronomic traits as well as seed composition traits in corn. It is suited for improving traits with high heritability

and species that can produce sufficient seed per plant to grow a yield trial. Species with selfincompatibility or some other constraint of sexual biology  are also suited to this method of breeding.

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2 Full-sib family selection

Full sibs are generated from biparental crosses using parents from the base population. The families are evaluated in a replicated trial to identify and select superior full-sib families, which are then recombined to initiate the next cycle.

Steps

Cycle 0

_ Season 1. Select random pairs of plants from the base population and intermate, pollinating one with the other. Make between 100 and 200 biparental crosses. Save the remnant seed of each full-sib cross.

_ Season 2. Evaluate full-sib progenies in multilocation replicated trails. Select promising half-sibs.

_ Season 3. Recombine selected full-sib. Cycle 1 Same as cycle C0

Genetic issues The genetic gain per cycle is given by:

where sFS is the phenotypic standard deviation of the full-sib families.

Application

Full-sib family selection has been used for maize improvement. Selection response per cycle of about 3.3% has been recorded in maize.

3 Selfed families selection

An S1 is a selfed plant from the base population. The key features are generations S1 or S2 families, evaluating them in replicated multi-environment trials, followed by recombination of remnant seed from selected families.

Steps

_ Season 1. Self pollinate about 300 selected S0 plants. Harvest the selfed seed and keep the remnant seed of each S1.

_ Season 2. Evaluate S1 progeny rows to identify superior progenies.

_ Season 3. Random mate selected S1 progenies to form C1 cycle population. Genetic issues The main reason for this scheme is to increase the magnitude of additive genetic variance. In theory the genetic gain is given by:

where sA1 2 is the additive genetic variance among S1, and sPSA is the phenotypic standard deviation among S1 families. The additive genetic variation among S2 is

two times that of S1. The S1 and S2, theoretically, have the highest expected genetic gain per cycle for intrapopulation improvement. However, various reports

have indicated that, in practice, full-sib and test cross selection have produced greater genetic gain for both populations per se and the population crosses.

Application

The S1 appears to be best suited for self-pollinated Species.It has been used in

maize breeding. One cycle is completed in three seasons in S1 and four seasons in S2. Genetic gain per cycle 3.3% has been recorded.

Family selection based on test cross

The key feature of this approach to selection is that it is designed to improve both the population per se as well as its combining ability. The choice of the tester is most critical to the success of the schemes. Using a tester to aid in selection increases the duration of a cycle by one year. The choice of a tester is critical to the success of a recurrent selection breeding program. The commonly used testers may be classified into two:a narrow genetic base tester, and a broad-genetic base tester. Broad-base testers are used for testing GCA in the population under improvement, whereas narrow genetic base testers are used to evaluate SCA and possibly GCA. Generally, plants are selected from the source population and selfed in year 1. Prior to intermating, the selected plants are crossed as females to a tester in year 2. Intermating of selected plants occur in year 3.

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Family selection methods

Family selection methods are characterized by three general steps:

Creation of a family structure.

Evaluation of families and selection of superior ones by progeny testing.

Recombination of selected families or plants within families to create a new base population for the next cycle of selection. Generally, the duration of each step is one generation, but variations exist.

1 Half-sib family selection methods

The basic feature of this group of methods is that half-sib families are created for evaluation and recombination, both steps occurring in one generation. The populations are created by random pollination of selected female plants in generation 1. The seed from generation 1 families are evaluated in replicated trials and in different environments for selection. There are different kinds of half-sib family selection methods, the simplest one being ear-to-row selection.

This method is applicable to cross-pollinated species.

Steps

_ Season 1. Grow source population and select desirable plants based on phenotype according to the traits of interest. Harvest plants individually. Keep remnant seed of each plant.

_ Season 2. Grow replicated half-sib progenies  from selected individuals in one environment. Select best progenies and bulk to create progenies of the next cycle. The bulk is grown inisolation and random mated.

_ Season 3. The seed is harvested and used to grown the next cycle. Alternatively, the breeder may bulk the remnant seed of S0 plants whose progeny have been selected and used that to initiate the next cycle.

Genetic issues

The expected genetic gain from half-sib selection is given by:

where sPHS is the standard deviation of the phenotypic variance among half-sibs. Other components are as before. The tester is the parental population, and hence selection or control is over only one sex. The genetic gain is hence reduced by half.

Genetic gain can be doubled by selfing each parent to obtain S1, then crossing to obtain half-sibs.

Application

Half-sib selection is widely used for breeding perennial forage grasses and legumes. A polycross mating system is used to generate the half-sib families from selected vegetatively maintained clones. The families are evaluated in replicated rows for 2–3 years. Selecting traits of high is effective. Modification The basic or traditional ear-to-row selection method did not show much gain over mass selection. An improvement was proposed by J.H. Lohnquist in which creation of family structure, evaluation and

recombination are conducted in one generation. The half-sib families are evaluated in replicated trials in many environments. The approach was to better manage the environmental and G _ E interactions.

Steps

_ Season 1. Select desirable plants from the source population. Harvest these open-pollinated individually.

_ Season 2. Grow progeny rows of selected plants at multiple locations and evaluate for yield performance. Plant female rows within seed from individual half-sib families alternating with male rows planted with bulked seed from the entire population. Select desirable plants from each progeny separately. Bulk the seed to start next cycle.

Genetic issues

The genetic gain has two components – among earrows across environments and

within families.The total genetic gain is given by: where swe is the square root of the plant-to-plant within plot variance. Others components are as before.

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