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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