Clonal cultivars

Seeds are used to produce most commercial crop plants. However, a significant number of species are propagated by using plant parts other than seed. By using vegetative parts, the cultivar produced consists of plants with identical genotypes and is homogeneous. However, the cultivar is genetically highly heterozygous. Some plant species are sexually reproducing but are propagated clonally by choice. Such species are improved through hybridization, so that when hybrid vigor exists it can be fixed and then the improved cultivar propagated asexually. In seed propagated hybrids, hybrid vigor is highest in the F1, but is reduced by 50% in each subsequent generation. In other words, whereas clonally propagated hybrid cultivars may be harvested and used for planting the next season’s crop without adverse effects, producers of sexually reproducing species using hybrid seed must obtain a new supply of seed, as previously indicated.

Apomictic cultivars

Apomixis is the phenomenon of production of seed without the benefit of the union of sperm and egg cells.The seed harvested are thus genetically identical to the mother plant. Hence, apomictic cultivars have the same benefits of clonally propagated ones, as previously discussed. In addition, they have the convenience of vegetative propagation through seed. Apomixis is common in

perennial forage grasses.

Multilines

Multilines are developed for self-pollinating species.These cultivars consist of a mixture of specially developed genotypes called isolines  because they differ only in a single gene. Isolines are developed primarily for disease control, even though these cultivars,potentially, could be developed to address other environmental stresses. Isolines are developed by using the techniques of backcrossing in which the F1 is repeatedly crossed to one of the parents  that lacked the gene of interest.

Genetic structure of cultivars and its implications

The products of plant breeding that are released to farmers for use in production vary in genetic structure and, consequently, the phenotypic uniformity of the product. Furthermore, the nature of the product has implications in how it is maintained by the producers regarding the next season’s planting.

Homozygous and homogeneous cultivars

A cultivar may be genetically homozygous and, hence, produce a homogeneous phenotype or product. Selfpollinated species are naturally inbred and tend to be homozygous. Breeding strategies in these species are geared toward producing cultivars that are homozygous. The products of economic importance are uniform.

Furthermore, the farmer may save seed from the current season’s crop  for planting the next season’s crop, without loss of cultivar performance, regarding yield and product quality. This attribute is especially desirable to producers in many developing countries where the general tradition is to save seed from the current season for planting the next season. However, in developed economies with well-established commercial seed production systems, intellectual property rights prohibit the re-use of commercial seed for planting the next season’s crop, thus requiring seasonal purchase of seed by the farmer from seed companies.

Heterozygous and homogeneous cultivars

The method of breeding of certain crops leaves the cultivar genetically heterozygous yet phenotypically homogeneous. One such method is hybrid cultivar production, a method widely used for production of, especially, outcrossing species such as corn. The heterozygous genetic structure stems from the fact that a hybrid cultivar is the F1 product of a cross of highly inbred parents. Crossing such pure lines produces highly heterozygous F1 plants. Because the F1 is the final product released as a cultivar, all plants are uniformly heterozygous, and hence homogeneous in appearance. However, the seed harvested from the F1 cultivar is F2 seed, consequently producing maximum heterozygosity and heterogeneity upon planting. The implication for the farmer is that the current season’s seed cannot be saved for planting the next season’s crop for obvious reasons. The farmer who grows hybrid

cultivars must purchase fresh seed from the seed company for planting each season. Whereas this works well in developed economies, hybrids generally do not fit well into the farming systems of developing countries where farmers save seed from the current season for planting the next season’s crop. Nonetheless,the use of hybrid seed is gradually infiltrating crop production in developing countries.