Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 247

The genotype of the sporophytic generation is highly heterozygous while the genotypes of gametes of a single plant are all different. The genetic structure of a cross‐pollinated species is characterized by a high level of heterozygosity. However, this is not to say that at each locus heterozygosity occurs. Especially when the allele frequency of certain genes is high (see Chapter 3), a plant may very well be homozygous for that locus. Another source of some homozygosity may be due to occasional selfing in a plant. Unlike allogamous species in which formation of new gene combinations are discouraged, cross‐pollinated species share a wide gene pool from which new combinations are created to form the next generation.

It is instructive to state that in autogamous crops in principle the whole genotype is transmitted through the generations (i.e. they are “immortal”). Homozygous plants reproduce genetically identically. Consequently, the unit of selection in a mixture of homozygous lines is genotype. In contrast, in allogamous crops, the unit of selection is the single gene. The gene in this case is “immortal.” Genotypes perish (lose their identity) at each round of sexual reproduction. The only way the genotype can become immortal and be the unit of selection in allogamous crops is when they are clonally propagated, as is the case in potato.

Allogamous species may to a varying extent undergo self‐fertilization. In that case the progeny usually suffers from inbreeding depression. Deleterious recessive alleles that were suppressed because of heterozygous advantage have opportunities to become homozygous and therefore become expressed. However, such depression is reversed upon cross‐pollination. Hybrid vigor (the increase in vigor of the hybrid over its partially homozygous and distinct parents) is exploited in hybrid seed production (see Chapter 18). In addition to hybrid breeding, population‐based improvement methods (e.g. mass selection, recurrent selection, and synthetic cultivars) are common methods of breeding cross‐pollinated species.