Breeding Methods for Outcrossing Plant Species: II. Hybrid Cultivars

Abstract

Abstract Hybrid cultivars are considered to be the most advanced population structure to enhance crop performance from the capture of genetic uniformity and polyheterozygosity (i.e., hybrid vigor). To be economically feasible, however, the crop value of the hybrid cultivar as compared with the OP must also exceed the increased costs of hybrid seed production. Recurrent selection is used to increase the frequency of desirable alleles in incipient parent lines and also to achieve fixation (homozygosity). Inbreeding depression is often encountered following successive cycles of self- or sib-matings to attain allele fixation. Sib-mating is employed instead of self-pollination to retard the fixation process and allow repulsion to be converted into coupling linkages. Heterosis, or hybrid vigor, is considered to be the diametric end of the inbreeding depression spectrum. In other words, they are the extremes of the same genetic/physiological/developmental phenomenon. Heterosis is a dynamic statistic that is highly dependent on genotypes, environments, phenotypes, and the method by which phenotypes are measured. The “dominance” and “overdominance” theories were advanced in the early 20th century to explain the mechanistic bases of heterosis. More recent molecular studies have demonstrated that heterosis can be attributed to both theories when considering a broad spectrum of examples. Following the assembly of germplasm, a program of pedigree or recurrent selection is invoked to produce two or more inbred lines that exhibit high mutual combining abilities. The most prevalent strategy is to employ recurrent selection for GCA in early generations followed by selection for SCA. Selection for GCA in early generations greatly reduces the need to perform labor- and space-intensive diallels and half-diallels. Alternatively, reciprocal recurrent selection may be used for all or most of the program to simultaneously select for mutual combining ability. Sib- and half-sib mating may be employed as an alternative to self-pollination to retard the rate of drive to fixation and allow more opportunities for recombination events to occur. Doubled haploids can greatly reduce the time required for inbred parent development, but this strategy limits recombination and is subject to technical and resource barriers. Opportunities for achieving efficiencies with MAS are substantial in recurrent selection and selecting for heterosis. While production of hybrid corn seeds is relatively easy, most crop species that feature hermaphroditic flowers and are capable of self-pollination present more of a challenge for hybrid seed production. Genetic variants that embody changes in floral structure and function such as gynoecy, male sterility, and self-incompatibility have been adapted as tools for the production of large quantities of genetically pure hybrid seeds. Constructing parental genotypes that possess high combining ability concomitantly with genes for reproductive traits presents more challenges to the plant breeder.