It certainly has not declined as might be anticipated if detrimental alleles were purged from the inbred lines during the breeding program. Although the performance of inbred lines has been continually improved, the contribution of heterosis to yield has changed very little. More recently, Duvick ( 8) has analyzed the heterotic response of increasingly improved inbred lines used in the hybrid corn industry over many years. East ( 2), while reflecting on nearly 30 years of research on the topic, noted that the apparent purging of detrimental alleles during an inbreeding regime did not diminish the potential for a heterotic response when the end products were again joined in a hybrid state. The presence of superior alleles A and B in the hybrid leads to a better phenotype due to complementation, giving the impression of overdominance.Īlthough the dominance model has maintained a “dominance” among practitioners of the field, there are several observations on heterosis that do not seem to be easily explained by this concept. The superior phenotype in the F 1 hybrid can be attributed to a small chromosomal region, which contains two or more different loci (e.g., a and b) that are linked in repulsion phase. When brought together in the F 1 hybrid, allele B and B′ can interact to cause a superior phenotype compared with both the parental BB and B′B′ homozygous states. The homozygous alleles at the b locus are different between the inbred parent 1 (BB) and 2 (B′B′). This complementation could cause the F 1 hybrid to exhibit a superior phenotype than the better of its parents. In the F 1 hybrid, at each locus, the superior allele A, B, or C will complement the inferior alleles a, b, or c. Inbred parents 1 and 2 carry slightly deleterious homozygous alleles (a and c in parent 1 b in parent 2). It is hypothesized in the diagrammed models that a phenotype or trait is controlled by multiple linked or unlinked loci (e.g., a, b, and c). They also suggest that heterosis was selected over evolutionary time for characteristics that impact reproductive success.įig. They conclude that most traits that exhibit heterosis do so as a result of heterozygosity of the controlling genomic regions to produce traits superior to the better parent. ( 7) examined an extensive set of quantitative traits in partial hybrids of domesticated tomato and a wild relative. In this issue of PNAS, the work of Semel et al. Although the latter explanation is simple and easily envisioned, results that seemed to favor interactions of diverse alleles have been repeatedly found. As genetic knowledge increased, the concept that inferior alleles of different genes in the two parents were complemented in the hybrid ( 6), thus leading to the superior characteristics, gained favor. An early view was that the combination of different alleles in an organism resulted in a superior state for growth and vigor compared with the presence of identical alleles ( 3). The genetic basis of heterosis has been debated for nearly a hundred years without an emerging consensus ( 3– 5) ( Fig. Inbreeding depression refers to the decline in the quantitative measure of these characters upon self-fertilization or other forms of homozygosis of alleles (inbreeding). In plants, this is basically achieved by a greater proliferation of cells in some but not all tissues ( 2). Hybrid vigor, or heterosis, is the increase in stature, biomass, and fertility that characterizes the progeny of crosses between diverse parents such that the F 1 is superior to the better of the two parents. The study of hybrid vigor and inbreeding depression traces back to Charles Darwin, who was the first scientist to examine the phenomenon in a systematic manner ( 1).
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