Causes of heterosis and genetic incompatibility

Heterosis in F1 hybrids arises from genetic interactions between the paternal and maternal alleles of the same loci (via dominance and overdominance) and/or different loci (via positive intergenic epistasis) (4). Genetic incompatibility similarly originates from allelic interactions at the same loci (via underdominance) and/or different loci (via negative intergenic epistasis). At any locus, if the paternal and maternal alleles differ, then either both of them are derived from their common ancestral allele at their coalescence or only one of them is derived whereas the other is ancestral.

Because fitter alleles tend to be partially or completely dominant over less fit alleles (36), when homozygous individuals from different populations hybridize, dominance can cause the hybrid to outperform the average of the two parents and result in heterosis. Because the occurrence of heterosis by dominance requires a change from the ancestral state in only one parent, it should rise in proportion to mating distance D. Overdominance, underdominance, positive intergenic epistasis, and negative intergenic epistasis can obviously occur in the hybrid between two derived alleles that are respectively homozygous in the two parents. Should overdominance between an ancestral and a derived allele occur, the derived allele will likely stay in the heterozygous state in one population; hence, heterosis is unlikely to occur upon hybridization. Similarly, should positive intergenic epistasis exist between an ancestral and a derived allele, this positive effect is already seen in one parent and thus is not heterotic. Should underdominance or negative intergenic epistasis occur between an ancestral and a derived allele, the derived allele will likely be selectively removed from the population and therefore is unlikely to contribute to genetic incompatibility between the two parents. Therefore, the effects from overdominance, underdominance, positive intergenic epistasis, and negative intergenic epistasis should most likely increase in proportion to D2.

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