Heterozygosity in apomictic organisms.

Abstract

Pamilo, P. 1987. Heterozygosity in apomictic organisms. —Hereditus 207: 95–101. Lund, Sweden. ISSN 0018–0661. Received November 19, 1986 New mutations generally occur as heterozygotes. In the absence of recombination. e.g., in apomictic organisms, the affected loci remain heterozygous and the number of heterozygous loci per individual increases. Temporal changes of such multilocus heterozygosity are here examined in a diploid model in which mutation occurs at a rate u per gene per generation between two functional alleles and at a rate v from a functional to a non-functional (recessive lethal) allele. The main emphasis is in examining the expected temporal changes of functional heterozygosity. i.e., of the proportion of loci heterozygous for two functional alleles. This proportion crucially depends on the ratio u/v and on selection against the heterozygotes with one non-functional allele. If the non-functional alleles are completely recessive, the frequency of functional heterozygotes reaches a maximum value and later declines. However, if the non-functional alleles slightly reduce the fitness when heterozygous, their frequency is kept low by selection and the functional heterozygosity increases. Similar conclusions hold for polyploid apomictic organisms, but the poly-ploid models are here examined only under the assumption of complete recessiveness of the non-functional alleles. The single-locus stochastic model of selectively neutral variation shows that a monophyletic apomictic line can retain the ancestral genotype with a high frequency for quite a long time. This challenges the interpretation that such genotypic monomorphism is maintained by selection.