View Program - 2022 Population, Evolutionary, and Quantitative Genetics Conference

Medea alleles act maternally to kill embryos that don’t inherit them. Recent work has shown that polymorphic Medea loci, long known from flour beetles, are particularly abundant in the genomes of Caenorhabditis nematode species whose mating systems involve high rates of selfing. In several cases, loci harbor multiple antagonistic Medea alleles, such that each allele kills embyros homozygous for the alternate allele. This superficially looks like overdominance, and the elements occur on ancient haplotypes, suggestive of balancing selection. At the same time, these species have genomes that are mosaics of hyper-polymorphic and nearly monomorphic regions, and it’s unclear whether and how Medeas contribute to this broader pattern of heterogeneous genetic diversity.
I derived analytical results for dynamical models of antagonistic Medea elements and their paternal-effect counterparts, Peel elements. The evolutionary behavior of these alleles is profoundly affected by partial selfing, and by details of the mating system (e.g., monoecy vs androdioecy). The major finding is that partial selfing generates positive frequency dependence, such that a weakly penetrant allele that is at high frequency in a population can prevent invasion and displacement by a much stronger allele. The positive frequency dependence precludes overdominance as an explanation for the ancient haplotypes as there are no stable internal allele frequency equilibria for a single population. Instead, ancient haplotypes in Caenorhabditis genomes may result from abundant weak Medeas that act as barriers to gene flow among populations. Antagonistic Medeas effectively behave as local adaptation loci, locally fixed and resisting homogenization in the face of gene flow, despite the lack of phenotypic benefits for their bearers.

Medea elements are on ancient haplotypes but not for the reason you’d think