Journal
GENETICS
Volume 207, Issue 2, Pages 711-727Publisher
GENETICS SOCIETY AMERICA
DOI: 10.1534/genetics.117.300151
Keywords
sex determination; heterogamety; genetic drift; drift-induced selection
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Funding
- Faculty of Arts and Sciences (FAS) Division of Science, Research Computing Group at Harvard University
- National Science Foundation (NSF) graduate research fellowship
- Finnish Center for Excellence in Biological Interactions
- Leverhulme Early Career Fellowship by the Leverhulme Trust
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Evolutionary transitions between male and female heterogamety are common in both vertebrates and invertebrates. Theoretical studies of these transitions have found that, when all genotypes are equally fit, continuous paths of intermediate equilibria link the two sex chromosome systems. This observation has led to a belief that neutral evolution along these paths can drive transitions, and that arbitrarily small fitness differences among sex chromosome genotypes can determine the system to which evolution leads. Here, we study stochastic evolutionary dynamics along these equilibrium paths. We find non-neutrality, both in transitions retaining the ancestral pair of sex chromosomes, and in those creating a new pair. In fact, substitution rates are biased in favor of dominant sex determining chromosomes, which fix with higher probabilities than mutations of no effect. Using diffusion approximations, we show that this non- neutrality is a result of drift-induced selection operating at every point along the equilibrium paths: stochastic jumps off the paths return with, on average, a directional bias in favor of the dominant segregating sex chromosome. Our results offer a novel explanation for the observed preponderance of dominant sex determining genes, and hint that drift-induced selection may be a common force in standard population genetic systems.
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