Abstract
Genetic incompatibility is believed to be the major cause of postzygotic reproductive isolation. Despite huge efforts seeking for speciation-related incompatibilities in the past several decades, a general understanding of how genetic incompatibility evolves in affecting hybrid fitness is not available, primarily due to the fact that the number of known incompatibilities is small. Instead of further mapping specific incompatible genes, in this paper we aimed to know the overall effects of incompatibility on fertility and viability, the two aspects of fitness, by examining 89 gametes produced by yeast S. cerevisiae - S. paradoxus F1 hybrids. Homozygous F2 hybrids formed by autodiploidization of F1 gametes were subject to tests for growth rate and sporulation efficiency. We observed much stronger defects in sporulation than in clonal growth for every single F2 hybrid strain, indicating that genetic incompatibility affects hybrid fertility more than hybrid viability in yeast. We related this finding in part to the fast-evolving nature of meiosis-related genes, and proposed that the generally low expression levels of these genes might be a cause of the observation.
Highlights
Postzygotic reproductive isolation is a key step of forming new species, and the underlying genetic mechanisms can be multiple [1,2,3,4]
Our result seems to be contradictory to a previous study in which only,18% of the Sc2Sp F2 hybrids are sterile [24]
The overall frequency of tetrasomy in the F2 hybrids of that study is 31%, and the number is 8.5% in our case when a similar approach was used to measure tetrasomy (244 out of 89*32 = 2848 chromosome ends show both Sp and Sc signatures) [24], which leads to the proportions of exposed incompatibilities being dramatically different between the two studies
Summary
Postzygotic reproductive isolation is a key step of forming new species, and the underlying genetic mechanisms can be multiple [1,2,3,4]. The model of between-gene incompatibility proposed by Dobzhansky [5] and Muller [6] posits that independent evolution of two populations can generate alleles that are completely normal on native genetic background but deleterious on genetic background of the other population, rendering hybrid inviability or sterility This is an appealing idea because it predicts that new species can evolve as a time-dependent byproduct of geographical separation of populations, and has received a lot of attention. Available incompatibilities identified from different organisms show an equivocal pattern regarding their effects on the two components of fitness [16] It is the aim of this study to systematically test whether genetic incompatibility affects hybrid fertility and hybrid viability differently
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