A Screen for Recessive Speciation Genes Expressed in the Gametes of F1 Hybrid Yeast

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Abstract
Diploid hybrids of Saccharomyces cerevisiae and its closest relative, Saccharomyces paradoxus, are viable, but the sexual gametes they produce are not. One of several possible causes of this gamete inviability is incompatibility between genes from different species—such incompatible genes are usually called “speciation genes.” In diploid F1 hybrids, which contain a complete haploid genome from each species, the presence of compatible alleles can mask the effects of (recessive) incompatible speciation genes. But in the haploid gametes produced by F1 hybrids, recessive speciation genes may be exposed, killing the gametes and thus preventing F1 hybrids from reproducing sexually. Here I present the results of an experiment to detect incompatibilities that kill hybrid gametes. I transferred nine of the 16 S. paradoxus chromosomes individually into S. cerevisiae gametes and tested the ability of each to replace its S. cerevisiae homeolog. All nine chromosomes were compatible, producing nine viable haploid strains, each with 15 S. cerevisiae chromosomes and one S. paradoxus chromosome. Thus, none of these chromosomes contain speciation genes that were capable of killing the hybrid gametes that received them. This is a surprising result that suggests that such speciation genes do not play a major role in yeast speciation. A species is usually defined as such because it cannot exchange its genes with other species. Closely related species may attempt to breed but be unsuccessful. A common example of this occurs when a donkey mates with a horse. The offspring of this mating is a hybrid called a mule. Mules are sterile and cannot reproduce, so donkeys and horses are maintained as distinct species—they cannot exchange genes. Understanding what makes hybrids sterile could tell us how new species originate. Instead of mules, this study examines yeast hybrids that are sterile because the sex cells (the yeast equivalent of sperms or eggs) they produce are dead. One possible reason for this is that the genes from the different species fail to work together in the sex cells, killing them. To test this idea, I replaced individual chromosomes in one species' sex cells with chromosomes from another species. Surprisingly, this did not kill the gametes, showing that the genes from one species can work fine with the genes of another. Not all the genes could be tested in this way, but nevertheless it seems likely that the death of sex cells produced by yeast hybrids is caused by something other than failure of the genes from different species to work together.