Mystery in the mechanism of yeast speciation

A paper in PLoS Genetics studied what happens when individual chromosomes of S. cerevisiae are replaced with a homologous copy its sister species, S. paradoxus. Previous work from Ken Wolfe’s lab interpreted the differential loss of genes after the whole genome duplication in the Saccharomyces lineage played a role in speciation among the yeast species. Surprisingly (or not, depending on how you interpret the previous work) Greig did not find any lethality in haploid F1 offspring from a diploid synthetically constructed individuals. Certainly this is not the last word but it represents a nice experimental screen to identify interacting genotypes. What would be interesting in followup work would be more subtle dissection of epistatic interactions among the genes on the different chromosomes to score phenotypes other than complete inviability. This might help understand what pathways are operating differently.

Several theories have been proposed about the mechanism of speciation. Excellent books and reviews (and more reviews) exist so I am punting on reviewing that literature here, but I want to point out one theory, Dobzhansky–Muller interactions since that is what Greig was attempting to screen for. Dobzhansky–Muller InteractionDM interactions look like this diagramming of two loci: A and B. The ancestral organism has the diploid genotype of AA BB for the A and B locus. Then the populations is subdivided and independent mutations arise and in fix in A and B locus creating a new homozygote genotype each locus (in lowercase). Finally the skull and crossbones indicates matings between individuals from the separate sub-populations fails because the double homozygote of the derived alleles (aa and bb) had never been tested since the aa genotype had never been seen in the bb genetic background.

Much of speciation genetics has been filled with the search and experimental confirmation of pairs of genes that have D-M interactions and prevent hybridization among two populations. Examples of DM interaction have been shown in plants (Monkey flowers) although the exact genes and the nature and mechanism of their interaction is still not known. The first confirmed example of DM interactions and the specific genes that interact in Drosophila was discovered by Dan Barbash’s lab at Cornell and described in their 2006 Science paper.

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