Allopatric divergence, secondary contact, and genetic isolation in wild yeast populations.

In plants and animals, new biological species clearly have arisen as a byproduct of genetic divergence in allopatry. However, our understanding of the processes that generate new microbial species remains limited [1] despite the large contribution of microbes to the world's biodiversity. A recent hypothesis claims that microbes lack biogeographical divergence because their population sizes are large and their migration rates are presumably high [2, 3]. In recapitulating the classic microbial-ecology dictum that "everything is everywhere, and the environment selects"[4, 5], this hypothesis casts doubt on whether geographic divergence promotes speciation in microbes. To date, its predictions have been tested primarily with data from eubacteria and archaebacteria [6-8]. However, this hypothesis's most important implication is in sexual eukaryotic microbes, where migration and genetic admixture are specifically predicted to inhibit allopatric divergence and speciation [9]. Here, we use nuclear-sequence data from globally distributed natural populations of the yeast Saccharomyces paradoxus to investigate the role of geography in generating diversity in sexual eukaryotic microbes. We show that these populations have undergone allopatric divergence and then secondary contact without genetic admixture. Our data thus support the occurrence of evolutionary processes necessary for allopatric speciation in sexual microbes.

Mate choice assays and mating propensity differences in natural yeast populations.

In sexual microbes, mating occurs by fusion of individual cells. This complete fitness investment suggests that cell behaviour could potentially mediate prezygotic isolation between microbial species, a topic about which very little is known. To investigate this possibility, we conducted individual cell mate choice trials and mass-culture mating propensity assays with isolates from sympatric natural populations of the closely related yeasts Saccharomyces cerevisiae and Saccharomyces paradoxus. Although we found no evidence for active species recognition in mate choice, we observed a marked difference in mating propensity between these two species. We briefly discuss the possibility that this mating propensity difference may contribute to reproductive isolation between S. cerevisiae and S. paradoxus in nature.

Distribution and sequence analysis of a novel Ty3-like element in natural Saccharomyces paradoxus isolates.

Little is known about the transposable elements of species closely related to Saccharomyces cerevisiae. We present a novel transposable element in Saccharomyces paradoxus, a close congener of S. cerevisiae. Sequence analysis of this element, designated Ty3-1p, indicates that it is a homologue of the S. cerevisiae Ty3 element. Ty3-1p shares 82% nucleotide identity with an S. cerevisiae Ty3 element and appears to be structured identically to Ty3, containing two overlapping open reading frames, six retroviral-like domains, a J domain, and flanking sigma-like elements. A sigma element from Ty3-1p is 75% identical to a Ty3 sigma element. There is no evidence of horizontal transfer of Ty3 in Saccharomyces sensu stricto. We assess the distributions of Ty3p and Ty3 element insertions in natural population samples of S. paradoxus and S. cerevisiae. The S. paradoxus population sample exhibits Ty3p insertions present at a variety of sites at low frequency; this suggests that Ty3p elements are active in the sampled population. The S. cerevisiae population sample exhibits a uniform Ty3 hybridization profile in which all element insertions appear to be fixed. We comment on the possible causes of these contrasting observed distributions (GenBank Accession Nos AY198186 and AY198187).