CANCELLED - Origin of yeast's natural genome engineering system, mating-type switching

Thursday 21 November 2019, 1.00PM

Speaker(s): Kenneth H. Wolfe, Conway Institute and School of Medicine, University College Dublin, Dublin 4, Ireland

The whole-genome duplication (WGD) that occurred in an ancestor of Saccharomyces cerevisiae profoundly altered the biology of this yeast by forming hundreds of pairs of duplicated genes, many of which have diverged in function. We now know that the WGD was a hybridization between two divergent parental species, from different parts of the budding yeast phylogenetic tree. Interspecies hybrids are increasingly being detected by yeast genome sequencing projects, but the key thing that distinguishes the ancient WGD from most of the other (more recent) hybridizations is that the WGD hybrid managed to regain a complete sexual cycle, whereas most recent interspecies hybrids are unable to make viable ascospores. Based on observations on recently-formed naturally fertile interspecies hybrids of Zygosaccharomyces, we postulate that the key steps in regaining fertility in interspecies hybrids are (i) damage to one copy of the MAT locus, which converts a diploid hybrid cell (MATa/α) into a functional gamete (MATa or MATα), followed by (ii) mating-type switching, which allows two such gametes to mate and form a zygote in which every chromosome can pair with an identical partner during meiosis, making the hybrid species fertile. HO endonuclease, which catalyzes mating-type switching in the Saccharomycetaceae family, originated just before the WGD occurred and we propose that it was instrumental in the regain of fertility after WGD. We have found that HO is a domesticated member of a new type of mobile genetic element that is native to one of the two parental lineages that hybridized in the WGD.

Location: Dianna Bowles Lecture Theatre B/K/018