Communication and choice in yeast mating

Abstract

Dissertação apresentada para obtenção do grau de doutor em Biologia de Sistemas pelo Instituto de Tecnologia Química e Biológica da Universidade Nova de Lisboa.Cell-cell communication is essential for all organisms and a hallmark of multicellularity. In the budding yeast, Saccharomyces cerevisiae, mating occurs when two haploid cells of opposite mating types (a and α), communicate through secreted pheromones and the corresponding transmembrane receptors, to find each other and fuse. I focused on the mating system of S. cerevisiae and used a quantitative approach to ask how yeast cells communicate with each other. I show that α cells advertise their presence strongly and devote about 1% of their protein synthesis to making just enough α-factor pheromone to initiate this communication. The a cells can only respond accurately to a small range of secreted α-factor and express a protease, Bar1, to maintain the α-factor concentration within their gradient sensing regime. I argue that this is an efficient way to keep mating economic and robust. I then asked how yeast cells choose a partner and I propose that sexual identity, in yeast, is determined at the cell surface, by which receptors and pheromones each cell expresses. I also report that S. cerevisiae’s a cell receptor, Ste2, is surprisingly promiscuous and can respond to high concentrations of pheromones from distant species. I present evidence that the Ste2 receptors across the Ascomycota are not under positive selection, contrasting with most genes involved in speciation, and this can explain the cross-talk between different receptors and pheromones. I solve this paradox by arguing that most fungi cannot distinguish between self and non-self closely related species and that speciation in fungi is not happening at the receptor/pheromone level. I discuss these findings from a molecular and evolutionary perspective