Saccharomyces spores are born prepolarized to outgrow away from spore-spore connections and penetrate the ascus wall

Abstract

How non-spore haploid Saccharomyces cells choose sites of budding and polarize towards pheromone signals in order to mate has been a subject of intense study. Unlike non-spore haploids, sibling spores produced via meiosis and sporulation by a diploid cell are physically interconnected and encased in a sac derived from the old cell wall of the diploid, called the ascus. Non-spore haploids bud adjacent to previous sites of budding, relying on stable cortical landmarks laid down during prior divisions, but since spore membranes are made de novo it was assumed that, as is known for fission yeast, Saccharomyces spores break symmetry and polarize at random locations. Here we show that this assumption is incorrect: Saccharomyces cerevisiae spores are born prepolarized to outgrow, prior to budding or mating, away from interspore bridges. Consequently, when spores bud within an intact ascus, their buds locally penetrate the ascus wall, and when they mate, the resulting zygotes adopt a unique morphology reflective of re-polarization towards pheromone, which we dub the derrière. Long-lived cortical foci containing the septin Cdc10 mark polarity sites, but the canonical bud site selection program is dispensable for spore polarity, thus the origin and molecular composition of these landmarks remain unknown. These findings demand further investigation of previously overlooked mechanisms of polarity establishment and local cell wall digestion, and highlight how a key step in the Saccharomyces life cycle has been historically neglected.Competing Interest StatementThe authors have declared no competing interest.View Full Text