10 The Pathway of Cell and Nuclear Fusion during Mating in S. cerevisiae

Abstract

I. INTRODUCTION
In the yeast Saccharomyces cerevisiae, the process of conjugation begins with two haploid cells of opposite mating type (a and α) which ultimately fuse to form a single diploid zygote (for recent reviews, see Cross et al. 1988; Sprague and Thorner 1994). Along the way, the cells must recognize the presence and spatial position of the partner cell via the mutual exchange of sexual pheromones. Cells of opposite mating type adhere and begin the orderly removal of intervening cell wall and plasma membrane to complete cell fusion. As the cells fuse, the nuclei move together and fuse to produce a single diploid nucleus. The diploid zygote then reenters the cell cycle and mitosis gives rise to diploid buds. The diploid cells then may divide indefinitely or, under appropriate conditions, enter the meiotic pathway to produce haploid spores. The haploid spore progeny can then recapitulate the conjugation pathway. The conjugation pathway includes elements of signal transduction, cell polarity, membrane fusion, and microtubule function. As such, it is a microcosm whose elements are of particular interest to cell biologists. Because of the unique power of the genetics of the pathway, considerable progress has been made in the last decade. In this chapter, we review the pathway of conjugation in yeast, beginning with nuclear fusion, about which the most is known, and then finishing with the pathway of cell fusion. The material on nuclear fusion is reprinted with permission, with minor modification, from a recent review on the pathway of nuclear...