Originally published as Genetics Published Articles Ahead of Print on November 16, 2006.

Genetics, Vol. 175, 143-154, January 2007, Copyright © 2007
doi:10.1534/genetics.106.066381

SSP2 and OSW1, Two Sporulation-Specific Genes Involved in Spore Morphogenesis in Saccharomyces cerevisiae

Jing Li*, Seema Agarwal*,{dagger},1 and G. Shirleen Roeder*,{dagger},{ddagger},2

* Department of Molecular, Cellular and Developmental Biology, {dagger} Howard Hughes Medical Institute and {ddagger} Department of Genetics, Yale University, New Haven, Connecticut 06520-8103

2 Corresponding author: Department of Molecular, Cellular and Developmental Biology, Howard Hughes Medical Institute, Yale University, P.O. Box 208103, New Haven, CT 06520-8103.
E-mail: shirleen.roeder{at}yale.edu

Spore formation in Saccharomyces cerevisiae requires the synthesis of prospore membranes (PSMs) followed by the assembly of spore walls (SWs). We have characterized extensively the phenotypes of mutants in the sporulation-specific genes, SSP2 and OSW1, which are required for spore formation. A striking feature of the osw1 phenotype is asynchrony of spore development, with some spores displaying defects in PSM formation and others spores in the same ascus blocked at various stages in SW development. The Osw1 protein localizes to spindle pole bodies (SPBs) during meiotic nuclear division and subsequently to PSMs/SWs. We propose that Osw1 performs a regulatory function required to coordinate the different stages of spore morphogenesis. In the ssp2 mutant, nuclei are surrounded by PSMs and SWs; however, PSMs and SWs often also encapsulate anucleate bodies both inside and outside of spores. In addition, the SW is not as thick as in wild type. The ssp2 mutant defect is partially suppressed by overproduction of either Spo14 or Sso1, both of which promote the fusion of vesicles at the outer plaque of the SPB early in PSM formation. We propose that Ssp2 plays a role in vesicle fusion during PSM formation.