Intellectual Merit: Sexual reproduction is the predominant mode of reproduction in eukaryotic systems, from single-celled yeast cells to mammalian organisms. The goal of this project is to understand the diversity of mechanisms that regulate sexual reproduction in yeast, and to determine how changes in these mechanisms can lead to distinct modes of sexual reproduction. Much of our understanding of sexual biology has been derived from work on the model yeast, Saccharomcyes cerevisiae. However, it is now apparent that other hemiascomycete yeast exhibit elemental differences in their sexual lifestyles to that of S. cerevisiae. This project will analyze these differences by comparing mechanisms of sexual reproduction in multiple species from the Candida clade of hemiascomycetes. These represent a diverse collection of yeast species, including those with traditional sexual programs (mating occurs between two partners of opposite sex), those that are able to undergo self-fertilization, and those that may be completely asexual. This project will examine how signaling via sexual pheromones regulates self-fertilization (inbreeding) as well as outbreeding in different species of yeast. In addition, the mechanism of meiosis will be addressed in Candida lusitaniae. This species undergoes an efficient program of meiotic chromosome reduction but is lacking many of the key genes regulating meiosis in S. cerevisiae. Completion of this research will reveal how sexual lifestyles can be modified or re-programmed to promote alternative lifestyles amongst closely related species of yeast.
Broader Impacts: This project will introduce approaches that foster integration of science in education and research using two broad themes. First, high school students from underrepresented minorities will be given Microbiology Fellowships to come to Brown University and participate in pre-college summer classes as well as full-time research projects in the project leader's laboratory. The project leader will also follow-up these fellowships by attending the fellows' high school to meet and encourage other students with a general interest in research in the biological sciences. The second approach will be to further the research opportunities for undergraduate students at Brown. Summer fellowships will be used to stimulate undergraduate interest in microbiology and genetics, and in particular to mentor students from underrepresented minorities.
This project examined the regulatory mechanisms controlling sexual reproduction in different yeast species. Many yeast species are capable of sex, which typically involves mating between cells of different mating types followed by meiosis, which acts to halve the genetic content of the cell. The major part of this project focused on one species in particular, Candida lusitaniae, as this is one of the few Candida species established to undergo efficient mating and meiosis. C. lusitaniae is also a rare pathogen of humans, able to cause mucosal and systemic infection. Using a combination of genetics and transcriptional profiling we addressed how mating and meiosis are regulated in C. lusitaniae. The results were surprising in that we showed that Ste12, a conserved regulator of meiosis in multiple yeast species, not only regulated meiosis in C. lusitaniae but was also critical for mating. Conversely, we found that Ime2, a kinase involved in regulating meiosis in the model yeast S. cerevisiae, regulated both mating and meiosis in C. lusitaniae. Thus, we concluded that the programs regulating mating and meiosis are separate in S. cerevisiae (and many other yeast species) but have become fused in C. lusitanaie. We were also struck that the co-regulation of mating/meiosis in C. lusitaniae resembles that in the distantly related yeast species, S. pombe. We therefore propose that the fusion of mating and meiosis programs has occurred multiple times during evolution. The question remains as to what, if any, selective advantage the fusion of these programs provides to the species. One possibility is that it promotes propagation of cells in the haploid state over the diploid state, and that haploid cells may exhibit a fitness advantage over diploid cells. This possibility is currently being investigated by ongoing experiments.
