Many organisms reproduce through the fusion of gametes, like sperm and eggs. Gametes usually carry one copy of each chromosome and, when they combine in the process of fertilization, create a new cell, called a zygote, that has two copies of each chromosome. Over the last century the budding/brewers'/bakers' yeast, Saccharomyces cerevisiae, has become a powerful experimental system for learning about how genes work and how cells duplicate and separate their chromosomes and divide. However, very little is known about how yeast cells make gametes and how yeast fertilization works. Dr. McMurray's lab found that, contrary to prior assumptions, (i) yeast gametes are made "pre-oriented" in terms of the direction in which they grow prior to fertilization; (ii) if they forego mating and choose instead to start dividing with only a single set of chromosomes, they must first penetrate a cell wall that encases the gametes, and (iii) yeast gametes that fail to inherit all the chromosomes they are supposed to get are still capable of fertilization, and can produce a viable zygote so long as the other gamete has the missing chromosome(s). This project will elucidate the molecular bases of these three phenomena, providing major advances in understanding of gamete biology and how genetic information is transmitted during reproduction. An outreach program will partner local middle school students and Early Education majors in isolating wild yeast and looking for clues from gene sequences about how fertilization happens in various natural habitats, enriching curricula at both levels.
The long-term goal of this project is to understand the molecular mechanisms by which Saccharomyces gametes (spores) exit dormancy, escape from a specialized, inflexible cell wall, and subsequently mate or bud. The objectives of this project are to determine how spores are born "pre-polarized", how upon budding they penetrate the ascus wall (the residual wall of the diploid mother cell), and how a "maternal contribution" of RNAs allows spores to germinate and mate without accessing their genomes. The hypotheses are that 1) polarity factors deposited in spore membranes during sporulation persist at cortical locations near the ascus wall, 2) secreted cell-wall degrading enzymes allow local digestion of the ascus wall during budding, and 3) many RNAs made during sporulation are shared between spores, whereas those controlling mating type are segregated. Imaging and genetic manipulation in the Principal Investigator's lab and the lab of a collaborator will pinpoint when, where and which polarity factors are deposited during sporulation and which are required for polarity establishment. Similar imaging and mutation-based approaches will identify the cell wall degradation enzymes required for ascus wall penetration. In order to resolve the paradox of how spores share maternal molecules but segregate those required to establish identity, the Principal Investigator's lab will monitor the localization and temporal origin of essential and mating-type specific RNAs during sporulation, and manipulate candidate pathways controlling RNA segregation, including targeted RNA degradation, RNA methylation, clustering of RNAs by RNA-binding proteins, and production of anti-sense transcripts.
This project is co-funded by Integrative Organismal Systems, Molecular and Cellular Biosciences and the Rules of Life.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
