NSF Postdoctoral Fellowship in Biology FY 2021: Leveraging Frequent Monoicy-dioicy Transitions in Hornworts to Understand the Rules of Sex Determination

Schafran, Peter W, Ithaca NY

Investigators

Abstract

This action funds an NSF Plant Genome Postdoctoral Research Fellowship in Biology for FY 2021. The fellowship supports a research and training plan in a host laboratory for the Fellow who also presents a plan to broaden participation in biology. The title of the research and training plan for this fellowship to Peter Schafran is "Leveraging frequent monoicy-dioicy transitions in hornworts to understand the rules of sex determination" The host institution for the fellowship is the Boyce Thompson Institute and the sponsoring scientist is Dr. Fay-Wei Li. In animals sex determination is fairly consistent across diverse groups, such as the XY chromosomes that originated once in the common ancestor of all placental mammals including dogs, cows, humans, elephants, whales, etc., over 166 million years ago. In contrast, the evolution of separate sexes in plants has occurred many times and over much shorter time periods. One plant group that shows this is hornworts, in which about half of all species have separate sexes that evolved from hermaphroditic ancestors at least 14 separate times. To understand why sexual system evolution is so flexible in plants, this project will examine how genes and chromosome structure change each time a species evolves to have separate male/female individuals. New genomes will be generated from hermaphroditic and male/female hornwort species representing over 300 million years of evolution in order to determine common genes that may be prerequisites for formation of separate sexes. This project will also examine patterns of change in genome architecture over time, as sex-determining genes eventually cause the formation of sex chromosomes, which then may be more likely to degrade. Identifying these processes in hornworts will uncover mechanisms that may provide a model for evolution of separate sexes across all plants and a contrast for the starkly different pattern in animals. A better understanding of how chromosomes and genes function will enhance efforts to improve crop plants. This project will provide varied opportunities for training high school and undergraduate students in tissue culture, genome assembly, comparative genomics, and plant transformation. This project will leverage multiple monoicy-dioicy transitions across the hornworts in order to test whether common rules govern the evolution of sex-linked genes and sex chromosomes. Evidence will be drawn from a broad range of phylogenetic diversity, evolutionary time, genomic scales, and gene-phenotype associations. Three specific objectives are: 1) characterize the structure and composition of hornwort sex chromosomes; 2) identify candidate sex-determining genes by examining genes associated with sex chromosomes and under selection in multiple lineages; 3) functionally characterize candidate sex-determining genes by transgenically over-expressing them in the monoicous model hornwort Anthoceros agrestis. Multiple genome assembly techniques will be used to create 19 new hornwort genomes representing monoicous and dioicous species assembled to pseudochromosome level. Genetic analyses will examine gene orthologs from pairs of monoicous and dioicous species to identify those with evidence of selection across multiple independent transitions between monoicy and dioicy. For genes found to recurrently appear to be selected for on sex chromosomes, their over-expression in A. agrestis is expected to affect the successful development gametes/gametangia and their ability to form viable sporophytes. The expected results will contribute functional knowledge about specific genes, and about the associated chromosome structural and compositional variations that lead to formation of sex chromosomes. All data will be publicly available through GenBank as well as HornwortBase, a new platform for hornwort genomic analysis (www.hornwortbase.org). Results will be communicated through public lectures and open access publications. 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.