How to Make a Hermaphrodite: Insights Into Sexual System Evolution in the Genome of Cosexual Moss Syntrichia princeps
Kaylee A. Macomb
Department of Biology
Faculty Supervisor: Jenna Ekwealor
Transitions in plant mating systems have shaped evolutionary history, yet the genomic mechanisms driving these changes remain poorly understood. Mosses provide an excellent model for exploring this question because they vary widely: half of the species are unisexual, producing separate male and female plants, while the other half are cosexual, with both reproductive organs on the same plant. We present a new genome assembly of the cosexual moss Syntrichia princeps and compare it with published genomes of its close unisexual relatives S. ruralis and S. caninervis to test the hypothesis that S. princeps retains both female (U) and male (V) sex-determining chromosomes. Such retention could have arisen through a historical genome duplication (polyploidy) or chromosome gain/loss (aneuploidy), providing a genomic basis for cosexuality. Using computational analyses in Python and Bash, including genome-wide synteny mapping (MCScan), we found a 2:1 synteny pattern between S. princeps and the unisexual species, consistent with polyploidy. However, only 15–22% of the S. princeps genome retains this duplication pattern, suggesting subsequent diploidization, where redundant genes and chromosomes are lost. Different blocks on the 12th and largest chromosome of S. princeps show homology to U and V chromosomes, indicating recombination or fusion of ancestral sex-determining regions of sex chromosomes. Our findings provide evidence that cosexuality in S. princeps evolved through whole or partial genome duplication and recombination, preserving elements of both male and female sex-determining regions.