Edgar Walker
Sex specific evolutionary processes in Ceratodon purpureus
Mentors:  Dr. Stuart McDaniel and Dr. Ralph Quatrano, Biology Department, Washington University in St. Louis

In dioecious species, males and females may make different contributions to patterns of genetic variation within and among populations. Sexual dimorphism in transmission from one generation to the next will be reflected in sex specific genomic regions, such as sex chromosomes.

However, sex chromosomes are shaped by a unique set of evolutionary forces as a result of their hemizygous condition. Here we document sex-linkage of the des6 gene in the moss Ceratodon purpureus. It is expressed in early development in both males and females. The des6 gene is present in at least five copies on the male Y chromosome, and three copies on the female X chromosome. On the male chromosome, the sequence is conserved among all copies, while on the female chromosome one copy is ~5% diverged from the expressed copy. We propose that gene conversion maintains this sequence homogeneity among distinct copies.

Among populations males showed limited variation, suggesting a recent selective sweep. In females, we found evidence of two haplotypes, one containing an apparently functional divergent allele and one containing a degenerating divergent allele. The two haplotypes appear to be geographically widespread based on a broad survey of C. purpureus populations.

Overall these results suggest that evolutionary forces shaping sex-specific chromosomes in males and females of C. purpureus are similar to those acting on such chromosomes in other eukaryotic groups.