Rapidly-evolving low-complexity regions (LCRs) in a subset of sex-related genes

Jae-Hyeok Lee, Sunjoo Joo and Ursula Goodenough

Washington University in St. Louis, MO 63130, U.S.A.

Speciation involves not only physical isolation (e.g. geographic) but also selection processes upon specific traits that result in reproductive isolation. In the process of coupling the former to the latter, it would be benefited for a lineage to carry genes prone to offer up opportunities for reproductive isolation in the form of new alleles. Studies on the genes that directly participate in sex-related traits have frequently found that these genes are more divergent than control housekeeping genes. This biased divergence is open to two interpretations that are not mutually exclusive: 1) Sex-related genes might be "poised" to generate allelic diversity that enables the development of reproductive isolation; or 2) the sequence divergence might have been selected following the onset of reproductive isolation. In this study, we characterize the mode of increased divergence in several sex-related genes of Chlamydomonas , and focus on LCR endowment as a putative evolvability factor as predicted to incur repeat-slippage mutations. We identify 4 sex-related genes involved in mating and zygote development that carry abundant LCRs. We compare these genes with their orthologues in the closely related species C. incerta, and document that the LCRs indeed carry most of the amino-acid-substitutions and insertion/deletions (indels) that distinguish the two sets of genes. We report that similar mutational profiles have also accumulated as within-species polymorphisms in geographic isolates of C. reinhardtii, indicating that they cannot be explained as a post-speciation event. The functional outcome of inter-specific mutations in GSM1/GSP1 is under investigation using transgenic studies wherein C. incerta orthologues are introduced into C. reinhardtii.

e-mail address of presenting author: jleec@artsci.wustl.edu