Evolution of U/V sex chromosomes

Posted by Jessica Abbott

Homalothecium lutescens, by
HermannSchachner

For next week's lab meeting I thought it might be interesting to read a paper about the evolution of sex chromosomes in haploid species. I was recently on the examination committee for Frida Rosengren, who has worked on mosses, a typical group with haploid sex chromosomes. These haploid chromosomes are usually called U and V, to distinguish them from diploid ZW and XY system. The fact that they usually exist in a haploid state leads to some interesting predictions, for instance that degeneration of the male V chromosome should not occur. I haven't read this paper yet but I think it sounds interesting!

Title: The evolution of sex chromosomes in organisms with separate haploid sexes

Abstract: The evolution of dimorphic sex chromosomes is driven largely by the evolution of reduced recombination and the subsequent accumulation of deleterious mutations. Although these processes are increasingly well understood in diploid organisms, the evolution of dimorphic sex chromosomes in haploid organisms (U/V) has been virtually unstudied theoretically. We analyze a model to investigate the evolution of linkage between fitness loci and the sex-determining region in U/V species. In a second step, we test how prone nonrecombining regions are to degeneration due to accumulation of deleterious mutations. Our modeling predicts that the decay of recombination on the sex chromosomes and the addition of strata via fusions will be just as much a part of the evolution of haploid sex chromosomes as in diploid sex chromosome systems. Reduced recombination is broadly favored, as long as there is some fitness difference between haploid males and females. The degeneration of the sex-determining region due to the accumulation of deleterious mutations is expected to be slower in haploid organisms because of the absence of masking. Nevertheless, balancing selection often drives greater differentiation between the U/V sex chromosomes than in X/Y and Z/W systems. We summarize empirical evidence for haploid sex chromosome evolution and discuss our predictions in light of these findings.

Immler & Otto (2015) Evolution 69(3):694-708.