Abstract
The evolution of sex chromosomes is classically considered to be driven by sexually antagonistic selection. However, selection during the haploid gametic phase of the lifecycle has recently received theoretical attention as possibly playing a central role in sex chromosome evolution, especially in plants. In particular, selection for reduced recombination on the sex chromosomes may occur as a result of intense haploid selection in males favouring the linkage of haploid beneficial alleles to an incipient Y chromosome. Here, we examine the evolution of gene expression in flower buds and pollen of two species of Rumex to test for signatures of sexual antagonism and haploid selection acting during sex chromosome evolution. We find that genes with high ancestral pollen expression bias occur more often on sex chromosomes than autosomes, and that genes on the Y chromosome are more likely to become enriched for pollen expression bias. We also find that genes with low expression in pollen are more likely to be lost from the Y chromosome. We found no comparable pattern for gene expression in male flower bud tissue suggesting that sexual antagonism among diploid parents may be a less important force in shaping Y chromosome evolution in Rumex. Our results suggest that selection during the haploid gametophytic stage of the lifecycle may be a major contributor to plant sex chromosome evolution.