The influence of ploidy-specific expression on selection

More efficient selection is expected for haploid-expressed genes compared diploidexpressed genes. This is because recessive mutations can be masked from selection by a dominant allele in diploids but are always exposed to selection in haploids. The significance of this effect for haplodiploids was recognised by White in 1945, who predicted less efficient selection on genes with diploid-limited expression. I present the first empirical support for these predictions for haplodiploids on the buff-tailed bumblebee, Bombus terrestris. I found evidence for weaker purifying selection on diploid-biased genes compared to haploid-expressed genes. This has led to higher protein divergence rates and polymorphism levels in diploid-biased genes compared to haploid-expressed genes. In contrast, I found no evidence of greater positive selection on haploid-expressed genes, suggesting that most new, recessive mutations may be deleterious. In a second experiment I tested the effect of ploidy-specific selection in the plant Arabidopsis thaliana by comparing selection patterns between haploid pollen genes and diploid sporophytic genes. I detected evidence for a change in selection patterns possibly due to a loss of self-incompatibility. Divergence data indicate stronger positive selection within pollen genes during a period dominated by outcrossing, likely caused by pollen competition and haploid selection. Polymorphism data, on the other hand, reveal signs of relaxed selection within pollen genes, possibly due to high homozygosity levels, which reduce pollen competition and the masking of recessive mutations in diploid genes. In a third study I used the data produced for determining ploidy-biased genes in B. terrestris to infer expression patterns involved in caste determination. This is the first broad scale analysis on caste determination in bumblebees. One major finding was that the expression patterns of bumblebee workers more closely resemble those of queens when reproductive compared to higher eusocial Hymenoptera, possibly due to the more plastic nature of bumblebee worker castes.