As with reproductive genes generally, we expect a faster evolution of plant pollen genes compared to sporophytic genes. Haploid expression in pollen leads to advantageous and deleterious alleles not being masked by a dominant homologue. A combination of haploid expression and pollen competition have been suggested as being responsible for stronger purifying and positive selection on pollen genes. However, it is unlikely that this mechanism is so straightforward in the model plant

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... sis thaliana. Since becoming self-compatible roughly 1 MYA high selfing rates have caused high homozygosity levels which in turn can be expected to reduce pollen competition and the effect of masking in diploid expressed, sporophytic genes. In this study we investigated the relative strength of selection on pollen genes compared to sporophytic genes inA. thaliana. We were especially interested in comparing ancestral selection patterns whenA. thalianawas an obligate outcrosser with more recent selection patterns since the plant's loss of self-incompatibility. We present two major findings: 1) before becoming self-compatible both positive and purifying selection was stronger on pollen genes than sporophytic genes forA. thaliana; 2) an apparent shift in selection efficacy has taken place sinceA. thalianabecame self-compatible. We present evidence which indicates that selection on pollen genes has become more relaxed and has led to higher polymorphism levels and a higher build-up of deleterious mutations in pollen genes compared to sporophytic genes.