Sexual Selection and Rates of Molecular Evolution in Flowering Plants

While sexual selection is known to be a major contributor to animal evolution and diversification, its role in plant evolution is largely unknown.  However, the conditions for sexual selection are exceptionally favourable in plants because the number of pollen grains deposited on stigmas often greatly exceeds that needed to fertilize ovules.  In this presentation, we ask whether rates of molecular evolution in plants are affected by sexual selection.  We specifically compare rates of protein evolution for genes predominantly expressed in sexual tissues (pollen / ovules) compared to those mainly expressed in somatic tissue.  We find that genes expressed in sexual tissues have a significantly higher ratio of nonsynonymous to synonymous substitution rates (dn/ds) than do somatic genes (p=0.00015).  Moreover, we show that rates of protein evolution are correlated with the strength of sexual selection, as estimated by the number of ovules per ovary.  Genes from sexual tissues evolve faster in species with few ovules (and putatively stronger sexual selection) than do sexual genes from species with many ovules (and putatively weaker sexual selection).  Lastly we compare rates of protein evolution in genes expressed mainly in pollen with those expressed in ovules, in an attempt to distinguish between the two main forms of sexual selection: male-male competition versus female choice.