RESEARCH ARTICLE Analyses of expressed seq

Full list of author information is available at the end of the articleBackground Genes involved in sexual reproduction are found to evolve rapidly in diverse taxonomic groups (e.g., [1-3]). The phenomenon is especially well studied in animals, where rapidly evolving genes are found to be expressed in virtually all stages of reproduction, from mating to fertilization [1]. Large-scale evolutionary analyses show that, in general, genes expressed in animal reproductive organs show a higher divergence than genes expressed in other tissues [4-7]. Examples of rapid evolution are also seen in self/non-self recognition genes in plants [8,9] and in certain reproductive genes of a wide range of eukaryotic taxa, including algae and fungi [3,10-12]. The rapid evolution of reproductive genes is generally assumed to be adaptive [1] and in line with this assump-tion, positive selection as the driving force has been con-firmed for many fast-evolving reproductive genes (e.g., [5,8,13-16]). Multiple hypotheses have been proposed to explain the rapid evolution of reproductive genes in ani