Abstract
Males and females often have marked phenotypic differences, and the expression of these dissimilarities invariably involves sex differences in gene expression. Sex-biased gene expression has been well characterized in animal species, where a high proportion of the genome may be differentially regulated in males and females during development. Male-biased genes tend to evolve more rapidly than female-biased genes, implying differences in the strength of the selective forces acting on the two sexes. Analyses of sex-biased gene expression have focused on organisms that exhibit separate sexes during the diploid phase of the life cycle (diploid sexual systems), but the genetic nature of the sexual system is expected to influence the evolutionary trajectories of sex-biased genes. We analyze here the patterns of sex-biased gene expression in Ectocarpus, a brown alga with haploid sex determination (dioicy) and a low level of phenotypic sexual dimorphism. In Ectocarpus, female-biased genes were found to be evolving as rapidly as male-biased genes. Moreover, genes expressed at fertility showed faster rates of evolution than genes expressed in immature gametophytes. Both male- and female-biased genes had a greater proportion of sites experiencing positive selection, suggesting that their accelerated evolution is at least partly driven by adaptive evolution. Gene duplication appears to have played a significant role in the generation of sex-biased genes in Ectocarpus, expanding previous models that propose this mechanism for the resolution of sexual antagonism in diploid systems. The patterns of sex-biased gene expression in Ectocarpus are consistent both with predicted characteristics of UV (haploid) sexual systems and with the distinctive aspects of this organism's reproductive biology.
Highlights
In many animal and plant species, males differ markedly from females in morphology, physiology, and behavior. Most of these phenotypic differences are mediated by differential gene expression in the two sexes (Ellegren and Parsch 2007) and this differential gene expression may involve a significant proportion of the genome, as much as 75% in Drosophila for example (Assis et al 2012)
Analyses of sex-biased gene expression in Drosophila have shown that a large proportion of the transcriptome is differentially expressed in the two sexes (Ellegren and Parsch 2007; Jiang and Machado 2009; Assis et al 2012)
A similar observation was made for turkeys, where it was further shown that male-biased gene expression is significantly enhanced, across the genome, in dominant compared with subordinate males (Pointer et al 2013)
Summary
In many animal and plant species, males differ markedly from females in morphology, physiology, and behavior. Most of these phenotypic differences are mediated by differential gene expression in the two sexes (Ellegren and Parsch 2007) and this differential gene expression may involve a significant proportion of the genome, as much as 75% in Drosophila for example (Assis et al 2012). These sexually dimorphic patterns of gene expression evolve as a consequence of different selection pressures acting on males and females. There are marked morphological differences between male and female individuals of the species that were studied and analyses of species displaying different degrees of sexual dimorphism would be useful to test the correlation between this character and level of sex-biased gene expression
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