Gene Duplication and the Genome Distribution of Sex-Biased Genes

Figure 1

Predictions of the standard DC (Model II and Model V in Table 1), MSCI (Model I in Table 1) and SA (Model VII in Table 1) models on the location of sex-biased genes. In XY systems with DC, the X chromosome is hypertranscribed to equal the level of expression of both autosomal and X chromosome genes in females (not shown). Note that the X chromosome is wider (i.e., hypertranscribed) than the autosomes. Under this hypothesis, there will be physical constraints evolution of male-biased expression for highly expressed genes located on the X chromosome on the (blue lines), while this constraint does not exist for genes evolving female-biased expression (pink lines) or even for male-biased genes expressed at low levels. Consequently, most male-biased genes will be located on the autosomes. Because the ZW system do not have dosage compensation, male-biased genes will be detected on the Z chromosome, but a correction must be made, with the expectation then being that male- and female-biased genes will be equally distributed among the chromosomes. MSCI exists in both XY and ZW systems. Because most sex-biased genes are gonad specific, male- and female-biased genes are predicted to be preferentially located outside of the X and Z chromosomes, respectively; otherwise, they would be inactivated in the gonads. Sexual antagonism predicts that the X and Z chromosomes will carry most of the sexually antagonistic alleles (blue circles for genes beneficial to males and detrimental to females; pink circles for genes beneficial to females and detrimental to males), which has been confirmed in Drosophila. Under the classical model [27], resolution of this conflict would be achieved through the evolution of sex-biased and, optimally, sex-specific expression of the antagonistic gene. Therefore, the X and Z chromosomes are predicted to hold the most sex-biased genes. Gray is used to represent heterochromatin or inactivation of the X/Z chromosomes during MSCI.