Abstract
In species that have two sexes, a single genome encodes two morphs, as each sex can be thought of as a distinct morph. This means that the same set of genes are differentially expressed in the different sexes. Many questions emanate from this statement. What proportion of genes contributes to sexual dimorphism? How do they contribute to sexual dimorphism? How is sex-biased expression achieved? Which sex and what tissues contribute the most to sex-biased expression? Do sex-biased genes have the same evolutionary patterns as nonbiased genes? We review the current data on sex-biased expression in species with heteromorphic sex chromosomes and comment on the most important hypotheses suggested to explain the origin, evolution, and distribution patterns of sex-biased genes. In this perspective we emphasize how gene duplication serves as an important molecular mechanism to resolve genomic clashes and genetic conflicts by generating sex-biased genes, often sex-specific genes, and contributes greatly to the underlying genetic basis of sexual dimorphism.
Highlights
Sexual dimorphism occurs in species that produce differentiated sexes, most commonly, males and females
This fast evolution at the sequence and expression level and the fact that most male-biased genes are testis-specific [2, 90, 95, 97, 98, 116] suggest that once expression testisspecific expression is achieved, genes may be released from pleiotropic constraints and, be able to evolve more quickly responding to the specific selective pressures of the particular tissue at the expression and sequence level than more widely expressed genes [116]
The most obvious limitations of the first model is that it only explains the X chromosome deficit for highly expressed male-biased genes, and a limitation of both is that most male-biased genes are testis-specific [2, 61], a tissue where DC is reportedly absent or at least not mediated by the dosage compensation complex [29, 45, 143]
Summary
Sexual dimorphism occurs in species that produce differentiated sexes, most commonly, males and females. Subsequent analyses have confirmed these patterns in other species [12,13,14,15], and for DNA-mediated duplicates [14, 15], suggesting that the same evolutionary forces are probably favoring the relocation and sex-biased expression of duplicated genes After these evidences, new empirical and theoretical studies are attempting to integrate gene duplication in the sexual conflict resolution. X/Z chromosomes will spend two thirds of their time in the homogametic sex and only one third in the heterogametic sex These two circumstances have important consequences from a population and evolutionary point of view, especially for sexually antagonistic genes. As we will describe the evolution and distribution of sex-biased genes present several particularities that cannot be satisfactorily explained by any of the aforementioned biological phenomena unless gene duplication is introduced in the models (see several models in Table 1 and details in the text)
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