Abstract
BackgroundDespite virtually identical DNA sequences between the sexes, sexual dimorphism is a widespread phenomenon in nature. To a large extent the systematic differences between the sexes must therefore arise from processes involving gene regulation. In accordance, sexual dimorphism in gene expression is common and extensive. Genes with sexually dimorphic regulation are known to evolve rapidly, both in DNA sequence and in gene expression profile. Studies of gene expression in related species can shed light on the flexibility, or degree of conservation, of the gene expression profiles underlying sexual dimorphism.ResultsWe have studied the extent of sexual dimorphism in gene expression in the brain of two species of songbirds, the zebra finch (Taeniopygia guttata) and the common whitethroat (Sylvia communis), using large-scale microarray technology. Sexual dimorphism in gene expression was extensive in both species, and predominantly sex-linked: most genes identified were male-biased and Z-linked. Interestingly, approximately 50% of the male-biased Z-linked genes were sex-biased only in one of the study species.ConclusionOur results corroborate the results of recent studies in chicken and zebra finch which have been interpreted as caused by a low degree of dosage compensation in female birds (i.e. the heterogametic sex). Moreover, they suggest that zebra finches and common whitethroats dosage compensate partly different sets of genes on the Z chromosome. It is possible that this pattern reflects differences in either the essentiality or the level of sexual antagonism of these genes in the respective species. Such differences might correspond to genes with different rates of evolution related to sexual dimorphism in the avian brain, and might therefore be correlated with differences between the species in sex-specific behaviours.
Highlights
Despite virtually identical DNA sequences between the sexes, sexual dimorphism is a widespread phenomenon in nature
Sex-biased genes evolve rapidly, both in terms of their DNA sequence and in their gene expression profiles [5,13,14,15,16,17,18,19]. Due to their rapid evolution and the fact that they are often involved in reproduction [20,21,22] or are coding for species specific traits, like for example song and plumage in birds, sex-biased genes are likely to play an important role in sexual selection and speciation [5,23,24,25,26]
The Significance Analysis of Microarray (SAM) analyses showed that 509 of these 21081 ESTs (i.e. 2.4%) were significantly differentially expressed between the sexes at a false discovery rate of 3.9% (Table 1; delta parameter = 1.28), while the expected number of false discoveries in this data should be less than 20 ESTs
Summary
Despite virtually identical DNA sequences between the sexes, sexual dimorphism is a widespread phenomenon in nature. Genes with sexually dimorphic regulation are known to evolve rapidly, both in DNA sequence and in gene expression profile. Sex-biased genes evolve rapidly, both in terms of their DNA sequence and in their gene expression profiles [5,13,14,15,16,17,18,19] Due to their rapid evolution and the fact that they are often involved in reproduction [20,21,22] or are coding for species specific traits, like for example song and plumage in birds, sex-biased genes are likely to play an important role in sexual selection and speciation [5,23,24,25,26]. The sex-biased gene expression in the brain is perhaps of particular interest in this context, because it is linked to behavioural differences between males and females [20,21,22,30] and thereby to the evolutionary basis of sex-specific behaviours such and how these vary between species
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