Sex-biased expression of microRNAs in Drosophila melanogaster.

Most animals have separate sexes. The differential expression of gene products, in particular that of gene regulators, is underlying sexual dimorphism. Analyses of sex-biased expression have focused mostly on protein-coding genes. Several lines of evidence indicate that microRNAs, a class of major gene regulators, are likely to have a significant role in sexual dimorphism. This role has not been systematically explored so far. Here, I study the sex-biased expression pattern of microRNAs in the model species Drosophila melanogaster. As with protein-coding genes, sex-biased microRNAs are associated with the reproductive function. Strikingly, contrary to protein-coding genes, male-biased microRNAs are enriched in the X chromosome, whereas female microRNAs are mostly autosomal. I propose that the chromosomal distribution is a consequence of high rates of de novo emergence, and a preference for new microRNAs to be expressed in the testis. I also suggest that demasculinization of the X chromosome may not affect microRNAs. Interestingly, female-biased microRNAs are often encoded within protein-coding genes that are also expressed in females. MicroRNAs with sex-biased expression do not preferentially target sex-biased gene transcripts. These results strongly suggest that the sex-biased expression of microRNAs is mainly a consequence of high rates of microRNA emergence in the X chromosome (male bias) or hitchhiked expression by host genes (female bias).