Sex-biased genes are thought to drive phenotypic differences between males and females. The recent availability of high-throughput gene expression data for many related species has led to a burst of investigations into the genomic and evolutionary properties of sex-biased genes. In Drosophila, a number of studies have found that X chromosomes are deficient in male-biased genes (demasculinized) and enriched for female-biased genes (feminized), and that male-biased genes evolve faster than female-biased genes. However, studies have yielded vastly different conclusions regarding the numbers of sex-biased genes and forces shaping their evolution.
Now, researchers at the University of California, Berkeley have used RNA-Seq data from multiple tissues of D. melanogaster and D. pseudoobscura, a species with a recently evolved X chromosome, to explore the evolution of sex-biased genes in Drosophila.
- They compared several independent metrics for classifying sex-biased genes and find that the overlap of genes identified by different metrics is small, particularly for female-biased genes.
- They investigated genome-wide expression patterns and uncovered evidence of demasculinization and feminization of both ancestral and new X chromosomes, demonstrating that gene content on sex chromosomes evolves rapidly.
- They examined the evolutionary rates of sex-biased genes and showed that male-biased genes evolve much faster than female-biased genes, which evolve at similar rates to unbiased genes.
Analysis of gene expression among tissues revealed that this trend may be partially due to pleiotropic effects of female-biased genes, which limits their evolutionary potential. Thus, these findings illustrate the importance of accurately identifying sex-biased genes and provide insight into their evolutionary dynamics in Drosophila.
- Assis R, Zhou Q, Bachtrog D. (2012) Sex-biased transcriptome evolution in Drosophila. Genome Biol Evol [Epub ahead of print]. [article]