Colin Meiklejohn

Male-biased genes-those expressed at higher levels in males than in females-are underrepresented on the X chromosome of Drosophila melanogaster. Several evolutionary models have been posited to explain this so-called demasculinization of the X. Here, we show that the apparent paucity of male-biased genes on the X chromosome is attributable to global X-autosome differences in expression in Drosophila testes, owing to a lack of sex chromosome dosage compensation in the male germline, but not to any difference in the density of testis-specific or testis-biased genes on the X chromosome. First, using genome-wide gene expression data from 20 tissues, we find no evidence that genes with testis-specific expression are underrepresented on the X chromosome. Second, using contrasts in gene expression profiles among pairs of tissues, we recover a statistical underrepresentation of testis-biased genes on the X but find that the pattern largely disappears once we account for the lack of dosage compensation in the Drosophila male germline. Third, we find that computationally "demasculinizing" the autosomes is not sufficient to produce an expression profile similar to that of the X chromosome in the testes. Our findings thus show that the lack of sex chromosome dosage compensation in Drosophila testes can explain the apparent signal of demasculinization on the X, whereas evolutionary demasculinization of the X cannot explain its overall reduced expression in the testes.

Levels of nucleotide polymorphism in three paralogous Drosophila simulans genes, janusA ( janA), janusB ( janB), and ocnus (ocn), were surveyed by DNA sequencing. The three genes lie in tandem within a 2.5-kb region of chromosome arm 3R. In a sample of eight alleles from a worldwide distribution we found a significant departure from neutrality by several statistical tests. The

... Molecular Evolution of the janus and ocnus Genes in the Z). melanogaster Species Subgroup In D. melanogaster, janusA (janA)yjanusB {janB) and ocnus {peri) are located in a gene-dense region near the telomeric end of the right arm of chromosome 3. The genomic ...

Genes involved in male fertility are potential targets for sexual selection, and their evolution may play a role in reproductive isolation and speciation. Here we describe a new Drosophila melanogaster gene, ocnus (ocn), that encodes a protein abundant in testes nuclear extracts. RT-PCR indicates that ocn transcription is limited to males and is specific to testes. ocn shares homology with another testis-specific gene, janusB (janB), and is located just distal to janB on chromosome 3. The two genes also share homology with the adjacent janusA (janA) gene, suggesting that multiple duplication events have occurred within this region of the genome. We cloned and se- quenced these three genes from species of the D. melanogaster species subgroup. Phylogenetic analysis based on protein-encoding sequences predicts a duplication pattern of janA → janA janB → janA janB ocn, with the latter event occurring after the divergence of the D. melanogaster and Drosophila obscura species groups. We f...

Comparison of the gene-expression profiles between adults of Drosophila melanogaster and Drosophila simulans has uncovered the evolution of genes that exhibit sex-dependent regulation. Approximately half the genes showed differences in expression between the species, and among these, approximately 83% involved a gain, loss, increase, decrease, or reversal of sex-biased expression. Most of the interspecific differences in messenger RNA abundance affect male-biased genes. Genes that differ in expression between the species showed functional clustering only if they were sex-biased. Our results suggest that sex-dependent selection may drive changes in expression of many of the most rapidly evolving genes in the Drosophila transcriptome.