Stage-Specific Expression Profiling of Drosophila Spermatogenesis Suggests that Meiotic Sex Chromosome Inactivation Drives Genomic Relocation of Testis-Expressed Genes

Long M (2009) Stage-Specific Expression Profiling of Drosophila Spermatogenesis Suggests that Meiotic Sex Chromosome Inactivation Drives Genomic Relocation of Testis-Expressed Genes. PLoS Genet 5(11): e1000731. doi:10.1371/journal.pgen.1000731 Stage-Specific Expression Profiling of Drosophila Spermatogenesis Suggests that Meiotic Sex Chromosome Inactivation Drives Genomic Relocation of Testis-Expressed Genes Maria D. Vibranovski 0 Hedibert F. Lopes 0 Timothy L. Karr 0 Manyuan Long 0 Harmit S. Malik, Fred Hutchinson Cancer Research Center, United States of America 0 1 Department of Ecology and Evolution, The University of Chicago , Chicago , Illinois, United States of America, 2 The University of Chicago Booth School of Business , Chicago , Illinois, United States of America, 3 The Biodesign Institute, Arizona State University , Tempe, Arizona , United States of America In Drosophila, genes expressed in males tend to accumulate on autosomes and are underrepresented on the X chromosome. In particular, genes expressed in testis have been observed to frequently relocate from the X chromosome to the autosomes. The inactivation of X-linked genes during male meiosis (i.e., meiotic sex chromosome inactivation-MSCI) was first proposed to explain male sterility caused by X-autosomal translocation in Drosophila, and more recently it was suggested that MSCI might provide the conditions under which selection would favor the accumulation of testis-expressed genes on autosomes. In order to investigate the impact of MSCI on Drosophila testis-expressed genes, we performed a global gene expression analysis of the three major phases of D. melanogaster spermatogenesis: mitosis, meiosis, and postmeiosis. First, we found evidence supporting the existence of MSCI by comparing the expression levels of X- and autosomelinked genes, finding the former to be significantly reduced in meiosis. Second, we observed that the paucity of X-linked testis-expressed genes was restricted to those genes highly expressed in meiosis. Third, we found that autosomal genes relocated through retroposition from the X chromosome were more often highly expressed in meiosis in contrast to their Xlinked parents. These results suggest MSCI as a general mechanism affecting the evolution of some testis-expressed genes. - Funding: This work was supported by a USA National Science Foundation CAREER award (MCB0238168) and USA National Institutes of Health R01 grants (R01GM065429-01A1 and R01GM078070-01A1) to ML and by a grant to TLK from the Royal Society and the Biotechnology and Biological Sciences Research Council. MDV was supported by Pew Latin America Fellowship and Brazilian Counsel of Technological and Scientific Development (CNPq). HFLs research was supported by The University of Chicago Booth School of Business. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Sex chromosomes evolve differently than autosomes due to their distinct characteristics such as a lack of recombination between the homologues (X and Y), a different number of chromosome copies between sexes, and the proportion of heterochromatin [1]. Genomic and proteomic studies in Drosophila, mammals and worms reveal that male-biased genes, i.e. those that are more highly expressed in males than in females, are underrepresented in the X chromosome [27]. One possible mechanism contributing to this biased chromosomal distribution is the directional movement of testis-expressed genes out of the X chromosome. In Drosophila and mammals, there is a significant excess of genes retroposed from the X chromosome to the autosomes, and these genes are more likely to be expressed in testis [811]. Recently, in Drosophila, this observation was found to also apply for DNA-based gene duplication [12,13]. Two evolutionary hypotheses have been proposed to explain the underrepresentation of genes expressed in males on the X chromosome. First, the meiotic sex chromosome inactivation (MSCI) hypothesis, can explain both the excess of retroposed genes from the X chromosome to the autosomes as well as the paucity of X-linked testis-expressed genes [9,14]. According to this hypothesis, the inactivation of X-linked genes during male meiosis favors the accumulation of testis-expressed genes in autosomes where such genes can be expressed during the meiotic stage [14]. A second hypothesis, based on a model proposed by Rice [15] and by Charlesworth and co-authors [16], states that sex biased expression is driven by sexually antagonistic forces, i.e., the case of opposing selection pressure on the two sexes. In this model developed through the investigation of polymorphic equilibrium, dominant mutations with beneficial fitness effects in females, but detrimental effects in males, will have a higher probability of being fixed on the X chromosome [15], while recessive sexually antagonistic mutations will have a higher probability of being fixed on the autosomes [16]. The opposite pattern is expected for male-beneficial, female-detrimental mutations: dominant and recessive alleles will have higher probability of being fixed on the autosomes and on the X chromosome, respectively [15]. Recently, however, Patten and Haig [17] showed that malebeneficial mutations could also be fixed on the X chromosome even for some cases of dominant alleles. In addition, is important During the course of Drosophila evolution, genes expressed in males have accumulated on the autosomes. Meiotic sex chromosome X inactivation in males was proposed, among other hypotheses, as a selective force favoring the accumulation of testis-expressed genes on the autosomes. Under such a model, the inactivation of X-linked genes would favor the accumulation of testis-expressed genes in autosomes, wherein these genes would still be expressed. In this study, we observed meiotic expression reduction for X-linked genes in D. melanogaster through a global gene expression analysis in different phases of spermatogenesis, in agreement with MSCI. In order to test the effects of MSCI on the chromosomal distribution of testis-expressed genes, we analyzed their expression pattern throughout spermatogenesis. First, X chromosome underrepresentation was restricted to testis-biased genes over-expressed in meiosis. Second, we observed that the autosomal genes retroposed from the X chromosome more often showed complementary expression in meiosis to their X-linked parents. These results support MSCI in Drosophila, suggesting its mechanistic role in the evolution of testis-expressed genes. to note that Gibson and colleagues theoretical and empirical findings [18] also suggest the X chromosome to be enriched in polymorphism for sexually antagonistic alleles. In order to relate the sexual antagonism hypothesis to the malebiased gene chromosomal distribution, one must assume a m (...truncated)