Evolution of Sex Chromosomes in Insects

Abstract

Sex chromosomes have many unusual features relative to autosomes. Y (or W) chromosomes lack genetic recombination, are male- (female-) limited, and show an abundance of genetically inert heterochromatic DNA but contain few functional genes. X (or Z) chromosomes also show sex-biased transmission (i.e., X chromosomes show female-biased and Z-chromosomes show male-biased inheritance) and are hemizygous in the heterogametic sex. Their unusual ploidy level and pattern of inheritance imply that sex chromosomes play a unique role in many biological processes and phenomena, including sex determination, epigenetic chromosome-wide regulation of gene expression, the distribution of genes in the genome, genomic conflict, local adaptation, and speciation. The vast diversity of sex chromosome systems in insects—ranging from the classical male heterogametic XY system in Drosophila to ZW systems in Lepidoptera or mobile genes determining sex as found in house flies—implies that insects can serve as unique model systems to study various functional and evolutionary aspects of these different processes.

Investigating Natural Variation inDrosophilaCourtship Song by the Evolve and Resequence Approach

“…However, it is worth noting that there is a deficit of genes with male-biased gene expression on the X chromosome in D. melanogaster (Parisi et al 2003;Ranz et al 2003). The reasons for this deficit are not yet entirely clear (Kaiser and Bachtrog 2010), but if it turns out that functional variation affecting male-specific sexually selected traits is underrepresented on the X, this may shed light on the causes of this deficit. Though drawing a firm conclusion is as yet premature, this result is a tantalizing example of insights that could be attained if methods of high-throughput mapping of genotype to phenotype are refined in model systems.…”

Production of all female progeny: an evidence for the presence of male sex determination factor on Y chromosome

“…Similarly, because the Y chromosome is never under selection in females, it is assumed to accumulate the genes that are beneficial to males (Kaiser and Bachtrog, 2010). Consistent with this, most of the genes on the Y chromosome in D. melanogaster are required for sperm motility and, in turn, male fertility (reviewed in Bernardo Carvalho et al, 2009).…”

“…bees, ants and wasps. Sex chromosome variations among different groups are further aided by different level of sex chromosome differentiation (reviewed in Kaiser and Bachtrog, 2010). Tribolium castaneum is an excellent model for the order Coleoptera, which follows an XY sex determination system.…”

The evolution of sex-biased gene expression in Drosophila serrata

“…The lack of significance may also be a statistical power issue given the relatively small number of genes in this subset of the data. However, if dosage compensation in D. serrata is gene-specific rather than chromosome-wide, as in birds (Mank et al 2008a;Itoh et al 2010) and some insects (Kaiser and Bachtrog 2010), then overexpression of X-linked female-specific genes may never have occurred. In that case, there would have been no selection for increased expression of these genes in males and thus no correlated response in females.…”

“…It remains evident that as new genomes are studied, novel phenomena are uncovered, which expose variation in the specific assumptions underlying some of these hypotheses (Prince et al 2010). Thus, further studies of sex-biased expression in additional taxa are required (Kaiser and Bachtrog 2010;Meiklejohn and Presgraves 2012). For this reason, I…”