Figure 1. Sex chromosomes in Aedes aegypti and Anopheles gambiae.

A) Sex-determining chromosomes in Ae. aegypti are homomorphic and karyotypically indistinguishable. A small 1.2-1.3 Mb region of divergence represents the M (blue line) or m (red line) locus near the centromere [22]. At the molecular level, however, a much broader 100Mb region showed sequence differentiation between the M-bearing and m-bearing chromosomes surrounding the M/m locus (e.g., [22]), indicated by the blue gradient and red gradient, respectively. The M locus harbors male-specific genes including the male-determining factor Nix and the myo-sex gene encoding a myosin heavy-chain protein necessary for male flight. White bars/lines within the M/m loci indicate gaps of various sizes. B) An. gambiae sex chromosomes are well differentiated X and Y chromosomes. As the Y chromosome has not been assembled at the chromosomal level, we compared the repeat and gene content of the non-recombining region of the Y (NRY, [18]) with those of the 24.39 Mb X chromosome assembly (vectorbase.org). The NRY is largely repetitive and only 2.2% of it is not masked by RepeatMasker (www.repeatmasker.org [97]), including five experimentally verified genes (An. gambiae Y genes or gYGs) such as the male determining factor gYG2/Yob [17–19]. Approximately 72.6% of the assembled X chromosome is comprised of single or low copy sequences that are not masked by RepeatMasker (www.repeatmasker.org) and it contains 1073 protein-coding genes.