Abstract
Aedes aegypti is the principal vector of several important arboviruses. Among the methods of vector control to limit transmission of disease are genetic strategies that involve the release of sterile or genetically modified non-biting males (Alphey 2014), which has generated interest in manipulating mosquito sex ratios (Gilles et al. 2014; Adelman and Tu 2016). Sex determination in Ae. aegypti is controlled by a non-recombining Y chromosome-like region called the M locus (Craig et al. 1960), yet characterisation of this locus has been thwarted by the repetitive nature of the genome (Hall et al. 2015). In 2015, an M locus gene named Nix was identified that displays the qualities of a sex determination switch (Hall et al. 2015). With the use of a whole-genome BAC library, we amplified and sequenced a ~200kb region containing this male-determining gene. In this study, we show that Nix is comprised of two exons separated by a 99kb intron, making it an unusually large gene. The intron sequence is highly repetitive and exhibits features in common with old Y chromosomes, and we speculate that the lack of recombination at the M locus has allowed the expansion of repeats in a manner characteristic of a sex-limited chromosome, in accordance with proposed models of sex chromosome evolution in insects.
