Sequencing and Characterization of a Maternal-Effect Sex Determining Autosomal Inversion in the Hessian Fly
The unusual sex-determination system of the Hessian fly provides an excellent opportunity to investigate early sex chromosome evolution, sex determination, and chromosome behavior. The female Hessian fly has two copies of each X chromosome- one copy from each parentwhereas the male has only the copies contributed by his mother. However, the Hessian fly has no heterogametic sex; both the mother and father contribute a copy of each somatic chromosome (two X chromosomes and two autosomes) to each of their gametes. The sex-determining karyotype in males is es- tablished through elimination of paternal X chromosomes during early embryogenesis. Whether an embryo discards its paternal X chromosomes, resulting in male develop- ment, or retains these chromosomes, resulting in female development, depends on the genotype of the mother. An inversion on the long arm of Autosome 1 (A1) has sup- pressed recombination around a sex-determining master switch, causing it to take on the role of a maternal-effect neo-W chromosome. In a ZW sex-determination system, sex is determined by the female gamete; the female is the heterogametic sex (ZW) whereas the male is homogametic (ZZ). We refer to A1 with the sex-determining inversion as W′ (prime representing the maternal effect) and A1 lacking the inver- sion as Z. Female-producing females (ZW′) contribute W′ to half of their offspring, which become female-producing females, and Z to the other half, which become male- producing females (ZZ). The presence of W′ in the mother prevents the elimination of paternal X chromosomes in her offspring, resulting in the female karyotype. The offspring of mothers lacking W are typically all male; however, there is a third form of A1 that results in the production of both male and female offspring. The sequence of W′, its evolutionary history, and the mechanism by which it prevents paternal X chromosome elimination are unknown. As a first step in addressing these unknowns, W′ and Z from both a New World and an Old World Hessian fly population were sequenced and characterized. The W′ and Z sequences reveal that the inversion occurred prior to the divergence of the New World and Old World populations and that relatively few changes have since accumulated within the inversion sequence of either population. Additionally, a region of A1 outside of the inversion on Scaffold A1.36 has been identified in which recombination between Z and W′ has been suppressed; this region may also have a role in sex determination. Genes were annotated for both the inversion scaffolds and Scaffold A1.36. Candidate genes for the sex-determination master switch have been selected from these regions based on their predicted functions and differences between the Z and W′ sequences of the genes.