Sequencing and Characterization of a Maternal-Effect Sex Determining Autosomal Inversion in the Hessian Fly
Abstract
The unusual sex-determination system of the Hessian fly provides an excellent oppor- tunity to investigate early sex chromosome evolution, sex determination, and chromosome behavior. The female Hessian fly has two copies of each X chromosomeone 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 established through elimination of paternal X chromosomes during early embryogenesis. Whether an embryo discards its paternal X chromosomes, re- sulting in male development, 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 suppressed 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 Al with the sex-determining inversion as W' (prime rep- resenting the maternal effect) and Al lacking the inversion 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 Al 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 di- vergence 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 Al 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 determina- tion. Genes were annotated for both §he 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.
Degree
Ph.D.
Advisors
Stuart, Purdue University.
Subject Area
Genetics
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