Genomics of a gall midge: Avirulence and sex determination in the Hessian fly (Mayetiola destructor).

摘要

The Hessian fly, Mayetiola destructor, is economically important as one of the most destructive insect pests of wheat. This study focused on two topics related to the Hessian fly: its interaction with the host plant and its sex determination. The contents of this thesis were described in four chapters. In chapter one, I introduced some features of the Hessian fly, including the life history and damage, the avirulence, the chromosome behavior, and the sex determination pathway. In chapter two, I established the position of Avr gene vH6 in the Hessian fly genome sequence. Linkage analysis mapped 18 markers on chromosome X2. Fluorescent in situ hybridization (FISH) mapping determined that four markers are on the short arm and the other fourteen are on the long arm. vH6 was mapped in the proximal region of long arm, where recombination rate is reduced. Using a recombinant inbred line population refined the position of vH6 within three overlapping BAC clones, which correspond to a 485-505 kb DNA sequence in the Hessian fly genome. In chapter three, I determined the DNA sequence of a chromosome inversion, In(A1q1), which acts as the maternal master switch to drive the postzygotic sex determination in the Hessian fly (Benatti et al. 2010). In(A1q1) was estimated to be 2.1 Mb in length and 149 genes were predicted within the inversion. Interestingly, two predicted genes (gene 1 and gene 149) are disrupted by the breakpoints in the inverted chromosome. Transcription analysis demonstrated that gene 1 was maternal and expressed in a sex-specific mode. No differential expression was observed between females and males with regards to gene 149. In chapter four, I focused on characterization of the Hessian fly homologues of Drosophila sex determination genes, including sxl, tra-2, and dsx. Transcription analysis indicated that only dsx, not sxl and tra-2, was discriminative in females and males, supporting the hypothesis that the sex determination in insects evolves from bottom to top. In addition, the failure of identifying tra in the Hessian fly and some other nematocera species leads to a conjecture that sex determination is divergent at the level upstream of dsx between Drosophila and the Hessian fly.