Mate recognition in moths : a model of speciation

Abstract

Understanding the processes that have given rise to the diversity of organisms that we see on the planet today is an important aspect of evolutionary biology. Central to understanding the molecular processes underlying speciation is the identification of model systems comprising closely related species where the signals underlying diversification events may still be detectable. Recognising an appropriate mate is an essential part of the evolutionary process and may play a pivotal role in speciation events. In the insect world, sex pheromones form the basis of an extremely sensitive and precise mate recognition system where male moths respond only to the sex pheromone produced by a conspecific female. The work presented in this thesis describes the use of the New Zealand native brown-headed and greenheaded leafroller moths, Ctenopseustis and Planotortrix, as a model system to understand the molecular events underlying mate recognition and speciation. Within the Ctenopseustis and Planotortrix genera, the specificity of the sex pheromones is determined solely by differences in the position of a single double bond. We propose that the desaturase enzymes involved in the double bond formation, are likely to play a key role in creating the species-specific sex pheromones. This work presents the isolation of desaturase gene transcripts from two Ctenopseustis and three Planotortrix species. Analysis of these sequences revealed the presence of a desaturase gene family consisting of at least six distinct groups in all species, rather than the presence of individual specialised desaturase genes in each species. The influence of selection on the gene family was investigated using a number of computational approaches. This analysis indicated that the desaturase genes are currently under strong purifying selection, which implies that these genes are under strong structural and functional constraint. Further analysis using lineage selection models revealed a role for positive selection during the formation and functional diversification of the desaturase gene family. Differences in transcriptional regulation of the desaturase gene family between species were investigated by quantitative real-time RT-PCR. This analysis revealed the differential regulation of desaturase gene expression between species and correlates specific desaturase expression with sex pheromone production. Specific methods to determine the precise biochemical function of the desaturases were investigated. Preliminary results indicate the presence of desaturase activities specific to sex pheromone production rather than general lipid metabolism. This work highlights the importance of regulatory networks as an evolutionary mechanism and supports the hypothesis that speciation may occur via changes in regulatory networks controlling key pathways.