After my PhD, I took a couple of months off to return to Singapore before starting a postdoctoral position in the lab of Professor Sean B. Carroll at the University of Wisconsin, Madison. Madison is a really beautiful place in summer, but probably too cold for a Singaporean like me during winter. Doing a postdoc in Sean’s lab is a fantastic experience. With very talented fellow postdocs and experienced technicians as colleagues, it was an intellectually stimulating environment. Thinking about broad ideas in biology is encouraged and there was support to take on ambitious long term projects. It is the scientific playground that many former Carroll lab students and postdocs learnt their trade and flourish.
In Sean’s lab, I have developed an independent research program to study the ecological factors and molecular mechanisms that affect various stages of the speciation process. I am interested in understanding how ecological factors can influence the speciation process and lead to the formation of species. Evolutionary changes in traits that affect both ecological divergence and mate choice are hypothesized to produce reproductive isolation which will lead to the formation of new species (“ecological speciation”). However, few examples of such dual traits (also known as ‘magic traits’) have been demonstrated, and the genetic and molecular bases of their evolution have not been identified. To understand the genetic mechanisms and evolution of dual traits, I investigated Drosophila species with varying degrees of reproductive isolation. This research would not be possible without the chemistry expertise and scientific knowledge of my collaborator, Professor Jocelyn Millar, who at times serves as a co-mentor.
Project 1: A single gene affects both ecological divergence and mate choice in Drosophila
(Chung et al. 2014 Science)
The first project involved a pair of Australian Drosophila species, D. serrata and D. birchii, which have overlapping ranges but exhibit strong premating reproductive isolation. While D. serrata is a habitat generalist, D. birchii is a rainforest specialist which is sensitive to desiccation, probably due to adaptation to the humid rainforest. I showed that these species differ in the amounts of methyl-branched cuticular hydrocarbons (mbCHCs), a class of compounds on the cuticle of the fly which has been shown to have pheromonal activities and have been suggested to be involved in waterproofing the insect cuticle. I showed that mbCHCs are produced in the insect oenocytes (cells that produce CHCs) by a fatty acid synthase, mFAS. I found that the differences in mbCHCs levels between these two species are due to loss of mFAS expression in D. birchii as a result of multiple cis-regulatory differences. To investigate the function of mFAS in desiccation resistance and mating success, I created transgenic lines of D. serrata in which mFAS expression of in oenocytes was knocked down by RNAi. Importantly, mFAS-RNAi in D. serrata flies resulted in a significant decrease in desiccation resistance and male mating success. Moreover, we could rescue both traits by applying synthetic mbCHCs, providing further evidence that mbCHCs act as a dual trait in D. serrata. This was the first published study that identified a single gene underlying the production of a dual trait, and provided evidence that evolutionary changes at the mFAS locus occurred between these two closely related species.
Fig 2. Loss of mbCHCs strongly reduces desiccation resistance in D. serrata