Nikolai Kandul
Ph.D. 2005, Harvard University, USA
Thesis title: Speciation in the genus Agrodiaetus (Lepidoptera, Lycaenidae): karyotypic diversification and reinforcement of pre-zygotic isolation.
Advisor: Naomi E. Pierce
2005 - present, Duke University, USA
Topic: Mapping behavioral speciation gene(s) in Drosophila pseudoobscura.
Advisor: Mohamed A. F. Noor
The establishment of reproductive isolation between previously interbreeding populations cause one species to split into two. This process, i.e. speciation, generated the immense biodiversity. Genetic changes that contribute to the reproductive isolation between closely related species could directly lead to speciation. A few genes, for example Nup96, Tu, Hmr, Lhr and Ods, affecting hybrid sterility, hybrid inviability, and assortative fertilization have been identified over the past decade. These finding have supported the Dobzhansky-Muller model of continually accumulating incompatibilities between diverging species. However, the identified genes associated with post-mating reproductive isolation. Pre-mating reproductive isolation, like behavioral differences, ecological and mating preferences, usually evolves earlier than post-mating isolation. Thus, although pre-mating isolation may be generally more relevant to speciation, comparatively little progress has been made in elucidating its genetic basis.
The objectives of my research are to identify and characterize genes contributing to the pre-mating isolation between the sister species Drosophila pseudoobscura and D. persimilis. Both these species are native to North America. D. pseudoobscura and D. persimilis are sympatric (co-occur) in California, but the range of D. pseudoobscura extends far further east into Texas and Colorado. Females from populations of D. pseudoobscura that co-occur with D. persimilis exhibit greater mate discrimination against D. persimilis males than females from other populations. In contrast, males of D. pseudoobscura do not discriminate against heterospecific matings, even after being exposed to and rejected by heterospecifics. The enhanced discrimination of D. pseudoobscura females contributes to the reduction in gene flow between these closely related hybridizing species and may have been favored by direct natural selection to prevent hybridization through reinforcement mechanism. As such, unlike all other such genes characterized to date, the genes I can potentially identify can justifiably be called "speciation genes."
Two gene regions conferring natural polymorphisms in the behavioral discrimination of D. pseudoobscura females were previously mapped to less than 400 kb using a QTL analysis. I am using natural recombination in marker-assisted backcrosses between the high and low discrimination lines of D. pseudoobscura to narrow these intervals to single candidate genes. The introgression of a small interval, 85 kb, containing high discrimination alleles into the otherwise low discrimination line confirmed the higher mate discrimination against D. persimilis males in the recovered females. I am currently in the process of cloning a good candidate gene found within this interval. At the same time, I am performing F2 crosses between the two lines different at the candidate gene to test for any effect from other genes.