Population and Evolutionary Genetics

Speciation; Evolution and Development

Hope Hollocher     Associate Professor Ph.D. Washington University Postdoctoral, University of Chicago

e-mail | labpage

Using Drosophila as a model system, my research combines population genetics, phylogenetics, molecular genetics and developmental biology to trace the changes responsible for morphological and reproductive differences between closely related species. As a whole, my work covers several different aspects of speciation from the role of behavioral isolation to the role of development in the formation of new species. Here I outline the two newest Drosophila systems that are being investigated in my laboratory to study speciation mechanisms.

The first project relates developmental changes in abdominal pigmentation to speciation in the Drosophila cardinigroup. The D. cardini group consists of species inhabiting different areas of tropical America. Included in this group are the species of the D. dunni subgroup, which are endemic to Puerto Rico and the islands of the Lesser Antilles, with each island having its own morphologically distinct species. Changes in abdominal pigmentation pattern show a regular cline along the arc of islands; lighter forms occur in Puerto Rico and progressively darker forms occur southward. Parallel clines in abdominal pigmentation exist within various cardini species on the mainland as well. Such clines imply the action of natural selection during differentiation. A primary goal of this project is to relate the evolutionary history of the species in the group to changes in abdominal pigmentation and reproductive isolation in order to elucidate the role these traits may have played during speciation. To this end, we are sequencing several different gene regions and performing microsatellite analysis to get a handle on the structuring of genetic variation within and between species. We are also developing specific DNA sequence search algorithms that will allow us to identify changes in control regions in candidate genes that can account for the changes we see in abdominal pigmentation expression patterns across species.

The second project characterizes hybrid sterility and hybrid sterility rescue in crosses between D. melanogasterand D. simulans. When Sturtevant first discovered that Drosophila melanogaster was actually two closely related species, Drosophila melanogaster and Drosophila simulans, he was thrilled with the prospect of being able to use these two species to understand the genetic basis of species differences, in particular, reproductive isolation. Although the prospects were high, reality turned out to be not so kind. The differences that had evolved between these two species proved to be genetically intractable since crosses yielded offspring that were either completely sterile or inviable (a geneticistís nightmare). With the recent discovery of naturally occurring strains that rescueinviability and sterility defects of hybrids, we are now able to fully realize Sturtevantís original desire to use the genetic tools available in a species such as D. melanogaster to understand the evolution of reproductive isolation. This project uses a combined developmental, molecular, and evolutionary approach to look at how germlinedevelopment has diverged between the two species. Most recently we have turned to microarray analysis to target genes involved in reproductive isolation.

 

A map of Puerto Rico and the Lesser Antilles showing female abdominal pigmentation patterns of the different species from D. dunni subgroup. Also depicted are the phylogenetic relationships between the species mapped onto the different islands.

 

A similar range of pigmentation variation is found within the Drosophila polymorpha species from southern Brazil. Flies on the right (A, C, & E) are males. Flies on the left (B, D, & F) are females.

 

We use immunofluorescence and confocal microscopy to track germ cell migration during early stages of Drosophila embryonic development. In this image, wild type germ cells destined for larval gonads are colored bright green.