Eukaryotes Genomes - DROSOPHILA PSEUDOOBSCURA

With over 2,000 known species, Drosophila offers a useful investigative platform for biologists of all sorts. Its interesting and diverse biology and ease of breeding in a variety of conditions has made Drosophila a favorite laboratory model organism.

Although bottled ‘populations’ of Drosophila genetic mutants quickly became the standard resource for geneticists, these lab strains were at first not useful to those researchers studying evolutionary processes. D. melanogaster and its sibling species Drosophila simulans, although currently distributed worldwide, arrived only recently from Africa and are, therefore, not the most ideal material for understanding historical mechanisms. To study a more natural situation, Theodosius Dobzhansky, a naturalist and geneticist, began to work with the then little-known species Drosophila pseudoobscura, whose natural habitat range largely covers the western part of North America. It has since been studied by many evolutionary biologists because of its geographical isolation from the main North American range of this species.

Drosophila pseudoobscura is the second of the Drosophila species to have its genome completely sequenced. The publication of the this sequence provides a snapshot of how genomes have changed over tens of millions of years and sets the stage for the analysis of more fly genomes.

Reinforcement is the process by which natural selection strengthens sexual isolation between incipient species, reducing the frequency of maladaptive hybridization and hence completing reproductive isolation. Although this model of speciation was once widely accepted, its plausibility and experimental support have been recently attacked. The complete genome sequence of D. pseudoobscura provides an example of speciation by reinforcement. Analysis suggests that females of D. pseudoobscura evolved increased sexual isolation from their sibling species, D. persimilis, by natural selection against maladaptive hybridization. D. pseudoobscura can hybridize with closely related species to generate fertile and viable offspring.

The sequencing of D. pseudoobscura is a milestone in comparative genomics. Comparison of the genome of this important model of speciation and development with that of its well-annotated sister species , D. melanogaster, will quickly become an indispensable tool for biologists. By using this genomic resource , we will be closer to tackling problems such as cracking the regulatory code and understanding the genetic basis of speciation. At a broader level, this exploratory analysis represents the beginning of a larger chapter as other species of Drosophila are currently in various stages of genome sequencing.

References:

http://www.hgsc.bcm.tmc.edu/projects/drosophila/
http://repositories.cdlib.org/lbnl/LBNL-55260/
Genome Research 15:1-18, 2005