Some fundamental questions about the origin of developmental and morphological variation can only be addressed by comparing very closely related species, or even individuals and populations within species.  To be useful for this type of research, the model species should meet many important conditions.  Ideally, they should combine low genetic divergence with a high degree of morphological differentiation.  They should be amenable to both genetic and molecular analyses.  And they should be easy to feed and easy to breed. 
    All these conditions are met superbly in the Southeast Asian Drosophila bipectinata species complex.  The six closely related species and subspecies in this complex show dramatic variation in two of our favorite traits – pigmentation and sex comb morphology.  D. parabipectinata, D. m. malerkotliana and D. p. nigrens are pigmented in a sexually dimorphic pattern where the last three abdominal segments are completely melanized in males, but not in females.  However, males of D. bipectinata, D. p. pseudoananassae and D. m. pallens lack such pigmentation, and are similar to females.  D. bipectinata and D. parabipectinata are also unique among the ananassae subgroup (but similar to several more distant evolutionary lineages) in having rotated, obliquely oriented sex combs.  Sex comb morphology is also variable within species, especially in D. bipectinata.  The fact that the same traits vary both within and among species makes the bipectinata complex an especially attractive model for investigating the genetic basis of morphological evolution.




    At the same time, genetic divergence among these species is extremely low – much lower than in the better known D. simulans species complex.  All species can be hybridized, and genetic analysis using molecular and morphological markers is possible.  We are now developing genetic and cytological maps for the bipectinata complex.  These maps will be used to map and identify the genes responsible for the interspecific and intraspecific differences in pigmentation and sex comb morphology.  Our work on the developmental genomics of these traits will provide the context in which the comparative data can be analyzed and interpreted. 

In addition to serving as a model of morphological evolution, the bipectinata species complex is useful for the study of speciation and the genetics of island populations.  Despite their very low genetic divergence, all four species are sympatric.  We are using phylogenetic and population-genetic approaches to reconstruct the evolutionary history of this species complex, and are beginning to characterize the mechanisms of prezygotic and postzygotic reproductive isolation.  Our field work in Southeast Asia is aimed at sampling intraspecific genetic, morphological, and behavioral variation, and trying to understand the extent of ecological differentiation among these closely related species.