INTROGRESSION BETWEEN CLOSELY RELATED SPECIES OF DROSOPHILA IN PANAMA.

Abstract

In a recent paper, Bigelow (1965) argues closely related but distinct animal species may interbreed in a hybrid zone where their respective distributions overlap, providing the genomes of the two species have become so coadapted hybridization fails make the gene pools progressively more similar. Thus persistence of a narrow hybrid zone over a long period of time demonstrates the biological integrity of the closely related species, and populations of hybrids zones should not be regarded as incipient species. Previously, Dobzhansky (1951) had observed that populations may continue to diverge despite gene exchange, the real problem being how much gene exchange between diverging populations is possible without arresting and reversing the divergence. The present work will describe a case which appears to corroborate Bigelow's argument. Drosophila metzii Sturtevant (1921) and Drosophila pellewae Pipkin and Heed (1964) belonging to the tripunctata species group of the subgenus Drosophila, are sympatric in the Isthmus of Darien, Panama. These species feed and breed together on fallen fruit and blossoms as part of large multispecific aggregations (Pipkin, 1965). A marked degree of sexual isolation exists, but there is some hybridization. Though the F1 hybrids between these species are often sterile in crosses inter se, female hybrids can backcross with either parental species. Recombination between the genomes of the respective species is limited by translocations involving four pairs of autosomes. However, a carina color polymorphism observed in D. pellewae of the Darien, where both species occur, and also present in a strain of D. pellewae from Rio Raposo, Colombia, where D. metzii is not found, is believed to be the result of introgression between the two species in the region of overlap. It is the purpose of this work to present evidence upon which this interpretation is based and to compare relationships between D. metzii, D. pellewae, and a third closely related allopatric species, D. leticiae Pipkin (1967) from the headwaters of the Amazon River.