Cytological evidence of introgression in the classical sense of Anderson (1949) and Stebbins (1950) has been presented for the sibling species Drosophila metzii and Drosophila pellewae in a population of the Isthmus of Darien, Panama. Salivary chromosome karyotypes of hybrids between the two species show multiple translocations involving the four large autosomes. A strain of D. metzii collected from a mixed population of the two species near El Real, Darien, Panama, displays both heterozygous and homozygous translocation figures and regions of asynapsis in salivary chromosomes of third instar larvae. A heterozygous translocation figure is seen in some salivary gland cells of larvae of the Darien strain of D. pellewae. Observations of variations in length of chromosomes in different individuals of the Darien D. metzii strain suggest restructuring arising presumably from crossing over in progenitors heterozygous for multiple translocations. The requirements necessary for introgressive hybridization outlined by Anderson (1949) and Stebbins (1950: 251-297) are fulfilled by the relationship between the sibling species Drosophila metzii and Drosophila pellewae. Each species has a characteristic range with a region of overlap in the Isthmus of Darien. Drosophila pellewae has been collected from Barro Colorado Island, Canal Zone, where it is extremely rare, to Rio Raposo, Colombia, where it is moderately common (Pipkin & Heed, 1964). In the north, D. metzii has been collected at Sonte Comapun (near Vera Cruz, Mexico) by Faberge (personal communication), at Turrialba, Costa Rica (Pipkin, 1965), and San Salvador, El Salvador (Heed & Wheeler, 1957), and to the south as far as Santa Marta on the northern coast of Colombia (Heed, personal communication). Mixed populations of these siblings occur in forests, feeding over the same fallen fruit or flowers. Pipkin (1968a) has presented evidence that the species do cross in nature and that a specific character, white carina color, has been transferred from D. metzii into populations of D. pellewae, where an obscure carina color polymorphism is seen both in the Darien and Rio Raposo populations. Although 280 individuals consisting of 60% D. pellewae, 40% D. metzii were collected from a single unusually large population in the Darien in 1961, gene exchange between the two species is greatly reduced, owing partly to translocations involving the four major autosomes (Pipkin, 1968a). A primitive karyotype consisting of five pairs of rod-shaped and one pair of dot-shaped chromosomes, characterizes both species according to analysis of larval ganglionic cells of strains originating 'Supported by NSF Grant GB-8770 and NIH Grant IR 101 GM 18409-01. The author is deeply indebted to Mr. Eric Schabtach, formerly of The Johns Hopkins University, now of The University of Oregon, Eugene, Oregon, not only for making the photographs used in this paper but also many other study photographs; to Dr. H. Ursprung, formerly of The Johns Hopkins University, for his generous permission to, use the Zeiss photomicroscope; to Mr. L. M. Powers, for culturing Drosophila metzii and Drosophila pellewae. 2 Post Office Box 138, Howard University, Washington, D. C. 20001. ANN. MissouRI BOT. GARD. 59: 417-434. This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:39:58 UTC All use subject to http://about.jstor.org/terms 418 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 59