BlOTROPlCA 34(1): 81-92 2002 zyxwvut Comparative Genetic Structure between Tropical Colombian and North American zyxwv Drosophila pseudoobscura Populations’ Diana Alvarez Unidad de Genetica (Grupo de Biologia Evolutiva, Laboratorio de Bioquimica, Biologia y Genetica Molecular de Poblaciones), Departamento de Biologia, Facultad de Ciencias, Pontificia Universidad Javeriana, CRA 7A No 43-82, Bogota DC, Colombia Mohamed A. F. Noor Department of Biological Sciences, Life Sciences Building, Louisiana State University, Baton Rouge, Louisiana 70803, U S A . and Manuel Ruiz-Garcia* Unidad de Genetica (Grupo de Biologia Evolutiva, Laboratorio de Bioquimica, Biologia y Genetica Molecular de Poblaciones), Departamento de Biologia, Facultad de Ciencias, Pontificia Universidad Javeriana, CRA 7A No 43-82, Bogota DC, Colombia ABSTRACT zyxwvuts Since the discovery of zyxwvutsr Drosophila pseudoobscura in the tropical highlands of the Colombian Andes during the 1960s, this population has been studied by many evolutionary biologists because of its geographical isolation from the main North American range of this species. We used five highly variable microsatellite loci (DPSX001, DPS2001, DPS3001, DPS3002, and DPS4001) to analyze the genetic structure of three Colombian populations and the genetic relationships with four North American populations. We found that the average heterozygosity was consistent among the three tropical Colombian populations zyxwvut (H = zyxwvu 0.665-0.675), but they had less variability than their North American coun- terparts. Nonetheless, the genic diversity found in the Colombian populations was higher than that found previously using other genetic markers. The average genic heterogeneity estimate among the Colombian populations (RST = 0.042), although statistically significant, was substantially lower than that found among the North American popu- lations (RST = 0.088). We identified alleles in the Colombian populations not reported in North American popula- tions, suggesting further divergence between the populations. We estimated that the populations on the two continents diverged ca 80,000 years ago, consistent with independent sequence analyses of these populations but contrary to some suggestions in the literature. Finally, we estimated an average effective population size of the Colombian pop- ulations to be on the order of 100,000. Key word: Drosophila pseudoobscura; microsatrllite loci; North America; population genetics; tropical Colombia. SINCE DOBZHANSKY ET AL.’s (1963) KARYOTYPIC STUDY of a relict Drosophila psedobscura popula- tion on the tropical high plateau of the Colombian Andes near Bogoth, many evolutionary biologists have expressed an interest in this divergent popu- lation (Prakash et a/. 1969, Ayala & Dobzhansky 1974, Orr 1987, Schaeffer & Miller 1991, Wang et al. 1997). This population is geographically iso- lated from the primary range of D. pseudoobscura in North and Central America by almost 2400 km. Only two third-chromosomal arrangements, Santa Cruz and Treeline (SC and TL), are present in this population out of more than 50 rearrangements surveyed in the central distribution range of this species (Guzman et af. 1994), although several new Received 28 June 2000; revision accepted 1 May 2001. Corresponding author. e-mail: mruiz@javercol. javeriana.edu.co chromosomal rearrangements in the Colombian populations have been discovered recently (Ruiz- Garcia et zyxw al. 2001). Furthermore, the males of this tropical relict population have only one of the four types of Y-chromosomes described by Dobzhansky (1937). Prakash et al. (1969) detected the loss of genetic variability in 24 allozyme loci in the Co- lombian population relative to North American populations. Further evidence of divergence was demonstrated by Ayala and Dobzhansky (1974), Singh et al. (1976), and Coyne and Felton (1977), who identified rare but unique alleles at the zy Xfh, Est-5, and Adh allozyme loci in the Colombian populations. Prakash (1972) also documented that hybrid males from crosses between Colombian fe- males and North American males were sterile, and he suggested that the peripatric Colombian popu- lation was the result of a temporally recent genetic revolution (perhaps in the 1960s) following the model proposed by Mayr (1954); however, Dobz- 81