Cytological differentiation and sexual isolation between populations of Drosophila nigricruria

Drosophila nigricruria has a distribution ranging from northern Mexico to Chile. It is a relatively rare species, primarily limited to cactus deserts or pine-oak associations at altitudes of 1200 to 2100 meters above sea level. The species comprises two cytological races with a zone of hybridization in the Oaxaca and Puebla regions of Mexico. Despite the zone of hybridization, gene flow for chromosome 4 genes is limited, or non-existent, between the two races. Sexual isolation tests were run using a population from the northern race, a hybrid population, and a population that is essentially southern. Sexual discrimination among these strains was on the same order of magnitude as that found between geographical populations of other species. There was no evidence to indicate that sexual isolation is any stronger between the cytological races of nigricruria than one might expect between any two geographical populations within a species.