PARTIALLY SUCCESSFUL ATTEMPT TO ENHANCE REPRODUCTIVE ISOLATION BETWEEN SEMISPECIES OF DROSOPHILA-PAULISTORUM

A strain descended from a single female captured in the Llanos of Colombia in 1958 produced fertile hybrids of both sexes when crossed to strains of the Orinocan semispecies of D. paulistorum. Since 1963, this strain, now referred to as New Llanos, gives sterile hybird males with Orinocan strains. New Llanos gives fertile hybrids with strains of the Interior semispecies. Since this latter effect was discovered only in 1964, there is not way to ascertain whether the original Llanos would have been interfertile with Interior in 1958. New Llanos shows little or no ethological isolation from either Orinocan or Interior semispecies, although these latter exhibit a fairly strong isolation from each other. From 1966 to 1974, artificial selection was carried on to erect an ethological isolation barrier between New Llanos and an Orinocan strain. For this purpose, 2 nonallelic recessive mutants were used. Homogamic matings were yielding phenotypically recognizable homozygotes, and heterogamic matings yielded wild-type heterozygotes. These latter were destroyed, while the homozygotes served as parents of subsequent generations. The selection was partially successful; strains showing a pronounced preference for homogamic matings, but not a complete ethological isolation, were obtained. The experimentally induced ethological isolation is about as strong as the weakest isolation observed between semispecies of this group that occur in nature.