Genetic analysis of hybrid sterility within the species Drosophila pseudoobscura

A local race of Drosophila pseudoobscura in the high Andes near Bogota, Colombia, is separated from the rest of the species in the U.S.A., Mexico, and Guatemala by a geographic gap. There is no premating isolation between Bogota and U.S.A. flies; hybrids are fertile as females; hybrid males are also fertile in the U.S.A. ♂ x Bogota ♀ cross, but sterile in the reciprocal cross. Some males in the progenies of backcrosses are fertile and others sterile. The genetic basis of the sterility is complex. One or several genes in both limbs of the X‐chromosome, and in the second and third chromosomes, are involved. While the F 1 hybrid males are either all fertile or all sterile depending upon which races are the female and male parents, the different recombination classes among the backcross hybrids differ only in that some of them are more and others less likely to be sterile. The most probable explanation is that the eggs deposited by hybrid females are influenced by the chromosome complement of the mother (maternal effect). Quantitative variations of the fertility‐sterility frequencies are however found also in the backcrosses in which the hybrid parent is the male. The sterility is here a threshold character. The Y‐chromosomes and the inversion polymorphisms in the third chromosomes of Bogota resemble those of Guatemalan populations. The Bogota local race is probably rather old, and genetically divergent from the main body of the species.