The Genetic Basis of Differences between Two Species of Drosophila

Abstract

Three [female][female] of D. americana were separately crossed to [male][male] of D. virilis. The [female] offspring was crossed back for at least 23 single pair generations to virilis [male][male] in such a way as to result in 3 separate lines of flies whose cytoplasm was ultimately derived from americana but whose chromosomes were presumably identical among each other and with a line of pure virilis. The 3 "virilized" hybrid lines were compared with the pure virilis line in order to test a possible autonomously reproduced influence of the americana-de-rived cytoplasm. 8 morph. and 2 physiol. characters diagnostic in the differentiation of the 2 spp. were found to be identical in the "hybrids" and pure virilis. Fertility tests also gave no significant differences. It is concluded that the differences between the 2 spp. do not depend on self-reproducing extrachromosomal elements but on their chromosomes.