An Analysis of the Chromosomes of the Two Sub-Species Drosophila Virilis Virilis and Drosophila Virilis Americana.

NCREASING attention is being focused on the use of Drosophila as a I convenient organism for investigating such fundamental problems as the formation of new species and interspecific sterility. Studies of the hybrid from the cross D. melarzogasterXD. simulans by PATAU (I935), and by HORTON (1939), and D. pseudoobscuraXD. miranda by DOBZHANSKY and TAN (1936), as well as studies of the hybrids from the interracial crosses of D. pseudoobscura by DoszHANsKY and STURTEVANT (1938), have amply demonstrated the advantages of the salivary gland technique in affording a new approach to such old problems, and making possible a more critical analysis of the differences in gene alignment. The present paper is a study of the differences in gene alignment of the two sub-species, D. virilis virilis and D. virilis americana as determined by the salivary gland chromosome analysis of the hybrid, and the differences in the chromosome configurations in the larval ganglion cells. Such a study has made necessary a revision of my preliminary map (HUGHES 1936) of the salivary gland chromosomes of D. virilis. These two sub-species are of special interest because, among other reasons, they can be crossed easily and yield hybrids which are partially fertile (SPENCER 1938). Both male and female hybrids are partially fertile when crossed inter se, or backcrossed to either parent sub-species. That two sub-species differing so widely in gene alignment, in the chromosome configuration of the ganglion cells, in phenotypic appearance, and physiological characteristics can be crossed is unusual for the genus Drosophila, and it is remarkable indeed that the hybrid is partially fertile. (Recently it has been reported that the female hybrid from the cross D. mirandaXD. pseudoobscura is slightly fertile under certain conditions) (MACKNIGHT 1939). The fortunate occurrence of partial fertility in the hybrids obviously will make it possible to follow the behavior of the genetic complexes of the two sub-species in subsequent generations. I am deeply indebted to PROFESSOR WARREN P. SPENCER of the College of Wooster for calling my attention to D. virilis americana, as well as for sending me stocks. To PROFESSOR F ANZ SCHRADER of Columbia Univer* The cost of the accompanying plate has been contributed by the Galton and Mendel Memorial Fund.