Genetic and cytological studies on the affinis group of Drosophila

Abstract

Introduction The genetic evidence available at present indicates that certain groups of genes tend to remain linked in the various members of the genus Drosophila (Donald 1936; Sturtevant and Tan, 1937; Sturtevant, 1938a, 1940; Sturtevant and Novitaki., 1941). These elements are comparable to the arms of the chromosomes of melanogaster and have accordingly been lettered A, B, C, D, E, and F for X, IIL, IIR, IIIL, IIIR, and IV, respectively. In view of the extensive cytological studies on the various members of the affinis group (Dobzhansky and Socolov, 1939; Miller, 1939; Novitski, unpublished; Sturtevant, unpublished), it has seemed pertinent to extend the homologies with the elements to the salivary gland chromosomes of this group. The high degree of similarity of the salivary gland chromosome arms of the various affinis group species examined renders it possible to make such a correlation within one species, in this case affinis, with the reasonable certainty that the results obtained will apply to other members of the group (athabasca, algonquin, and azteca). Sturtevant (1939) has homologized a number of mutant genes in affinis with those of melanogaster and pseudoobseura, applying to the linkage groups of affinis the same designations as those of their homologs in pseudoobsoura. In terms of the above mentioned elements, consequently, the affinis linkage groups, XL, XR, II, III, IV, and V correspond to A, D, E, C, B, and F. respectively. Miller (1939), in a study of the salivary gland chromosomes of algonquin, has arbitrarily lettered the units found therein: a two-armed X-chromosome (XL and XS), a single armed autosome (A), two two-armed autosomes (B and C), and a microchromosome (D). In order to avoid confusion, the symbols to be used until the complete correlation has been established will be those of Sturtevant for the linkage groups and those of Miller for the salivary gland chromosomes. The metaphase chromosomes of males of the affinis group include a V-shaped X-chromosome, a J-shaped Y-chromosome, two pairs of J-shaped autosomes, a pair of rods and a pair of dots (Metz, 1916; Sturtevant and Dobzhansky, 1936; Dobzhansky and Socolov, 1939; Miller, 1939). It seems clear that there is a correspondence between the number of large chromosome arms in the metaphase plate and the euchromatic arms In the salivary gland nucleus (Miller, 1939, for algonquin; Sturtevant, unpublished, for afinis; Novitski, unpublished, for athabasca; Novitski, in Sturtevant, 1940, for azteca.