Chromosomal Polymorphism in Drosophila Rubida

HE population geneticist is essentially interested in the variation of gene freTquencies in natural populations. Within chromosomal inversions blocks of genes are contained and in many species of Drosophila due to the presence of easily analysable giant chromosomes, the behavior of these blocks of genes can be easily studied in natural populations. Thus, of recent years chromosomal inversion polymorphism in the genus Drosophila has been extensively studied (see review by DA CUNHA 1955 and discussion by GOLDSCHMIDT 1958), the most extensive work having been done on the temperate species D. pseudoobscura Frolowa from northwestern America, D. robusta Sturtevant from eastern America and D. subobsczua Collin from Europe and the tropical species D. willistoni Sturtevant from South America. From this work it has been suggested that the genetical significance of inversions is to maintain coadapted gene sequences by the elimination of chromatids produced by crossing over within the limits of heterozygous inversions. PATTERSON and STONE (1952) list 17 species in the immigrans species group, the majority being from the Australian and Oriental geographical regions. In spite of the giant chromosomes of those members of the group which have been studied being very suitable for detailed analysis, anly D. immigrans has been examined for polymorphism. A number of inversions have been detected by FREIRE-MAIA, ZANARDINI and FREIRE-MAIA (1953) and BRNCIC (1955) in South America, GRUBER (1958) in Israel and TOYOFUKU (l957,1958a,b) in Japan. In 1958 Drosophila collections were made at Cairns in northeastern Australia and a new species of the immigrans species group discovered. This was subsequently described as D. rubida (MATHER 1960) and shown, like D. immigrans, to have excellent salivary chromosomes although the Cairns population appears to be free of inversions. In 1959 Drosophila collections were made in Papua-New Guinea at Port Moresby, Goroka and Lae (New Guinea), Kavieng (New Ireland) and Rabaul (New Britain), and the species was shown to be widespread (MATHER 1959). It was also established by qualitatively analysing a number of mass cultures that it is rich in chromosomal inversion polymorphism. For these reasons it was decided to study D. rubida more fully with a view to obtaining data on the polymorphism pattern of a polymorphic species from tropical Australasia.