A comparative study of the chromosomes in the incipient species of the Drosophila paulistorum complex

Summary Drosophila paulistorum Dobzhansky et Pavan is a complex of six races or incipient species. The races are mostly allopatric, but they are reproductively isolated sufficiently to permit them to exist also sympatrically in some places. The gene arrangements in the chromosomes of the races have been compared by means of examination of the giant chromosomes in the larval salivary glands; 28 strains of all races, and about an equal number of interracial hybrids have been studied.Chromosomal inversion polymorphism has been discovered in all races, even in the Guianan race of which only a single strain is available. Inversion heterozygotes are found in every one of the five chromosomal strands which the species has. Interracial hybrids tend to be heterozygous for more inversions than are present in the strains of the parental races. The Transitional race has however much the same gene arrangements as the widespread Andean — South Brazilian race.With the exception of the Transitional race, and of three other possible exceptions, each race has a collection of its own race-specific inversion polymorphs, not found in the other races. This very striking finding is discussed in connection with the hypothesis which envisages the origin of new species from marginal colonies at the periphery of the geographic distribution area of the ancestral species.The work reported in this article has been carried under Contract No. AT-(30-1)-1151, U.S. Atomic Energy Commission, mostly at the Department of Zoology, Columbia University, New York.     

Distribution among the Chromosomes of DROSOPHILA PSEUDOOBSCURA of the Genes Governing the Response to Light

Most strains of Drosophila pseudoobscura are neutral to light when tested in phototactic mazes. However, clear-cut photopositive and photogenative populations are obtained by selection over a series of generations. The genetic nature of the differences between the positive and negative populations has been studied in crosses in which the three large autosomes carried mutant markers. All chromosomes contain genes which influence the response to light. The third chromosome has the strongest effect, followed by the second, the X, and the fourth chromosomes. This seriation is not in proportion to the relative lengths of the chromosomes. Either the effective genes are not very numerous, or some of them exert stronger influences than others.