THE RELATION OF AN INVERSION SYSTEM TO RECOMBINATION IN WILD POPULATIONS

A chromosome inversion results in an alteration of the original gene order. The alteration produced by each inversion is readily recognizable in the salivary gland chromosomes of Drosophila because of the constant differences in appearance and arrangement of the bands that characterize them. This genus is accordingly a facile subject for a study of the geographical distributions and frequencies of different arrangements in wild populations. The number, kinds and frequencies of arrangements vary from species to species (Patterson and Stone. 1952; da Cunha, 1955). The inversions of Drosophila pseudoobscura, which we propose to discuss, are confined chiefly to two chromosomes, the acrocentric third and the metacentric X and, in the latter, to its right arm. Nineteen arrangements have been recorded in the third chromosome and another is inferred to have existed at one time, if it does not now. They generally overlap each other and are only rarely included, and their heterozygotes are correspondingly complex. Twelve arrangements of this chromosome are known in the populations we shall discuss. Only three inversions have been recorded in the right arm of the X. They are structurally independent of each other, but are not apparently separable by crossing over in nature, and thus form only one effective arrangement. It is associated as a unit to some genetic factor that causes an alteration of the sex ratio and produces mostly females in matings of males that carry it. Three inversions have been found in the second chromosome and one in the fourth. They occur only in low frequencies in the area we shall discuss, if at all, and can have but little effect on the operation of the inversion system. Five types of the Y chromosome are associated with these inversions btut their relation to the system is uinknown. They may need be taken into account ultimately. As far as known. only one type occurs in this area. Thus, the inversion system of D. pseudoobscura is more or less equally divided between two parts of the genome. The basic data for the whole system have been presented lby Dobzhansky (Dobzhansky and Epling, 1944). More recently discovered arrangements are discussed by Epling and Lower (1957). Dobzhansky, Holz and Spassky (1942) have estimated the relative lengths of the five chromosome arms. The third chromosome constitutes approximately 18 per cent of the total and the right arm of the X approximately 23.5 per cent. Thuis, when arrangements are heterozygous simultaneously in both chromosomes, crossing over will be greatly reduced in about 40 per cent of the euchromatin of the cell concerned. The arrangements of the third chromosome have been given names suggested by the places where they were first detected, such as Arrowhead (AR), Chiricahua (CH), Treeline (TL), Pikes Peak (PP) and Standard (ST). Those of the X chromosome are known as Standard (STX) and sex ratio (SR). The term Standard in both instances means only that these arrangements were taken as a base for comparisons. 1 Supported in part by the Board of Research of the University of California; Office of Naval Research, Contr. Nonr. 233 (11); The National Science Foundation Project C1028; and the U. S. Public Health Service, Grant C-3032 M and G.