RELATION BETWEEN ADAPTIVE VALUES AND COMPOSITION OF THE POPULATION INDROSOPHILA ANANASSAE

Abstract

Chromosomal polymorphism is very common in natural populations of Drosophila (da Cunha, 1955), and the study of this phenomenon has made remarkable progress in several species of Drosophila through the population cage technique. Since the experiments of Wright and Dobzhansky (1946), it has been shown that an equilibrium is reached as the result of the superior fitness of the inversion heterozygotes to the homozygotes. The adaptive values of the chromosomal types, however, do not necessarily remain constant but can be altered by environmental factors such as temperature, nutritional conditions, population density, and genotypic background (Wright and Dobzhansky, 1946; da Cunha, 1951; Dobzhansky and Spassky, 1954; Birch, 1955; Levine, 1955; Parsons, 1959; Vetukhiv and Beardmore, 1959; Van Valen et al., 1962). Furthermore, the kind of competitors and actual frequencies of all karyotypes in the same population can affect the adaptive values (Levene et al., 1954, 1958; Spiess, 1957). In Drosophila ananassae, artificial populations containing a terminal inversion of the third chromosome (A and B arrangements after Kikkawa, 1938) have been shown to reach an equilibrium with about 40% of the B arrangement at 250 C regardless of the initial frequencies, thus showing an establishment of balanced chromosomal polymorphism in the population. In the author's previous work the relative adaptive values of the three genotypes, AA, AB, and BB, fluctuated somewhat among different cages, notwithstanding that all populations were composed of identical strains, prepared at the same time and subjected to the same conditions (Tobari, 1962). A similar phenomenon was noted regarding In2L of D. ananassae (Moriwaki et al., 1953, 1954), although precise analyses have not yet been made. The present experiments are to elucidate factors which change the adaptive values of the karyotypes in the artificial populations.