MODIFICATION OF THE ADAPTIVE VALUES OF CHROMOSOMAL TYPES INDROSOPHILA PSEUDOOBSCURABY NUTRITIONAL VARIABLES

Abstract

Most natural populations of Drosophila pseudoobscura are polymorphic with respect to the gene arrangement in some of the chromosomes, especially the third chromosomes. Some of the wild flies are inversion heterozygotes and others are homozygotes. It has been shown both through observations on natural populations, and through experiments on artificial ones that the chromosomal polymorphism is adaptive and balanced (Wright and Dobzhansky, 1946; Dobzhansky and Levene, 1948). In at least some environments the adaptive values of inversion heterozygotes are higher than those of the homozygotes. However, the adaptive values of chromosomal types can easily be modified by environmental factors. In experiments conducted at 250 C., the adaptive value of heterozygotes for Standard and Chiricahua third chromosomes for certain California populations is about twice as high as that of the Chiricahua homozygotes; at 150 C. these chromosomal types are similar in adaptive value. Heuts (1947, 1948) found that the relative humidity may also modify the adaptive values. Heuts (l.c.) and Wallace -(1,948) showed that the adaptive value of a chromosomal type is a resultant of interaction of many physiological variables: The ability of larvae to survive under competition for a limited amount of food, fecundity, longevity, sexual activity, etc. Furthermore, a chromosomal type may be superior to another type in some of these properties, equal in others, and inferior in still others. The chromosomal types possess different ecological optima. Perhaps the least well known, and -yet doubtless very important, variables of Drosophila ecology are those concerned with the nutrition of the flies. Dobzhansky and Epling (1944) and Shihata (unpublished) found that adult D. pseudoobscura caught in their natural habitats contain yeasts and bacteria in their digestive organs. Shihata has isolated 32 species of yeasts from 92 flies collected in 5 localities in California. Carson (1951) found both adults and larvae feeding in slime fluxes on diseased oaks and other trees. Considerable differences in the nutritional values of different yeasts have been found by Burkholder (1943a and b), Christophersen and Holzweissig (1950), Hedrick and Burke (1950), and others. Wagner (1944, 1949) found that certain species of yeasts permit normal development 'of some but not of other species of Drosophila related to D. mulleri. Experiments on the influence of different species of yeasts and bacteria in the food on the adaptive value of the chromosomal types of D. pseudoobscura are reported in the present article.