Genetic Structure of Natural Populations Environment Interaction in Viability of Drosophila Melanogaster . Xix . Genotype

To investigate whether or not an excess of additive genetic variance for viability detected in southern natural populations of Drosophila melanogaster was created by diversifying selection, genotype-environment interaction was tested as follows. (1) Two karyotype chromosomes were used: 61 second chromosomes with the standard karyotype and 63 second chromosomes carrying In(2L)t. Their homozygote viabilities were larger than 50% of the average viability of random heterozygotes. (2) The effects of two factors (culture media and yeasts) were examined at three levels (the culture media: tomato, corn and banana; and the yeasts: sake, brewer's and baker's). The results of 16 three by three factorial experiments by the Cy method in the same karyotype groups for relative viabilities of homozygotes and heterozygotes elucidated the following findings: ( 1 ) there was no significant difference between the two karyotype groups, (2) the variance components of genotype-environment interaction were highly significant, (3) the variance component of heterozygotes was significantly smaller than that of homozygotes. From the experimental findings and previous results, diversifying selection in natural populations acting on viability polygenes to increase the additive genetic variance was suggested. The relation of the present result to protein polymorphism is also discussed. n the basis of the genetic load and genetic variance component analyses, 0 it has been suggested that, in southern populations of D. melanogaster, some type of balancing selection is operating at a minority of loci for the maintenance of genetic variability of viability, although it can almost completely be explained by mutation-selection balance in northern populations in Japan (MUKAI et al. 1974, 1982; MUKAI 1977; TACHIDA et al. 1983; KUSAKABE and MUKAI 1982, 1984a). As a candidate for the type of balancing selection, diversifying selection, or maintenance of genetic variability due to genotype-environment interaction, has been proposed, since microenvironmental variance of heterozygotes is significantly smaller than that of homozygotes especially in southern populations and since the main alternative hypotheses to' diversifying selection in balancing selection (overdominance and frequency-dependent selection) have been re' To whom correspondence should be addressed. Genetics 111: 43-55 September, 1985. 44 H. TACHIDA AND T . MUKAI jected on the basis of previous work (MUKAI 1977; MUKAI, TACHIDA and ICHINOSE 1980; MUKAI et al. 1982; MUKAI, KUSAKABE and TACHIDA 1983). DOBZHANSKY and LEVENE (1 955) reported the existence of genotype-environment interaction with respect to viability in some chromosomes of D. pseudoobscura. Furthermore, there are several papers that suggest the operation of diversifying selection in Drosophila natural populations (WILLS 1975; POWELL 1971; POWELL and WISTRAND 1978; MACKAY 1981). Thus, it is very likely that diversifying selection is operating to maintain the excess variance of viability detected in southern populations of D. melanogaster. In this paper, we will report the results of the experiment using a southern Japan (Ishigakijima) population of D. melanogaster in which viabilities were measured in various growing environments as in DOBZHANSKY and LEVENE (1 955). For this population, excess additive genetic variance has been reported (TACHIDA et al. 1983). MATERIALS AND METHODS The second chromosomes used in the present experiments were extracted from a population of D. melunoguster in IshigaklJima, Okinawa, Japan, with the mating system described by WALLACE (1956) using the stock C160 [In(ZLR)SMI, Cy/In(2LR)bwV’, with an isogenic background; see MUKAI (1964) for its detailed description]. In this report, the second chromosome balancer, In(2LR)SMI, will be referred to as Cy, the dominant marker gene for this multiple inversion. The collections of flies were made in 1979 and 1980, and approximately 500 second chromosomes were extracted each year. A preliminary test for estimating the homozygous viability of each chromosome was carried out by the Cy method (WALLACE 1956): Five Cy/+, females were mated with five Cy/+, males, where i indicates the line number. Four days after the cross was made, all of the parental flies (ten flies) were transferred to a second vial and after four more days they were discarded. The offspring were counted until the 18th day after the cross (or transfer) was made. The counts from the original and transferred vials were pooled. In the offspring, if there is no viability difference between genotypes, two types of flies segregate [Cy(Cy/+) and wild-type(+/ +)] in the expected ratio of 2: 1, since the Cy homozygotes are lethal. Viability was expressed as the ratio of 2 X [the number of wild-type flies] to [the number of Cy flies + 11 (HALDANE 1956). If we substitute Cy/+J males (wherej indicates a line number other than i) for Cy/+, males in the above cross, we can estimate the viability of a heterozygote +,/+J. The karyotype of each chromosome was determined by cytologically examining salivary gland chromosomes after making a cross to the cn bw stock with the standard karyotype. From these chromosomes, 63 chromosomes with the standard karyotype and viability indices of 0.5 or higher and 63 chromosomes that had both the inversion, In(2L)t , and viability indices of 0.5 or higher were randomly chosen and used for the experiments. T o exclude genes with drastic effects from the analysis, the above selection of experimental chromosomes was made. Before using these chromosomes for the experiments, they were backcrossed to C160 for two generations in order to substitute the nearly isogenic genetic background of C160 for that of these experimental lines. Following this procedure, including the process of extracting the chromosomes, the heterozygosity between the third chromosomes of C160 and those of the flies from the natural population became, on the average, 12.5%. The sex chromosomes do not have any genetic variation among different second chromosome lines. This backcrossing was done in order to minimize the effects of other chromosomes. These chromosomes were divided into 16 groups. Each group consisted of three or four chromosomes that had the standard karyotype and the same number of In(2L)t-carrying chromosomes. Homozygote and heterozygote viabilities of the chromosomes within each group were tested simultaneously by the Cy method in nine kinds of growing environments described below. Heterozygote crosses were made between the two successively numbered lines, i.e., Cy/+, X Cy/+,+l within each karyotype. The line with the last number was crossed to line 1. Nine kinds of growing environments were established in the following way: Three kinds of yeasts (sake, brewer’s and GENOTYPE-ENVIRONMENT INTERACTION