Max Levitan
1951 Genetics  
OST species of Drosophila so far studied in this respect are poly-M morphic for gene arrangements in their chromosomes. In D. pseudoobscura, D. persimilis, and D. funebris, this polymorphism is adaptive ( DOB-ZHANSKY 1943 , 1947 , 194Sa, 194Sb, 1948c DOBZHANSKY and EPLING 1944; DUBININ and TINIAKOV 1945 , 1946a , 1946b SPIESS 1950) . The adaptive nature of the chromosomal polymorphism is shown by three types of evidence. (1) The frequencies of the gene arrangements often show clear geographic
more » ... clear geographic and altitudinal gradients. (2) Cyclic seasonal changes in relative frequencies of gene arrangements are observed in some localities. (3) Experiments on the behavior of chromosomes with different gene arrangements in artificial populations in the laboratory show that inversion heterozygotes possess, as a rule, a much higher fitness than do the corresponding inversion homozygotes. CARSON and STALKER (1947 , 1949 ) STALKER and CARSON ( 1948 have discovered chromosomal polymorphism also in D. robusta Sturtevant, a species common in forested regions of the eastern United States. In addition to the usual paracentric inversions, D. robusta populations contain also at least two pericentric and two cytologically apparently terminal ones. Geographic and altitudinal clines have been demonstrated for certain gene arrangements, but seasonal cyclic variations are small or absent in the populations living near St. Louis, Missouri. The work reported in the present paper is a study of natural and experimental populatioiis of D. robusta in the New York City area. The evidence shows that the chromc~somal polymorphism in this species is also adaptive, inversion heterozygotes possessing higher adaptive values than do the homozygotes. The work was started during the summer of 1947. Preliminary collections showed that the gene arrangements 2L, 2L-1 and 2L-3 in the left arm of the second chromosome would be especially suitable for experimental study. Experimental populations with different combinations of the gene arrangements were created, and the ensuing changes in the relative frequencies of the chromosomal types in these populations were recorded by means of periodic sampling. In the spring and fall of 1948 and in the spring of 1949 systematic collecting was made in a single locality in New Jersey to detect possible seasonal variations in the incidence of the gene arrangements and to obtain a larger number of strains for further experimental populations. 1 Submitted in partial fulfillment of the requirements for the degree of Doctor of 2 Present address :
doi:10.1093/genetics/36.3.285 fatcat:nnyimmozhffp7dvdcff3k7nqry