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FURTHER GENETIC STUDIES OF THE SOUTH AMHERST POPULATION OF DROSOPHILA MELANOGASTER

The genetic structure of the natural population of Dr-osophila melanogaster found in South Amherst, Massachusetts, was first outlined in a report by Ives (1945) based on studies which were initiated in 1931 and which became quantified in 1938. Changes in the structure of this population have been reported and discussed in Ives (1954) and in Band and Ives (1961, 1963, 1968). In the 1938-1946 period a high frequency of lethal chromosomes was found in early fall collections from this population together with a very low rate of allelism between the lethal chromosomes. It seemed probable then that only by selection against these recessive lethals when they were in heterozygous condition was their frequency kept from reaching substantially higher levels. At no time in the 1938-1958 period was there a high enough rate of allelism to enable one to observe if homozygosis might also act as an appreciable restraining factor in this population. In the joint studies of Band and Ives a significant correlation was found with certain weather factors and it was suggested that to some extent summer rainfall and, even more, the average daily temperature range during the week before the flies are collected have a strong influence on the frequency of lethal and semilethal chromosomes found in a given collection. Experimental studies by Band (1963) have in fact demonstrated just such a selective effect by temperature range. The continuity of the South Amherst population has been indicated by the recurrent appearance of the uncommon eye color mutant, cardinal-3. That mutant was found in the offspring of the first (1931) South Amherst collection, which was from a very large population growing on the fermenting corn silage in a silo on a small dairy farm owned by the author's father, and was reported in Plough and Ives (1934). It has appeared in a low proportion of the third chromosomes from the South Amherst population in nearly every year that second chromosomes from that population have been analysed for lethals, including 1969. These observations suggest that the fall population, which is sampled by collections from the author's porch, is a consistently large breeding unit of a continuous population. That population may, however, consist of several much smaller breeding units early in the summer when flies do not always come to the porch site in numbers sufficient for a meaningful quantitative test. Late in 1965, a search was initiated by Dr. James E. Carver, Jr., working on a N.I.H. postdoctoral fellowship, for a place in South Amherst where flies could be collected routinely in quantity early in the growing season. He found such a place at the Markert apple storage, about 5 km (3 miles) from the author's residence. Close to the storage is a large, continuous pile of rotten apples, which appears from Dr. Carver's ecological observations (to be published separately), and from the genetic data to be reported here, to be a major source of the South Amherst population. During the 1966-1969 period, collections have been made from at least the first and the last flies appearing each year in the 'Supported in part by contract number AT (30-1)2467 with the United States Atomic Energy Commission and in part by grant number GB-5680 from the National Science Foundation. 2 This paper is dedicated to Professor Th. Dobzhansky on the occasion of his 70th birthday.