The genetic basis of natural variation. IV. On the natural distribution of cve polygenes of Drosophila melanogaster.

N an attempt to extend the use of standard methods of genetic analysis to those I naturally important genes which generally act in combination, posterior crossvein formation in D. melanogaster has been chosen for study. The pioneering work of WADDINGTON (1 953, 195 7) relating crossveinless phenocopy sensitivity to crossvein defects under normal conditions has been extended by BATEMAN (1959), MOHLER (1957), and MILKMAN (1960b, c) . All have found polygenes, extractable by selection from wild populations, which lower a fly's ability to make posterior crossveins. These polygenes have yielded to various kinds of genetic analysis, though not without difficulty. Standard methods of analysis have been developed for the cue polygenes and for the developmental processes they control. This has made possible the investigation of cue polygenes in natural populations and the detailed study of an array of developmental labilities in wild Drosophila strains. This paper reports the findings of such a study. Remarkable concealed genetic variability has been demonstrated by TIMOFEEFF-RESSOVSKY (DOBZHANSKY 1950) and others ( LERNER 1954). Where TIMOFEEFF-RESSOVSKY was able to demonstrate many previously unsuspected simple recessives, DUBININ and LERNER (LERNER 1954) called attention to an even subtler and correspondingly more extensive genetic basis of natural variation: genes with individually small effects which act only in rare combinations to produce deviant phenotypes. Their findings renewed optimism for finding a common analytic ground between genetic variation and natural phenotypic variation. The original experiment (MILKMAN 1960a) from which the present work stems was performed in order to gain insight into the natural importance of the cue polygenes and to develop this representative system for the consideration of potential genetic variability in natural populations. It was known (MILKMAN 1960c; BATEMAN 1959) that cue phenodeviants, while rare, are ubiquitous, and that from these phenodeviants one may select true-breeding, polygenic, cue strains with no great difficulty. But could one demonstrate such cue phenodeviants