A Reconsideration of the Mode of Development of the Bar Eye of Drosophila Melanogaster.

HE sex-linked semi-dominant mutation, Bar, of Drosophila melanoT gaster (TICE 1914) when homozygous reduces the facet number of the compound eye from approximately 770 (the facet number of the wild type) to about 75. When heterozygous, it reduces the facet number to about 350. Its discovery afforded geneticists an effective tool for the quantitative study of gene action. Only one year after the announcement of its discovery, ZELENEY and MATTOON (1915) showed it to be sensitive to selection-that is, to changes in the genotypic environment. SEYSTER (1919) showed that in Bar facet number varied inversely with temperature and that temperature was effective only during the larval period. To explain his data, SEYSTER postulated “the existence of a chemical factor or determiner which acts as an inhibitor of facet formation and that, at a higher temperature, the speed of reaction is much greater than at a lower.” This pioneer work of SEYSTER, which unfortunately was interrupted by the first World War, opened the floodgates for a number of experiments designed to measure the effect of temperature on the development of the Bar eye, with the intention of finding an explanation of the mode of action of the Bar mutant. KRAFKA, in a series of three papers, published in 1920, confirmed and extended SEYSTER’S work to show that temperature was effective during only a limited portion of the larval period. He suggested “ . . . the decrease in facet number to be due to an inhibitor, the temperature coefficient of which differs from that of the normal facet-producing reaction . . .” and that “ . . . the Bar eye factor comes into play after about three-fourths of the larval period is finished.” Subsequent work showed that the temperature effective period (T.E.P.) varies in duration (percentage of larval life) and position from temperature to temperature (DRIVER 1926 and others). For example, at 2ooC the T.E.P. occupies approximately 25 percent of larval life, while at 27OC it includes only about 15 percent (DRIVER 1931). More recently, MARGOLIS (1934, 1935a, 1935b, 1936) very carefully repeated the temperature work on Bar. His conclusions in the main confirm those of the earlier workers. MARGOLIS (1935”) presented in schematic