The Meiotic Origin of Temperature-Induced Crossovers in DROSOPHILA MELANOGASTER Females.

I T has been known since the early work of PLOUGH (1917, 1921) that alterations in temperature can increase recombination frequencies in females of Drosophila melanogaster. Two distinct, yet related, problems have arisen from such studies. First, does the additional frequency of crossing over represent gonial or meiotic events, or both? Second, does temperature act at the time of crossing over or before? Initially, PLOUGH proposed that the effect is a meiotic one, occurring in the earliest oocyte and most probably at the time of exchange. In a subsequent paper ( 1 924) he expressed the somewhat altered view that the sensitive stage was the gonial cell and that induction occurred prior to the meiotic event. WHITTINGHILL’S studies (1 964) indicated to him that at least a component of heat-induced crossovers might be oogonial in origin, whereas THOMPSON (1964) not only favored a gonial origm, but also suggested that heat treatment permanently affects the female, leading to increased recombination frequencies among her progeny for the duration of her egg production. A permanent effect must have its origin in the stem cells. The present experiments are directed toward answering only the first question, i.e., the origin of temperature-induced crossovers. Genetic tests are available which permit one to arrive at an unambiguous answer. Two such tests have been utilized here, one employing the phenomenon of secondary nondisjunction and the other nonrandom disjunction. Both are special types of meiotic segregation, the former occurring at anaphase I, the latter at anaphase 11. In each case the frequency of segregation is directly related to the amount of meiotic exchange. When exchange is increased by temperature, one kind of segregation will result if meiotic exchange is involved; another kind if it is gonial exchange. Hence, the effect of increased exchange on meiotic segregation provides a means of discriminating between the two different types of origin. The results indicate an exclusively meiotic origin.