EXPERIMENTS ON THE INFLUENCE OF SELECTION PRESSURE ON IRRADIATED POPULATIONS OF DROSOPHILA MELANOGASTER

Abstract

It may be supposed that the social level and the level of medical care in human populations is of influence upon the genetical consequences of irradiation of these populations. In order to study this question, model experiments with Drosophila melinogaster were started towards the end of 1956. Flies of experimental populations are for each generation given an acute x-ray dose of 1.5 kiloroentgens. From eggs laid thereafter freshly hatched larvae are collected and transferred to vials with a standard amount of food where the larvae develop. The number or larvae thus transferred is different in different populations; in some only 25 larvae, in others 200 larvne. In the former type there is practically no competition for food, but this competition is strong in the latter type. Besides this control of selection pressure there is, in other populations, also a control that individual females do not contribute more than a limited number of progeny for the next generation. All populations emanate from a common non irradiated homozygous strain. Genetical differences which later are found between the populations may therefore primarily be ascribed to the differences in selection pressure. In all these populations there occurred, in the beginning, difficulties to keep themmore » alive. With increasing number of generations these difficulties were rather soon overcome in the populations with strong selection BIOLOGY pressure. In the populations with low selection pressure the situation was much more grave: at one moment it seemed as if extinction was imminent in one of these populations. However. when a certain amount of accumulated dose was surpassed, the viabilities of the lowpressure populations increased to a level close to that of the high-pressure populations. Flies are randomly collected from the populations prior to the irradiation of the populations. With these flies, which thus are, themselves, nonirradiated, different kinds of tests are made. These tests include larval competitions, egglaying capacity, egg hntchability, fertility of males, frequency and spread into the populations of bad genes. So far no clearcut picture is arrived at from these tests. Several more generations must elapse before valid conclusions can be drawn. (auth)« less