It has been shown in previous work that heterozygotes between irradiated and non-irradiated populations of D. melanogaster experience a marked reduction in competitive ability. The effects of allowing the nucleus of a heterozygote gene pool to develop are here investigated and it is found that the total number emerging from a cage containing irradiated, non-irradiated, and heterozygote flies is considerably increased and the number of heterozygotes themselves shows almost a 100% increase. It is concluded that the opportunity given to the heterozygotes to form an integrated gene pool allows them to overcome much oftheir competitive disability.Further experiments involving three irradiated populations and the three possible hybrids between them which could be formed at each generation showed that these hybrid populations, although having similar radiation histories and containing similar genetic material, had a competitive ability reduced by over half or even three-quarters in many cases, compared with the parental population.It appears that it is the nature of the gene pool rather than its heterozygosity or the individual genes contained in it which largely determines the fitness and competitive ability of the population.
HE genetic effects of ionizing radiations have now been studied extensively in a wide variety of systems. The study of their cumulative effects in populations has also been given increasing attention in recent years although, because of experimental difficulties, on a more restricted number of organisms. The effect of radiation on a population will be to alter its genetic constitution and, therefore, presumably, to affect the biological fitness. In order to relate these changes in fitness to specific genetic damage three prime conditions must be met. First, the genetic constitution must be exactly known at the commencement of the experiment; second, the accumulation of genetic damage must be closely followed; and third, direct and exact comparisons with control populations must be continuously possible.These three criteria are satisfied in the following way in the present experiments. The flies are first generation hybrids between two inbred lines and are, therefore, genetically almost identical and at maximum fitness with minimum genetic load. They are homozygous for the second chromosome genes light or straw and are, therefore, phenotypically distinct, but at each generation hetero-
The effect of 1500 r X.rays, delivered for several generations to populations of D. melanoga8ter, on the sex ratio of the flies emerging has been examined. The results have been compared with those from similar populations living in the same environment but not given any radiation treatment. The mean values of four irradiated populations, when compared with their unirradiated counterparts, showed an initial drop in sex ratio followed by a marked recovery particularly between generations 7 and 12. When composite populations were set up, combining males from one irradiated population with females from another at each generation, this recovery in sex ratio was not observed. It was concluded that this latter result ruled out chromosomal phenomena and indicated a biometrical explanation as the most likely one for the observed rise in sex ratio with accumulated ancestral radiation in integrated gene pools.
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