A selection response plateau for radiation resistance in the cotton boll weevil

Twenty generations of family selection in the cotton boll weevil for 14-day postirradiation survival to 10,000 rads of gamma irradiation has increased survival to nearly 90% as compared with about 35% in the unselected control population. Mean survival time has increased to 21.2 days in the selected population, as compared with 12.8 days in the unselected control. Nearly all of the response to selection occurred in the first 12 generations of selection, with no significant improvement beyond that point. A relaxed selection line was established in generation 12 and has been maintained as a population cage with discrete generations since that time. A comparison in generation 17 between the relaxed selection population and the selected population where both populations were managed in the same way indicated that none of the increase in resistance had been lost due to relaxation of selection (89.2% survival in the relaxed population as compared with 86.0% in the selected population). The rapid increase in response to selection followed by a quick plateau and no decline in the mean following relaxation of selection support the hypothesis that the increased resistance to irradiation resulted from changes in allelic frequencies for a relatively small number of genes. Alleles for increased resistance were either fixed by the selection process or, if still segregating, were not negatively correlated with fitness. Estimates of heritability for other fitness traits indicate selection should be effective for several other traits of importance in the efficiency of a mass rearing program.