David T. North

Effects of gamma irradiation on the mechanisms of sperm transfer in Trichoplusia ni

Abstract The mechanisms involved in the transfer of sperm by unirradiated and irradiated cabbage loopers were studied by live dissection of the female immediately after the initiation of mating. This technique allows for observation of the entire mating and sperm transfer process. Failure of gamma-sterilized (30 krad.) adult male cabbage loopers to transfer sperm to the spermatheca was due to the inability of the irradiated males properly to incorporate sperm into the spermatophore bulb. The time required for mating was significantly longer for gamma-sterilized male moths. The timing of the insertion of the spermatophore, inflating the spermatophore bulb, and the inclusion of sperm into the bulb was altered in irradiated moths. Most irradiated males studied everted their endophallus early and ejaculated into the bursa copulatrix. Since eupyrene sperm have no motility until reaching the spermatheca, they have no way of being utilized for fertilization. A fraction of the apyrene sperm, on the other hand, find their way from the bursa copulatrix to the spermatheca since this type of sperm is motile at the time of ejaculation. Apyrene sperm being anucleate cannot effect fertilization so that radio-sterilization of the cabbage looper is essentially due to lack of sperm utilizable by the female. Irradiated males ejaculated the same number of eupyrene and apyrene bundles as the controls. These studies indicate that the elicitation of the ovipositional response is initiated by one or more of the male accessory secretions which must be incorporated into the spermatophore bulb to be effective.

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

SummaryTwenty 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.

Selection for mating propensity in irradiated populations of the cotton boll weevil

SummaryTwelve generations of family selection for 10-day post-irradiation male mating propensity resulted in significant divergence between the selected and unselected control populations. Much of this divergence was the result of a decline in the control population which was believed to have been a function of both inbreeding and environmental effects. Significant correlated responses as measured by differences in the two populations for linear time trends of performance on generations of selection were observed for 10-day post-irradiation survival, percent survivors mating at 10 days, and 7-day egg production of unirradiated females.