Genetic population replacement for insect control: a new method for estimating fitness and generation time of continuously-breeding competing strains.

A model of complete underdominance that applies to population replacement for insect control by compound autosomes or compound; free arm strains, has been used to develop a new technique for estimating fitness and generation time in continuously-breeding competing populations, without resorting to measurement of birth rate, survivorship etc. The method is statistical and uses successive intervals of various sizes in an estimation equation. Estimates of fitness and generation time are revealed as a result of convergence of data from competitions in which a strain either becomes fixed or is eliminated in a mixed population. The technique has been applied to data from Drosophila melanogaster cage competitions with believable results. Difficulties resulting from the frequency dependence of the estimates over time and the inherent cyclicity of the population competition data are evaluated. Fitness estimates from this method of successive intervals are lower than those from another unstable equilibrium method. The former technique measures fitness in population at carrying capacity in which density-dependence is prominent, whereas the latter method is applicable only to populations in which density-dependence is negligible. The implications to insect control of an estimation procedure which yields fitness values for continuously-breeding populations under conditions of density dependence are discussed.

Chromosome replacement in mixed populations of compound-2L; free-2R and standard strains of Drosophila melanogaster : An example of unstable genetic isolation.

Crosses between compound-2L; free-2R (free-arm) and standard strains of Drosophila melanogaster produce two classes of inviable aneuploid hybrids in equal proportions: monosomic 2L and trisomic 2L. The lethal period for monosomics occurs during embryogenesis while the trisomics survive to late pupae. Since the hybrids are inviable, standard and free-arm strains within a mixed population remain genetically isolated. Genetic isolation in the absence of mating isolation offers an extreme example of unstable equilibrium. Relative fitness data indicate that an unstable equilibrium will be established between free-arm and standard strains at a ratio of 2.5∶1. Indeed, in three cage experiments established at initial ratios of 3∶1, free arms to standards, laboratory (Oregon R) or native (Okanagan S) standard strains were completely replaced in approximately 100 days by free-arm lines derived either from laboratory or from native genetic background. In contrast, one cage established at an initial ratio of 4∶1 failed to show replacement and for 92 days remained at approximately the initial ratio. Subsequent genetic analysis of flies removed from this cage identified the presence of an anomalous strain through which genetic information was transferred reciprocally between the free-arm and standard lines. The second chromosomes carried by this strain consisted of a free-2R and a standard second on the right arm of which was attached a duplication for all of 2L. While the origin of the 2L·2R+2L chromosome was uncertain, genetic and cytological examinations revealed that it represented the reciprocal crossover product expected from an exchange that generated a F(2R). Additional crosses disclosed that the transmission frequency of the asymmetrical pair of second chromosomes, as well as their right-arm crossover products, was disproportionately in favor of the short arm. Since unequal transmission was invariably greater from female parents, this phenomenon was viewed as further evidence in support of the drag hypothesis.

Exploring the Potential of Compound;free-Arm Combinations of Chromosome 2 in DROSOPHILA MELANOGASTER for Insect Control and the Survival to Pupae of Whole-Arm Trisomies.

Genetic tests of second-chromosome compound;free-arm combinations ("free arms") in Drosophila melanogaster indicate that the egg hatch is approximately 50% that of standard lines and adult recovery is approximately 40%. Free-arm strains are genetically isolated from both compound-chromosome lines and standards.A large proportion of the hybrid progeny arising from crosses between free arms and standards or free arms and compounds, survive to the pupal stage. Cytological examinations reveal that these hybrids are trisomic for one arm of chromosome 2. Such hybrid progeny may place an added constraint upon the competition between free-arm and standard strains by competing for food, but not contributing to the adult population. The fitness data, the genetic isolation characteristic and the possible impact of hybrid progeny all suggest that free arms may prove to be a valuable genetic tool for insect population control. Preliminary cage-competition experiments to test this prediction have demonstrated that free arms are able to displace standards at ratios as low as 3:1, which is close to the theoretical equilibrium predicted by the fitness data (2.5:1).

Population control of caged native fruitflies in the field by compound autosomes and temperature-sensitive mutants.

A genetic technique for insect population control has been tested in cages under field conditions at two different locations in British Columbia. The method entails the population replacement of standard insects by those bearing compound autosomes using the principle of negative heterosis, thus permitting control or elimination through conditional mutations. Both native- and laboratory-derived compound strains of the fruitfly Drosophila melanogaster were tested in population cages against standards in the laboratory and at the two field sites. Those compound-bearing insects originating from the wild were the most successful, both in the laboratory and the field, in displacing standards from the cages down to a minimum initial ratio of 5 compounds to 1 standard. The importance is stressed of collecting strains from the wild, and performing the necessary genetic manipulations as rapidly as possible, prior to releasing the rearrangement in the field for control purposes.

Genetic control of insect population. I. Cage studies of chromosome replacement by compound autosomes in Drosophila melanogaster.

A genetic method for insect control was evaluated using the test organism, Drosophila melanogaster. The technique involved the displacement under a system of continuous reproduction, of standard strains by those carrying compound autosomes. The eradication of the replacements could subsequently be achieved through the use of temperature-sensitive lethal mutations.-While certain compound autosome strains failed to displace standards in population cages, even at the initial release ratio of 25:1, others were highly successful. Indeed, for some strains when the ratio of compounds to standards was as low as 9:1, the population rapidly went to fixation in favor of the compound line.-Hatchability was found to be an insufficient index of fitness to estimate the initial ratios of compounds to standards that would guarantee fixation of the former. Differences in other fitness components, such as development time, were detected that could seriously modify displacement, especially with continuous overlapping generations. The importance of examining the fitness of various compound lines and selecting the most competitive in cages, prior to field tests, cannot be overemphasized.