Simulation of instability in genetic sexing strains: effects of wild-type contamination in association with other biological parameters

Abstract

Genetic methods to eliminate females prior to releasing sexually sterilized males in sterile insect technique programmes generally involve the translocation onto the Y chromosome of a readily selectable gene (here designated F). However, such strains often show instability with regard to sexual dimorphism. Concerted attempts to determine the most probable causes of such instability have rarely been successful, and appropriate countermeasures have generally not been implemented. We have developed a computer model which simulates the progression of instability in such strains. Here we simulate the effects of contamination in association with a range of genetic and biological parameters and assess the patterns and rates of progression of the ensuing instability. A single event of contamination alone was found to contribute relatively little to the progression of instability and invariably resulted in an equilibrium within the population. However, strong effects were produced by reduced fitness and mating competitiveness of both the /// genotype and the translocation carriers, a reduction in the parameters involving the /// genotype resulting in the virtual elimination of ///, while a reduction in the translocation parameters had the opposite effect of virtually eliminating the F/ -phenotype. The sex ratio was affected only by a change in the relative fitness of the /// genotype and the translocation. Several characteristic patterns contributed to distinguish clearly the effects of contamination from those of male recombination in unstable genetic sexing strains. With such knowledge appropriate measures may be taken to counter or alleviate the effects of instability in mass reared genetic sexing strains.

Effects of contamination with wild-type males, virgin females or mated females, on the stability of the T:Y(ωp†)30C genetic sexing strain of Ceratitis capitata (Diptera: Tephritidae)

“…The percentage of aberrant females and aberrant males in G5 reached stable levels close to 50% by generations 12-16. This was similar to the predictions of Busch-Petersen & Baumgartner (1992) when simulating a hypothetical genetic sexing strain with a fitness disadvantage of the mutant allele of 8.5% and of the translocation carrier of 38%.…”

“…The pupal colour ratio changed markedly after male contamination, more so at the higher level of contamination when the percentage of brown pupae reached more than 80% in generations 10-16. This was a much greater increase than observed in the simulations of Busch-Petersen & Baumgartner (1992), which assumed equal fitness of all genotypes and karyotypes. A fitness advantage of the brown gene, when not attached to the Y chromosome, seemed to be indicated.…”

“…In experiment G3, in which the proportion of contaminants was five times greater than in G4, the replicates were much much more similar; the percentage of brown females seemed to reach a plateau in all three replicates at around 40%. A plateau at around 10% was predicted in computer simulations by Busch-Petersen & Baumgartner (1992) under conditions of equal fitness.…”

“…The three types of contamination of the T:Y(zup + )30C genetic sexing strain (wild-type males, virgin females and fertilized females) each produced a different pattern and progression of instability. A summary of results is presented in table 1 which will now be discussed in relation to computer simulations carried out by Busch-Petersen & Baumgartner (1992), using the Gensex model (Busch-Petersen & Baumgartner, 1991).…”

“…The pattern of breakdown to be expected from contamination by wild-type flies has been studied in computer simulations by Busch-Petersen & Baumgartner (1992). In the experiments reported here, the aim has been to determine the patterns and progress of instability under various conditions of controlled contamination with wild-type material.…”