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.
Genetic systems have been developed in several insect species for separating males and females prior to releasing sterilized males in pest control programmes using the sterile insect technique. The systems generally depend on translocating a readily selectable gene onto the Y chromosome. A potential source of instability in such a system is genetic recombination in the male. Although such recombination was originally thought to be absent in most cyclorrhaphous Diptera, low levels have recently been found. We have developed a computer model which simulates the progression of instability in the presence of male recombination, which can be used to assess the influence of rate of recombination in combination with a range of associated genetic and biological parameters. Male recombination alone or fitness of the Y-linked translocation were found to contribute relatively little to the rate of progression of instability. By contrast reduced fitness or mating competitiveness associated with the selectable gene had a strong effect. The sex ratio and the ratio of carriers to non-carriers of the selectable gene showed patterns characteristic of the parameters modelled. The relevance of such data to the development of suitable strains for genetic sex-separation and the replacement of strains under mass rearing conditions are discussed.
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