Development and evaluation of a conditionally lethal transgenic pink bollworm

A new area-wide pest control strategy using the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), genetically transformed with a conditionally lethal gene, is under development. Conditional lethality of several transgenic pink bollworm strains was demonstrated in a series of laboratory rearing experiments. Pink bollworms were transformed with genetic constructs using the RIDL technology (Release of Insects with a Dominant Lethal gene) for development of an autocidal biological control system for possible supplement or replacement of radiation based sterile insect release. LA1124 is a lethal construct controlled by a tetracycline repressible transactivator protein (tTA), in which binding of tTA to its specific target sequence tetO drives production of more tTA. In the absence of tetracycline, this leads to lethality by high expression of tTA. When tetracycline is present, tTA does not bind tetO, and so the positive feedback cycle is not established and tTA remains at a low, non-lethal level for more detail on the tetO-tTA system). Tetracycline (in the form of chlortetracycline or CTC) is a normal part of the pink bollworm artificial diet, so such a strain of pink bollworm could readily be incorporated into the current mass-rearing system. Eight laboratory rearing experiments have been conducted with the LA1124 lines where pink bollworm heterozygous for the RIDL gene were crossed to wild type pink bollworm to produce an F1 generation with a 1:1 ratio of progeny of two genotypes: one that is heterozygous for the RIDL gene and one that is wild type. These were reared with and without CTC and mortality was scored at larval, pupal, and adult stages. This experiment was designed to simulate progeny mortality that would occur from the mating of a homozygous RIDL moth with a wild type pink bollworm after release of RIDL moths in a cotton field, while also including an internal wild type control (the wild type siblings of the heterozygous F1 transgenics). To date, over 12,000 individuals have been tested and significant levels of mortality were shown for heterozygous RIDL progeny (genotype = 1124/+) reared on CTC free diet. Most of the mortality occurs in the prepupal-pupal stage with mean levels of mortality ranging from 60 to 92%. Rates of mortality in the control treatments (1124/+ progeny reared on CTC diet, and +/+ progeny reared on CTC free diet) were low, ranging from 2-15%. Other work underway includes: testing adult longevity of LA1124 lines (important for both mass-rearing and field efficacy estimates); testing the mortality rates of constructs with autocidal effector genes other than tetO-tTA (e.g., Nipp1Dm in LA476, and cyclin B in lines LA1062 and LA1064) when crossed to LA1124 lethal lines with the tetO-tTA gene. The strategy is to combine the lethal effects of two separate effector genes into a single PBW strain to learn if mortality rates can be increased or if lethality will occur at earlier larval stages than for the LA1124 strain construct alone