Introgression of resistance genes between populations: a model study of insecticide resistance in Bemisia tabaci.

Introgression is a key process in conservation biology, genetic modification of (crop) species and in the evolutionary ecology of many species. Here we consider the case of introgression of insecticide resistance in the whitefly, Bemisia tabaci. B. tabaci is a species complex consisting of a range of biotypes, known to have a high degree of inter-biotype reproductive isolation. In areas where insecticide resistant and susceptible biotypes of B. tabaci coexist, introgression of the resistance gene will have considerable consequences for whitefly control. Using a stochastic branching process model we calculate the relative importance of life-history traits in determining the probability of introgression given that a hybridization event has occurred. We show that a fitness cost expressed through the average number of eggs laid, has the largest effect on the introgression probability as compared to fitness costs expressed through other life-history parameters. These results change when we consider a reproductive isolation mechanism, for which we show that the fitness cost expressed through the male survival and mating probability have the largest effect on the probability of introgression.