Synthetic Genetic Elements take over

A recent article in Science demonstrates propagation of a synthetic genetic element that replaces the endogenous genotype in drosophila in under 10 generations (subscription required). A freely available synopsis is here at The Scientist. The idea is that if, for example, a mosquito that is genetically programmed not to carry the malaria parasite were set off into the wild to replace disease carrying mosquitos it would not be successful because there is no reproductive advantage. Associating the genotype closely with this synthetic genetic element would make this genotype dominate. The system works by encoding two microRNAs (the 'toxin') that are expressed in the germline of the female. These microRNAs repress the expression of an gene essential for embyogenesis, so the system also encodes for a microRNA resistant form of that gene (the 'antidote'). Therefore a female carrying this genetic element can only produce viable offspring who carry the genetic element. In any case apparently if introduced at a minimal threshold level of 25% this element rapidly replaces the other genotypes. I'm still not sure exactly how this happens as you would think that a female that has less viable offspring than one that doesn't carry this element would make it less reproductively fit.