Making mosquitoes harmless

Abstract

Besansky and Collins ~ draw attention to the insect symbiont Wolbachia, which is maternally inherited and causes sterility (incompatibility) in matings of infected males by uninfected females, but not in the reciprocal cross. Thus the infected state is at a selective advantage, and Besansky and Collins suggest that 'Desirable traits, such as those controlling vector competence, could potentially be introduced into infected females who would then drive the trait to fixation as well as the infection'. Coincidentally, I recently proposed approximately the same idea 2, but with the important proviso that the desirable gene would first have to be inserted into Wolbachia before introducing the symbiont into an Anopheles mosquito. If the gene were left in its normal chromosomal location, fertile matings between uninfected wild males and released infected females or their progeny would 'recombine' the vector competence gene with the infected cytoplasm so the end result would be a vectorially competent Wolbcchia-infected population. The problem of obtaining tight 'linkage' between transport systems and the genes they are meant to transport is not a new one. In matings between Culex pipiens, populations which are described as bi- directionally cytoplasmically incompatible, there is generally a low level of fertility 3, and it was this potential for recombination that discouraged me in the 1970s from pursuing this system for transport of desirable chromosomal factors into mosquito populations. It was inconceivable in those days to propose re-locating the gene into Wolbachia but perhaps now this does not seem so very different from cloning a gene into a plasmid. However, replication of the appropriately transformed Wolbachia clone, in vitro, may prove difficult as these symbionts have been described as 'fastidious '4. Two other points to consider are: ( I ) The gene to be introduced would have to be dominant, ie. to actively block vectorial competence of the mosquito, because the vector competence allele would still exist at its normal chromosomal location, and (2) If the gene were introduced into any other maternally inherited entity, it might remain 'linked' to the Wolbachia as the symbiont spreads in the wild population, but occasional transmission through the sperm could break down such linkage. A transformed mitochondrion could probably not be used because there is no reason to expect that such an organelle would outcompete the pre-existing mitochondrial population. The advantage of Walbachia for use against Anopheles vector populations is that, as far as is known, this symbiont has not yet found its way into Anopheles (data of J. Yen, cited in Ref. 5). However, judging by its performance in several different types of insect 6, and with a little help from an expert with a syringe, it might well succeed in colonizing the empty niche provided by Anopheles cytoplasm, including that of eggs, and in causing unidirectional incompatibility, which is the means by which Walbachia causes its own genes to be selectively replicated.