In the near future, population biologists will be increasingly called upon to assess the potential of a large number of different genetically modified mosquito (GMM) strains to reduce pathogen transmission by natural mosquito populations. Adopting a standardized methodology for GMM fitness assessment will allow researchers to compare results from different laboratories and rapidly identify constructs and GMM strains that are most likely to be of applied use in the field. In this article we provide an operational definition for fitness, review the complexity of fitness, discuss lessons that can be learned from past genetic-based mosquito control programmes, and propose a methodology for rapidly and effectively assessing the fitness of GMMs compared to wild-type mosquitoes. Fitness is best understood as success at producing offspring. Because it can vary across identical genotypes, fitness is often considered as the average contribution to succeeding generations. Herein, we refer to the relative fitness of GMMs because they will be compared to their wild-type counterparts. Fitness is dynamic and measuring it is complicated. It can be influenced by variation in environment and genetic background. Based on conclusions from past mosquito population reduction projects, mating competitiveness and processes by which the size of populations are regulated will be important considerations for population replacement strategies. An examination of published results from fitness assessment of three transgenic mosquito lines indicates that to avoid the effects of inbreeding and fitness depression, transgenic lines should be outbred with wild-type strains before measuring fitness, and that transgenes may not necessarily confer a fitness cost. As a methodology for assessing GMM fitness we advocate three phases of cage competition experiments, beginning in the laboratory and ending in large field enclosures. For all three we recommend introgression of transgenes into the genetic background of the proposed target field population. Control cages should be included to assess common environmental effects. Relative fitness can be estimated from the frequency of transgene genotypes in subsequent generations. In the first phase, outbred GMMs would be introduced into laboratory cages at equal frequencies with mosquitoes from the target field population. Phase two would be the same experiment, except that cages would be held at the proposed release site and GMMs would compete against mosquitoes collected directly from the field. In the third phase, mosquitoes would be released into large replicate outdoor enclosures and competed against field-collected conspecifics. The process would begin with many GMM candidate lines and end with one or very few lines that will be seriously considered for use in disease prevention.
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