Structured and unstructured continuous models for Wolbachia infections

We introduce and investigate a series of models for an infection of a diplodiploid host species by the bacterial endosymbiont Wolbachia. The continuous models are characterized by partial vertical transmission, cytoplasmic incompatibility and fitness costs associated with the infection. A particular aspect of interest is competitions between mutually incompatible strains. We further introduce an age-structured model that takes into account different fertility and mortality rates at different stages of the life cycle of the individuals. With only a few parameters, the ordinary differential equation models exhibit already interesting dynamics and can be used to predict criteria under which a strain of bacteria is able to invade a population. Interestingly, but not surprisingly, the age-structured model shows significant differences concerning the existence and stability of equilibrium solutions compared to the unstructured model

Interspecific transfer of Wolbachia into the mosquito disease vector Aedes albopictus

Intracellular Wolbachia bacteria are obligate, maternally inherited endosymbionts found frequently in insects and other invertebrates. The evolutionary success of Wolbachia is due in part to an ability to manipulate reproduction. In mosquitoes and many other insects, Wolbachia causes a form of sterility known as cytoplasmic incompatibility (CI). Wolbachia-induced CI has attracted interest as a potential agent for affecting medically important disease vectors. However, application of the approach has been restricted by an absence of appropriate, naturally occurring Wolbachia infections. Here, we report the interspecific transfer of Wolbachia infection into a medically important mosquito. Using embryonic microinjection, Wolbachia is transferred from Drosophila simulans into the invasive pest and disease vector: Aedes albopictus (Asian tiger mosquito). The resulting infection is stably maintained and displays a unique pattern of bidirectional CI in crosses with naturally infected mosquitoes. Laboratory population cage experiments examine a strategy in which releases of Wolbachia-infected males are used to suppress mosquito egg hatch. We discuss the results in relation to developing appropriate Wolbachia-infected mosquito strains for population replacement and population suppression strategies

Fitness studies: developing a consensus methodology

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 preventio

Evolutionary history of a mosquito endosymbiont revealed through mitochondrial hitchhiking

Due to cytoplasmic inheritance, spread of maternally inherited Wolbachia symbionts can result in reduction of mitochondrial variation in populations. We examined sequence diversity of the mitochondrial NADH dehydrogenase subunit 4 (ND4) gene in Wolbachia-infected (South Africa (SA), California and Thailand) and uninfected (SA) Culex pipiens complex populations. In total, we identified 12 haplotypes (A–L). In infected populations, 99% of individuals had haplotype K. In the uninfected SA population, 11 haplotypes were present, including K. Nuclear allozyme diversity was similar between infected and uninfected SA populations. Analysis of nuclear DNA sequences suggested that haplotype K presence in uninfected SA Cx. pipiens was probably due to a shared ancestral polymorphism rather than hybrid introgression. These data indicate that Wolbachia spread has resulted in drastic reduction of mitochondrial variability in widely separated Cx. pipiens complex populations. In contrast, the uninfected SA population is probably a cryptic species where Wolbachia introgression has been prevented by reproductive isolation, maintaining ancestral levels of mitochondrial diversity. Molecular clock analyses suggest that the Wolbachia sweep occurred within the last 47 000 years. The effect of Wolbachia on mitochondrial dynamics can provide insight on the potential for Wolbachia to spread transgenes into mosquito populations to control vector-borne diseases