Date Published: January 5, 2017
Publisher: Public Library of Science
Author(s): Perran A. Ross, Itsanun Wiwatanaratanabutr, Jason K. Axford, Vanessa L. White, Nancy M. Endersby-Harshman, Ary A. Hoffmann, Elizabeth Ann McGraw.
Aedes aegypti mosquitoes infected with Wolbachia bacteria are currently being released for arbovirus suppression around the world. Their potential to invade populations and persist will depend on interactions with environmental conditions, particularly as larvae are often exposed to fluctuating and extreme temperatures in the field. We reared Ae. aegypti larvae infected with different types of Wolbachia (wMel, wAlbB and wMelPop-CLA) under diurnal cyclical temperatures. Rearing wMel and wMelPop-CLA-infected larvae at 26–37°C reduced the expression of cytoplasmic incompatibility, a reproductive manipulation induced by Wolbachia. We also observed a sharp reduction in the density of Wolbachia in adults. Furthermore, the wMel and wMelPop-CLA infections were not transmitted to the next generation when mosquitoes were exposed to 26–37°C across all life stages. In contrast, the wAlbB infection was maintained at a high density, exhibited complete cytoplasmic incompatibility, and was transmitted from mother to offspring with a high fidelity under this temperature cycle. These findings have implications for the potential success of Wolbachia interventions across different environments and highlight the importance of temperature control in rearing.
Aedes aegypti mosquitoes transmit some of the most important arboviral diseases worldwide. They are widespread in tropical and subtropical regions , inhabiting urban environments where they have adapted to breed in artificial containers . Dengue and Zika are among the viruses they transmit and these are rapidly increasing their burden on global health. Dengue alone infects as many as 390 million people each year, and up to half of the world’s population is at risk of infection . Zika is an emerging threat that is experiencing an epidemic following an outbreak in Brazil in 2015 [3, 4]. A vaccine for dengue has recently been licensed  but no vaccines for Zika are commercially available and there are risks associated with deployment of the licensed dengue vaccine . Efforts to reduce the spread of dengue and Zika therefore rely on the direct control of Ae. aegypti populations. Though permanent mosquito eradication is unlikely to be achieved, several genetic and biological approaches are being utilized to reduce the burden of arboviruses .
We demonstrate for the first time that the wMel and wMelPop-CLA infections of Ae. aegypti exhibit incomplete cytoplasmic incompatibility when immature stages experience cyclical temperatures of 26–37°C during development. We also show that these infections are not transmitted to the next generation when infected mosquitoes experience these conditions over their entire lifecycle. wMel infected mosquitoes are currently being deployed in several countries for the control of arboviruses . Immature Ae. aegypti may experience extreme temperatures in the field that are similar to the conditions used in our study [33, 54]; the thermal sensitivity of the wMel and wMelPop-CLA infections could therefore reduce their ability to establish and persist in natural populations. In contrast, the wAlbB infection retains its ability to induce complete cytoplasmic incompatibility under the same conditions, while maternal transmission fidelity remains relatively high. Densities of wAlbB are also stable, suggesting that it will also provide effective arboviral protection [22, 59]. The robustness of wAlbB when exposed to high maximum temperatures could make this infection more suited for field release in environments where temperatures in breeding sites fluctuate in comparison to wMel.