Sensitivity of wMel and wAlbB Wolbachia infections in Aedes aegypti Puducherry (Indian) strains to heat stress during larval development

Abstract

Background ICMR-Vector Control Research Centre, Puducherry, India, developed two colonies of Aedes aegypti infected with wMel and wAlbB Wolbacia strains called Ae. aegypti (Pud) lines for dengue control. The sensitivity of wMel and wAlbB strains in Ae. aegypti (Pud) lines to heat stress was studied. Methods wMel and wAlbB infected and uninfected Ae. aegypti larvae (first to fourth instars) were reared in the laboratory to adults at 26 °C, 30 °C, 36 °C and 40 °C constant temperatures and also 26–30 °C, 26–36 °C and 26–40 °C diurnal cyclic temperatures. The adults were tested for Wolbachia infection. Experiments were also carried out rearing the larvae under simulated field conditions in summer (April and June) under sunlight using fully open and half open bowls and also under sunlight and natural shade. Results At 36 °C and 40 °C constant temperatures, complete larval mortality was observed. At 30 °C and 26 °C, no larval mortality occurred, but Wolbachia density was relatively low in wMel infected males compared to control (maintained at 26 ± 1 °C). At diurnal cyclic temperature of 26–40 °C, Wolbachia density was reduced in males of both the (Pud) lines, but not in females. At 26–36 °C, reduction in Wolbachia density was observed in wMel males but not in wAlbB males. At 26–30 °C, no significant reduction in Wolbachia density was observed with wMel and wAlbB strains. In simulated field conditions (April), under sunlight, the daytime water temperature reached a maximum of 35.7 °C in both full and half open bowls. No larval mortality occurred. Wolbachia frequency and density was reduced in wMel-infected Ae. aegypti (Pud) males from both type of bowls and in females from full open bowls, and in wAlbB males from half open bowls. In June, rearing of larvae under sunlight, the first-instar larvae experienced a maximum daytime water temperature of > 38 °C that caused complete mortality. No larval mortality was observed in bowls kept under shade (< 32 °C). Conclusions Exposure of larvae to higher rearing temperatures in the laboratory and simulated-field conditions reduced the densities of wMel and wAlbB strains particularly in males, but the impact was more pronounced for wMel strain. The actual effect of heat stress on the stability of these two Wolbachia strains needs to be tested under natural field conditions. Graphical Abstract

Wolbachia wAlbB inhibit dengue and Zika infection in the mosquito Aedes aegypti with an Australian background

“…High temperatures may have contributed to the heterogeneity in this study [19] and the low prevalence of wMel in Nha Trang, Vietnam [21]. Side by side comparisons show the wAlbB strain to be more robust to heat stress than the wMel strain [23,39], however the utility of a Wolbachia strain is also determined by factors including the competitiveness of infected mosquitoes with uninfected wild mosquitoes and persistence of Wolbachia infections in the mosquito eggs. Characterisation of Wolbachia strains with varying traits, including thermal tolerance, could allow for the optimisation of Wolbachia strategies that are tailored to a particular DENV endemic region and to mitigate potential effects of increasing global temperatures [40].…”

Modelling the ecological dynamics of mosquito populations with multiple co-circulating Wolbachia strains

“…Some Wolbachia features regulating the success of these mechanisms include immune system preactivation in the vectors, induction of cytoplasmic incompatibility (CI) rendering offspring unviable, imperfect maternal transmission of Wolbachia , loss of Wolbachia infection (LWI) due to high temperature, and superinfection by a second Wolbachia strain 9 – 12 . Based on these features, there are some tradeoffs exhibited by different Wolbachia strains, i.e., some strains induce CI (which is good) but also have LWI due to high temperature (which is bad) and vice versa 13 , 14 .…”

Modelling the ecological dynamics of mosquito populations with multiple co-circulating Wolbachia strains

“…Some Wolbachia features regulating the success of these mechanisms include immune system preactivation in the vectors, induction of cytoplasmic incompatibility (CI) rendering offspring unviable, imperfect maternal transmission of Wolbachia, loss of Wolbachia infection (LWI) due to high temperature, and superinfection by a second Wolbachia strain [9][10][11][12] . Based on these features, there are some tradeoffs exhibited by different Wolbachia strains, i.e., some strains induce CI (which is good) but also have LWI due to high temperature (which is bad) and vice versa 13,14 .…”

“…Presently, the wMel-Wolbachia strain is commonly used in the field, with releases in Australia 15 , Indonesia 16 , Brazil 17 , Colombia 18 , the United States of America and China 19 . The wAlbB Wolbachia strain was later introduced in Malaysia 20 , Thailand 21 , Taiwan 22 , India 14 and wMelPop in Vietnam 23 , while other strains are yet to be field-tested. Single-strain Wolbachia experimental studies have shown that most crosses between Wolbachia-infected arthropods and wild-type mosquitoes induce unidirectional CI, that is, loss of fertility of a wild-type female mating with a Wolbachia-infected male mosquito, but not the reverse 2,[24][25][26] .…”

“…Single-strain Wolbachia experimental studies have shown that most crosses between Wolbachia-infected arthropods and wild-type mosquitoes induce unidirectional CI, that is, loss of fertility of a wild-type female mating with a Wolbachia-infected male mosquito, but not the reverse 2,[24][25][26] . In addition, most Wolbachia-infected mosquitoes greatly lose their infection under high temperatures 12,27 except those infected with the CI-inducing wAlbB and wAu-Wolbachia strains, which does not induce CI 3,10,14 . For double-strain Wolbachia experimental studies, CI is typically bidirectional, that is, any mismatch in Wolbachia strain among mating vectors results in infertility; however, CI does not affect crosses involving wAu-Wolbachia-infected males with other Wolbachia-infected females 3,6,28,29 , opening up a tantalising possibility of two different strains of Wolbachia-infected mosquitoes co-existing (Figure 1).…”