Riboflavin instability is a key factor underlying the requirement of a gut microbiota for mosquito development

Abstract

We previously determined that several diets used to rear Aedes aegypti and other mosquito species support the development of larvae with a gut microbiota but do not support the development of axenic larvae. In contrast, axenic larvae have been shown to develop when fed other diets. To understand the mechanisms underlying this dichotomy, we developed a defined diet that could be manipulated in concert with microbiota composition and environmental conditions. Initial studies showed that axenic larvae could not grow under standard rearing conditions (27 °C, 16-h light: 8-h dark photoperiod) when fed a defined diet but could develop when maintained in darkness. Downstream assays identified riboflavin decay to lumichrome as the key factor that prevented axenic larvae from growing under standard conditions, while gut community members like Escherichia coli rescued development by being able to synthesize riboflavin. Earlier results showed that conventional and gnotobiotic but not axenic larvae exhibit midgut hypoxia under standard rearing conditions, which correlated with activation of several pathways with essential growth functions. In this study, axenic larvae in darkness also exhibited midgut hypoxia and activation of growth signaling but rapidly shifted to midgut normoxia and arrested growth in light, which indicated that gut hypoxia was not due to aerobic respiration by the gut microbiota but did depend on riboflavin that only resident microbes could provide under standard conditions. Overall, our results identify riboflavin provisioning as an essential function for the gut microbiota under most conditions A. aegypti larvae experience in the laboratory and field.

Diet–Microbiota Interactions Alter Mosquito Development

“…However, microbiota-produced B vitamins may be more important to aquatic animals like mosquito larvae because of their instability in liquid media, where light, oxygen, high temperature, or changes in pH can rapidly degrade them into forms that cannot be used ( Sheraz et al, 2014 ; Schnellbaecher et al, 2019 ). Further, work using holidic diets that excluded individual micronutrients showed direct evidence that microbes promote the growth of mosquito larvae through the production of various B vitamins ( Wang et al, 2021 ). In addition, this work suggests that completely axenic rearing of mosquito larvae was previously thwarted by the degradation of B vitamins via photodegradation in the aquatic rearing environment ( Correa et al, 2018 ; Wang et al, 2021 ).…”

“…Further, work using holidic diets that excluded individual micronutrients showed direct evidence that microbes promote the growth of mosquito larvae through the production of various B vitamins ( Wang et al, 2021 ). In addition, this work suggests that completely axenic rearing of mosquito larvae was previously thwarted by the degradation of B vitamins via photodegradation in the aquatic rearing environment ( Correa et al, 2018 ; Wang et al, 2021 ).…”

The Axenic and Gnotobiotic Mosquito: Emerging Models for Microbiome Host Interactions

“…Finally, using data from the axenic and decolonized mosquito models, Wang et al verified and replicated the observation that supplementing the diet of axenic larvae with B vitamins, in this case riboflavin, can rescue development. They further showed that axenic larvae undergo gut hypoxia when reared under appropriate conditions, verifying that microorganisms are not required to produce anoxic conditions (Wang et al, 2021a). Thus, axenic studies of mosquito larvae have come full circle, highlighting the role of the microbiome in supplying essential nutrients, B-vitamins, such as folate and riboflavin, as the key function of the microbiome in mosquito larval development.…”

Mosquito-bacteria interactions during larval development trigger metabolic changes with carry-over effects on adult fitness

“…The mechanisms underlying the effects of mosquito-microbiota interactions on immature development remain to be fully elucidated. It has been proposed that bacteria in the larval gut provide the essential micronutrient riboflavin (vitamin B2), whose lack results in gut hypoxia and developmental arrest [29][30][31]. Other studies suggested that the role of bacteria during larval development is primarily nutritional [28], possibly by contributing to folate biosynthesis and/or by enhancing energy storage [32].…”