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

Vitamin B2 deficiency because of the decay of riboflavin is a major driver for the requirement of gut microbiota in the survival of mosquitos. Gut microbiota are major players in the health of animals, but because of diverse diets, large multi-member microbial groups make them difficult to study. To account for this, authors of this paper studied mosquitos from the larval stage through adulthood. Mosquito Larvae hatch with no gut microbiota and acquire theirs by feeding in their environment—different environments means that the gut microbiota also varies between species. Aedes aeqypti, or the Yellow Fever Mosquito, can be found worldwide in tropical and subtropical regions. The A. aeqypti species goes through four instars (phase between two molting periods) and then pupate (cocoon) and emerge as an adult. 

Why a gut microbiota is necessary is a difficult topic to understand because there are so many factors. This study aimed to sort out why mosquito larvae required a gut microbiota in some previous studies, but not others. The authors used diets from those previous studies and determined what dietary components allowed the Larvae to survive. 

In previous studies, the researchers reared A. aeqypti in the lab under standard conditions and fed them a variety of nutrient rich diets in cultures with no other organisms (i.e. no opportunity for the larvae to develop gut microbiota). These axenic mosquitos failed to grow past the first instar, suggesting mosquitos require a gut microbiota to survive. For this study, researchers found axenic A. aeqypti did develop fully when fed diets composed of autoclaved bovine liver powder (LP), brewer’s yeast (YE), and E. coli (EC) all embedded in agar (a gel-like substance used to make cultures). The authors conducted multiple trials using variations of the diet (LP:YE, YE:EC, in combination with a  holidic medium (H) made with essential nutrients). They also trialled an H medium lacking components that vertebrates are thought to require but not insects.

Results indicated that Riboflavin (a source of B2 vitamin) is a major reason Larvae died before the first instar. Riboflavin decays rapidly, leaving the mosquitos with no source of B2 vitamin and causing a deficiency. The gut microbiota, particularly E. coli continuously synthesizes Riboflavin, combating that deficiency.

This article was published by the Proceedings of the National Academy of Sciences of The United States of America on April 7, 2021.

Summary by Jannah Zinker