Variation in Size and Progression through Larval Stages of Drosophila melanogaster Fruit Flies in the Absence of Dietary Folic Acid

Folic acid is involved in DNA synthesis and methylation, and other key reactions. Animals, including humans and fruit flies, cannot synthesize the vitamin and so require outside sources of it for proper development. Microbial symbionts alone in fruit flies can provide enough active forms of folic acid (folates) for survival on diets lacking folate. However, larvae who use folates only from microbes are less likely to survive to the pupal stage, compared to larvae fed the vitamin. In addition, the amount of folates in the tissues of these larvae are the same but vary more than larvae consuming folates. We hypothesized that this overall decrease in survival is because only some larvae are able to survive folate‐free diets well, because of individual variation in larval microbiomes that result in larvae receiving different amounts of folate. An alternative hypothesis is that most larvae survive folate‐free diets well, until the end of the larval stage when most “run out of” microbially‐derived folates. To help distinguish between these two possibilities, we aimed to see if the rate that larvae develop in media without folate varies widely or whether larvae fare similarly in this environment. Eggs were transferred to holidic media made without folic acid. Larvae were observed for 1.5 weeks and photographed under a stereomicroscope. Two‐dimensional larval size was calculated and larval instar stage was determined. The preliminary data show that both of these varied significantly in the absence of dietary folic acid. Age at which an instar stage was reached was wide‐ranging. The size of a larva for its given age and larval instar also varied. By day 10, about one‐fourth of the larvae had reached the third instar, however more than one‐third of the larvae were still first instars. The data support the primary hypothesis that some larvae cannot develop normally without dietary folic acid, while others appear to. Future studies will determine if this is due to differences in microbial flora, causing different amounts of folate to be available to the larvae. Such studies examining how individual differences in microbiomes affect the host in model organisms such as Drosophila can inform the understanding of important host‐microbiome interactions in humans and other animals.