Bertanne Visser, Cécile Le Lann, Caroline M. Nieberding, Mark Lammers, Daniel A. Hahn, Hans T. Alborn, Thomas Enriquez, Mathilde Scheifler, Jeffrey A. Harvey, Jacintha Ellers

Fat reserves, specifically the accumulation of triacylglycerols, are a major energy source and play a key role for life histories. Fat accumulation is a conserved metabolic pattern across most insects, yet in most parasitoid species adults do not gain fat mass, even when nutrients are readily available and provided ad libitum. This extraordinary physiological phenotype has evolved repeatedly in phylogenetically dispersed parasitoid species. This poses a conundrum because it could lead to significant constraints on energy allocation toward key adult functions such as survival and reproduction. Recent work on the underlying genetic and biochemical mechanisms has spurred a debate on fat accumulation versus fat production, because of incongruent interpretation of results obtained using different methodologies. This debate is in part due to semantics, highlighting the need for a synthetic perspective on fat accumulation that reconciles previous debates and provides new insights and terminology. In this paper, we first clarify the distinct metabolic pathways involved in the complex process of lipogenesis and propose updated, unambiguous terminology for future research in the field, including “fatty acid synthesis” and “lack of adult fat accumulation”. Second, we discuss the benefits and drawbacks of the main methods available to measure fatty acid synthesis and adult fat accumulation. Most importantly, gravimetric/colorimetric and isotope tracking methods provide complementary information, provided that they are applied with appropriate controls and interpreted correctly. Third, we compiled a comprehensive list of fat accumulation studies performed during the last 25 years. Last, we present avenues for future research that combine chemistry, ecology, and evolution into an integrative approach, which we think is needed to understand the dynamics of fat accumulation in parasitoids.