Why do many parasitoids lack adult triglyceride accumulation, despite functioning fatty acid biosynthesis machinery?

Abstract

Ruther et al (2021) evaluated fatty acid synthesis in several parasitic wasp species to test if the general finding that lipogenesis in parasitoids is lacking is upheld (Visser et al 2010 PNAS). As proposed by Visser & Ellers (2008), parasitoids can readily assimilate the triglyceride stores produced by their host. When large triglyceride stores are carried over from larval feeding into adulthood (i.e., up to 30 to 40% of the parasitoid’s dry body weight; Visser et al., 2018, 2021), de novo lipid synthesis from adult feeding is either unnecessary or too costly to maintain, leading to trait loss (Ellers et al., 2012). To test the hypothesis that many parasitoids do not synthesize substantial quantities of fat stores as adults, a previous study used feeding experiments on a wide taxonomic range of insects, including parasitoid wasps, parasitoid flies, a parasitoid beetle, and 65 non-parasitoid species (Visser et al., 2010 and references therein). What is striking is that when compared to non-parasitoid insects, 24 out of 29 evolutionarily distinct parasitoid lineages (Coleoptera, Diptera and Hymenoptera; Visser et al., 2010) did not accumulate significant lipid quantities in adulthood even when fed surplus carbohydrates. When little to no lipids are synthesized de novo by adult parasitoid wasps, this can lead to significant constraints on energy allocation toward key adult functions, such as maintenance, dispersal, and reproduction (Jervis et al., 2008). To our minds, the most important question is ‘why don’t parasitoids accumulate substantial quantities of fat as adults like other insects do, and what does this mean for their life histories?’

The Drosophila‐parasitizing wasp Leptopilina heterotoma: A comprehensive model system in ecology and evolution

“…Fat synthesis, accumulation and plasticity therein in L. heterotoma is currently the core theme of our own research and there are many exciting prospects for further research on this topic (Visser et al, 2022 ). For example, we still need to better understand how plasticity of fat synthesis and accumulation affects life histories and fitness (see the subsection below).…”

“…So far, explicit tests for plasticity of fat synthesis and accumulation have only been done with L. heterotoma (using genetically similar individuals). To determine if plasticity of fat synthesis and accumulation evolved also in other parasitoids, many more parasitoid species now need to be tested (Visser et al, 2022 ). We can also now start digging into the genomics and transcriptomics of plastic fat synthesis in L. heterotoma to understand the mechanisms at play in generating distinct fat accumulation phenotypes.…”