Larval lipid synthesis
As larvae, parasitoids develop inside or on other arthropods, but their adult stage is free-living. The loss of lipid synthesis evolved concurrently with the parasitic lifestyle in adult parasitic beetles, flies and wasps (Visser & Ellers 2008; Visser et al 2010) and it was hypothesized that the loss of adult lipid synthesis resulted from selection against larval lipid synthesis. As lipid synthesis is costly, parasitoid larvae could avoid synthesizing their own lipids by taking them over directly from their host. In collaboration with Hans Alborn, stable isotope analyses and GC-MS techniques were used to test if larvae of several insect species were capable of synthesizing lipids. Our results revealed that there is concurrence in the ability for lipid synthesis between insect life stages, i.e. if the adults lack lipid synthesis, so do the larvae and if the adults synthesize lipids, the larvae can do so as well (Visser et al 2017).
Collaborators: Denis Willet, Hans Alborn
Maternal effects on offspring nutrient dynamics
In collaboration with the University of Lyon, Doriane Muller (Ingénieur d’Étude) and Nicolas Lefrique (M1 student) have worked on a project focusing on nutrient dynamics during embryonic and larval development of the parasitoid Eupelmus vuilleti. During this project, we showed that older mothers have less resources available for investment in their offspring and that egg provisioning is reduced when mothers age. Reduced provisioning negatively affects offspring nutrient dynamics during development and leads to adult female offspring that have fewer resources at emergence. We revealed that maternal age can affect the quantity of resources available for their offspring, which in turn could affect reproduction and survival of daughters (Muller et al 2017).
Collaborators: Doriane Muller, Nicolas Lefrique, Benjamin Rey, Emmanuel Desouhant, Jérôme Casas & David Giron
Resource competition in a hyperparasitoid
An important question in evolutionary ecology is how the environment shapes reproductive investment. Random mating within a population will, over time, lead to an equal investment of resources into males and females. Mating opportunities are, however, not always random, for instance in highly structured populations where dispersal is limited. Such conditions can lead to competition for resources that may severely distort sex ratios, and which can further limit benefits associated with discriminative behaviors. This project aimed at investigating if and how resource competition, in terms of mates and hosts, affects sex ratios, discriminative behaviors and competitive interactions of the solitary wingless hyperparasitoid Gelis acarorum that exploits patchily distributed hosts (Visser et al 2014).
Stress physiology and performance
Within their lifetime organisms are regularly exposed to a variety of stresses that are expected to be detrimental to performance. Stress does not necessarily lead to reduced performance, however, because in some cases a mild stress can induce acclimation responses that mitigate costs on performance following subsequent exposures. Such physiological acclimation, or hormesis, might be quite common, particularly in species that are repeatedly exposed to predictable stressors. Using soil pupating Caribbean fruit flies that are regularly exposed to oxygen limitation due to rainfall, this study showed that repeated anoxia exposure during early development protects against the negative consequences of this stressor later in life, both in terms of physiology and performance (Visser et al 2018).
Over the last 15 to 20 years, several scientists independently uncovered that parasitoids showed an atypical metabolic response to feeding, in which these insects fail to increase their lipid reserves after sugar-feeding. Unlike predatory or herbivorous insects, parasitoids develop on or inside a single host insect, ultimately killing their host upon entering their free-living adult life-stage. Lacking lipid synthesis is a remarkable deviation from typical nutrient metabolism, because core nutrient metabolic pathways are highly conserved and lipid synthesis is essential for key traits, such as growth, survival, and reproduction. Based on these earlier findings we hypothesized that evolution of the parasitoid lifestyle preceded or coincided the evolution of lack of lipogenesis, in other words, only parasitoids have lost lipid synthesis.
Using a comparative approach, we tested the hypothesis of concurrent evolution between the parasitoid lifestyle and loss of lipogenesis and showed indeed that lipogenesis was lost at least three times in parallel in parasitoid flies, beetles and wasps. We further found that some parasitoids had reverted their strategy to active lipogenesis. Although the vast majority of parasitoids lack lipid synthesis, active lipid synthesis predominates in generalist parasitoids that adopt a wide host range. Generalists are more likely to require active lipid synthesis, because host resource availability is highly variable and manipulation of host physiology unlikely (Visser & Ellers 2008; Visser et al 2010).
Following the key finding that lipogenesis was lost in the majority of parasitoid lineages, the question remained which mechanisms underlie regressed lipid synthesis. Sugar feeding typically leads to up regulation of a suite of genes involved in carbohydrate and lipid metabolism, such as the prime gene necessary for lipid synthesis, fatty acid synthase. Contrary to findings in other animals, the transcriptional response to feeding of the parasitoid Nasonia vitripennis did not reveal up regulation of fatty acid synthase. Furthermore, numerous genes involved in sugar metabolism were down regulated rather than up regulated (Visser et al 2012).
Little is known about the efficiency with which specific nutrient classes, such as lipids and essential fatty acids, are transferred to higher trophic levels. By studying the lipid dynamics of species at different trophic positions within the community of parasitoids associated with the gall wasp Diplolepis rosae, fatty acid composition was shown to remain stable across the trophic cascade. Only the fatty acid composition of the parasitoid Orthopelma mediator differed, suggesting this species is able to modify its fatty acid composition after consumption of host lipids despite lacking lipid synthesis (Visser et al 2013).
Regression of an essential metabolic trait, such as lipid synthesis, is bound to have striking effects on important life history traits. During my PhD research I further investigated how calorie-rich diets, in terms of carbohydrates (Ellers et al 2011) as well as lipids (Visser & Ellers 2012), affect life histories, revealing that parasitoids do not seem to benefit from a high-calorie diet.
Promotor: Jacintha Ellers
Compensated trait loss
An interesting aspect of lost lipid synthesis is that this phenotypic function is still required by the parasitoid. So unlike the loss of eyes in cave-dwelling organisms where vision is unnecessary, parasitoids still require lipids to survive and reproduce. However, lipogenesis is not performed by the parasitoid itself, but by the host that it exploits during development. The conceptual differentiation between these types of trait loss fuelled the development of a novel framework addressing lost traits that are compensated for by an interacting partner. Compensated trait loss is much more common than currently appreciated. Many distinct taxa, including bacteria, plants and humans, exhibit compensated trait loss in one or both interacting partners through mutualistic or parasitic interactions, such as the loss of photosynthesis in parasitic plants or the loss of vitamin c synthesis in frugivorous primates and other mammals. Of critical importance is that compensated trait loss severely tightens the dependence of interacting species; hence compensated trait loss may direct symbiotic interactions by preventing partners from splitting up (Ellers et al 2012).
Collaborators: Jacintha Ellers, Toby Kiers