Evolutionary loss of traits can result from negative selection on a specific phenotype, or if the trait is selectively neutral, because the phenotype associated with the trait has become redundant. Even essential traits may be lost, however, if the resulting phenotypic deficiencies can be compensated for by the environment or a symbiotic partner. Here we demonstrate that loss of an essential me-tabolic trait in parasitic wasps has evolved through environmental compensation. We tested 24 species for the ability to synthesize lipids de novo and collected additional data from the literature. We found the majority of adult parasitoid species to be incapable of synthesizing lipids, and phylogenetic analyses showed that the evolution of lack of lipogenesis is concurrent with that of parasitism in insects. Exploitive host manipulation, in which the host is forced to synthesize lipids to the benefit of the parasitoid, presumably facilitates loss of lipogenesis through environmental compensation. Lipogenesis re-evolved in a small number of parasitoid species, particularly host generalists. The wide range of host species in which generalists are able to develop may impede effective host manipulation and could have resulted in regaining of lipogenic ability in generalist parasitoids. As trait loss through environmental compensation is unnoticed at the phenotypic level, it may be more common than currently anticipated, especially in species involved in intricate symbiotic relationships with other species.

The diet of organisms generally provides a sufficient supply of energy and building materials for healthy growth and development, but should also contain essential nutrients. Species differ in their exogenous requirements, but it is not clear why some species are able to synthesize essential nutrients, while others are not. The unsaturated fatty acid, linoleic acid (LA; 18:2n-6) plays an important role in functions such as cell physiology, immunity, and reproduction, and is an essential nutrient in diverse organisms. LA is readily synthesized in bacteria, protozoa and plants, but it was long thought that all animals lacked the ability to synthesize LA de novo and thus required a dietary source of this fatty acid. Over the years, however, an increasing number of studies have shown active LA synthesis in animals, including insects, nematodes and pulmonates. Despite continued interest in LA metabolism, it has remained unclear why some organisms can synthesize LA while others cannot. Here, we review the mechanisms by which LA is synthesized and which biological functions LA supports in different organisms to answer the question why LA synthesis was lost and repeatedly gained during the evolution of distinct invertebrate groups. We propose several hypotheses and compile data from the available literature to identify which factors promote LA synthesis within a phylogenetic framework. We have not found a clear link between our proposed hypotheses and LA synthesis; therefore we suggest that LA synthesis may be facilitated through bifunctionality of desaturase enzymes or evolved through a combination of different selective pressures.

The Earth's climate is changing at a rapid pace. To survive in increasingly fluctuating and unpredictable environments, species can either migrate or evolve through rapid local adaptation, plasticity and/or bet-hedging. For small ectotherm insects, like parasitoids and their hosts, phenotypic plasticity and bet-hedging could be critical strategies for population and species persistence in response to immediate, intense and unpredictable temperature changes. Here, we focus on studies evaluating phenotypic responses to variable predictable thermal conditions (for which phenotypic plasticity is favoured) and unpredictable thermal environments (for which bet-hedging is favoured), both within and between host and parasitoid generations. We then address the effects of fluctuating temperatures on host–parasitoid interactions, potential cascading effects on the food web, as well as biological control services. We conclude our review by proposing a road map for designing experiments to assess if plasticity and bet-hedging can be adaptive strategies, and to disentangle how fluctuating temperatures can affect the evolution of these two strategies in parasitoids and their hosts.