Small-scale spatial resource partitioning in a hyperparasitoid community

Plant-herbivore-natural enemy associations underpin ecological communities, and such interactions may go up to four (or even more) trophic levels. Here, over the course of a growing season, we compared the diversity of secondary hyperparasitoids associated with a common host, Cotesia glomerata, a specialized larval endoparasitoid of cabbage butterfly caterpillars that in turn feed on brassicaceous plants. Cocoon clusters of C. glomerata were pinned to ~30 Brassica nigra plants by pinning them either to branches in the canopy (~1.5 m high) or to the base of the stem near the ground. The cocoons were collected a week later and reared to determine which hyperparasitoid species emerged from them. This was done in four consecutive months (June–September). Cocoons placed in the canopy were primarily attacked by specialized winged hyperparasitoids (Lysibia nana, Acrolyta nens), whereas cocoons on the ground were attacked by both winged and generalist wingless hyperparasitoids (Gelis acarorum, G. agilis), although this changed with season. There was much more temporal variation in the diversity and number of species attacking cocoons in the canopy than on the ground; the abundance of L. nana and A. nens varied from month to month, whereas P. semotus was only prevalent in August. By contrast, G. acarorum was abundant in all of the samples placed near the ground. Our results show that hyperparasitoids partition host resources at remarkably small vertical spatial scales. We argue that spatial differences in the distribution of natural enemies can contribute to the diversity patterns observed in the field.     

Multi level ecological fitting: indirect life cycles are not a barrier to host switching and invasion

Many invasive species are able to escape from coevolved enemies and thus enjoy a competitive advantage over native species. However, during the invasion phase, non‐native species must overcome many ecological and/or physiological hurdles before they become established and spread in their new habitats. This may explain why most introduced species either fail to establish or remain as rare interstitials in their new ranges. Studies focusing on invasive species have been based on plants or animals where establishment requires the possession of preadapted traits from their native ranges that enables them to establish and spread in their new habitats. The possession of preadapted traits that facilitate the exploitation of novel resources or to colonize novel habitats is known as ‘ecological fitting’. Some species have evolved traits and life histories that reflect highly intimate associations with very specific types of habitats or niches. For these species, their phenological windows are narrow, and thus the ability to colonize non‐native habitats requires that a number of conditions need to be met in accordance with their more specialized life histories. Some of the strongest examples of more complex ecological fitting involve invasive parasites that require different animal hosts to complete their life cycles. For instance, the giant liver fluke, Fascioloides magna, is a major parasite of several species of ungulates in North America. The species exhibits a life cycle whereby newly hatched larvae must find suitable intermediate hosts (freshwater snails) and mature larvae, definitive hosts (ungulates). Intermediate and definitive host ranges of F. magna in its native range are low in number, yet this parasite has been successfully introduced into Europe where it has become a parasite of native European snails and deer. We discuss how the ability of these parasites to overcome multiple ecophysiological barriers represents an excellent example of ‘multiple‐level ecological fitting’.     

Convergent development of a parasitoid wasp on three host species with differing mass and growth potential

Koinobiont parasitoids develop in hosts that continue feeding and growing during the course of parasitism. Here, we compared development of a solitary koinobiont endoparasitoid, Meteorus pulchricornis Westmael (Hymenoptera: Braconidae), in second (L2) and fourth (L4) instars of three host species that are closely related (Lepidoptera: Noctuidae) but which exhibit large variation in growth potential. Two hosts, Mamestra brassicae L. and Spodoptera littoralis Boisduval, may reach 1 g or more when the caterpillars are fully mature, whereas Spodoptera exigua Hübner is much smaller with mature caterpillars rarely exceeding 200 mg. Parasitoid survival (to pupation) in the two host instars was much higher on the larger hosts than on S. exigua. However, other fitness correlates in M. pulchricornis were very similar in the three host species. Development time was fairly uniform in L2 and L4 hosts of the three host species, whereas wasps were larger in L4 than in L2 hosts. However, M. pulchricornis developmentally arrested each of the hosts differently. The mass of dying L2 and L4 hosts after parasitoid larval egression (i.e., when they emerge from the dying caterpillar) varied significantly, with S. littoralis being by far the largest and S. exigua the smallest. These results reveal that M. pulchricornis is able to adjust its own development in response to species‐specific differences in host resources.     

An Evolutionary Perspective on Linoleic Acid Synthesis in Animals

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.