Coevolutionary fine-tuning: evidence for genetic tracking between a specialist wasp parasitoid and its aphid host in a dual metapopulation interaction

Systematic Mapping and Review of Landscape Fire Smoke (LFS) Exposure Impacts on Insects

Landscape fire activity is changing in many regions because of climate change. Smoke emissions from landscape fires contain many harmful air pollutants, and beyond the potential hazard posed to human health, these also have ecological impacts. Insects play essential roles in most ecosystems worldwide, and some work suggests they may also be sensitive to smoke exposure. There is therefore a need for a comprehensive review of smoke impacts on insects. We systematically reviewed the scientific literature from 1930 to 2022 to synthesize the current state of knowledge of the impacts of smoke exposure from landscape fires on the development, behavior, and mortality of insects. We found: (1) 42 relevant studies that met our criteria, with 29% focused on the United States of America and 19% on Canada; (2) of these, 40 insect species were discussed, all of which were sensitive to smoke pollution; (3) most of the existing research focuses on how insect behavior responds to landscape fire smoke (LFS); (4) species react differently to smoke exposure, with for example some species being attracted to the smoke (e.g., some beetles) while others are repelled (e.g., some bees). This review consolidates the current state of knowledge on how smoke impacts insects and highlights areas that may need further investigation. This is particularly relevant since smoke impacts on insect communities will likely worsen in some areas due to increasing levels of biomass burning resulting from the joint pressures of climate change, land use change, and more intense land management involving fire.

Host Specificity in Subarctic Aphids

Plants and herbivorous (or parasitic) insects form the majority of macroscopic life. The specificity of interaction between host plant and parasitic insect depends on the adaptations of both the host and the parasite. Over time, these interactions evolve and change as a result of an 'arms race' between host and parasite, and the resulting species-specific adaptations may be maintained, perpetuating these interactions across speciation events. This can lead to specialisation between species or clades. With speciation and species sorting over time, complex interactions evolve. Here, we elucidate a three-tier method to test these interactions using the aphids (Hemiptera: Aphididae) and plants of Churchill (Manitoba, Canada) as a model system. We analyzed these interactions by testing for three patterns in host specificity: monophagy, phylogenetic clustering, and cophylogeny. We defined monophagy strictly as one species feeding exclusively upon a single host plant species (an association likely driven by arms races in morphology, chemical resistance/tolerance, and visual appearance) and observed this in 7 of 22 aphid species. In all the remaining 'polyphagous' cases, there was a strong trend toward monophagy (80% of individuals were found on a single host plant species). Second, we observed two separate examples of phylogenetic clustering where groups of closely related aphid species fed upon individual plant species. Finally, we found no support for cophylogenetic relationships where both aphids and plants cospeciate to form congruent phylogenetic trees (evidence of coadaptation through an ongoing arms race). One explanation for uncovering species-specific interactions in a recently deglaciated, subarctic locality is that the species involved in the associations moved north together. Testing different levels of specificity in the most predominant species-species interactions on the planet will allow us to elucidate these patterns accurately and gives us insight into where to direct future research.

Comparative genetics of invasive populations of walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, in California

Coevolution may be an important component of the sustainability of importation biological control, but how frequently introduced natural enemies coevolve with their target pests is unclear. Here we explore whether comparative population genetics of the invasive walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, provide insights into the localized breakdown of biological control services in walnut orchards in California. We found that sampled populations of C. juglandicola exhibited higher estimates of genetic differentiation (FST) than co-occurring populations of T. pallidus. In contrast, estimates of both the inbreeding coefficient (GIS) and contemporary gene flow were higher for T. pallidus than for C. juglandicola. We also found evidence of reciprocal outlier loci in some locations, but none showed significant signatures of selection. Synthesis and applications. Understanding the importance of coevolutionary interactions for the sustainability of biological control remains an important and understudied component of biological control research. Given the observed differences in gene flow and genetic differentiation among populations of T. pallidus and C. juglandicola, we suspect that temporary local disruption of biological control services may occur more frequently than expected while remaining stable at broader regional scales. Further research that combines genomewide single nucleotide polymorphism genotyping with measurements of phenotypic traits is needed to provide more conclusive evidence of whether the occurrence of outlier loci that display significant signatures of selection can be interpreted as evidence of the presence of a geographic mosaic of coevolution in this system.

Aphid specialism as an example of ecological-evolutionary divergence

Debate still continues around the definition of generalism and specialism in nature. To some, generalism is equated solely with polyphagy, but this cannot be readily divorced from other essential biological factors, such as morphology, behaviour, genetics, biochemistry, chemistry and ecology, including chemical ecology. Viewed in this light, and accepting that when living organisms evolve to fill new ecological-evolutionary niches, this is the primal act of specialisation, then perhaps all living organisms are specialist in the broadest sense. To illustrate the levels of specialisation that may be found in a group of animals, we here provide an overview of those displayed by a subfamily of hemipteran insects, the Aphididae, which comprises some 1600 species/subspecies in Europe alone and whose members are specialised in a variety of lifestyle traits. These include life cycle, host adaptation, dispersal and migration, associations with bacterial symbionts (in turn related to host adaptation and resistance to hymenopterous wasp parasitoids), mutualisms with ants, and resistance to insecticides. As with polyphagy, these traits cannot easily be separated from one another, but rather, are interconnected, often highly so, which makes the Aphididae a fascinating animal group to study, providing an informative, perhaps unique, model to illustrate the complexities of defining generalism versus specialism.

Spatial and temporal genetic structure at the fourth trophic level in a fragmented landscape

A fragmented habitat becomes increasingly fragmented for species at higher trophic levels, such as parasitoids. To persist, these species are expected to possess life-history traits, such as high dispersal, that facilitate their ability to use resources that become scarce in fragmented landscapes. If a specialized parasitoid disperses widely to take advantage of a sparse host, then the parasitoid population should have lower genetic structure than the host. We investigated the temporal and spatial genetic structure of a hyperparasitoid (fourth trophic level) in a fragmented landscape over 50 × 70 km, using microsatellite markers, and compared it with the known structures of its host parasitoid, and the butterfly host which lives as a classic metapopulation. We found that population genetic structure decreases with increasing trophic level. The hyperparasitoid has fewer genetic clusters (K = 4), than its host parasitoid (K = 15), which in turn is less structured than the host butterfly (K = 27). The genetic structure of the hyperparasitoid also shows temporal variation, with genetic differentiation increasing due to reduction of the population size, which reduces the effective population size. Overall, our study confirms the idea that specialized species must be dispersive to use a fragmented host resource, but that this adaptation has limits.

Diversity, population structure, and individual behaviour of parasitoids as seen using molecular markers

Parasitoids have long been models for host-parasite interactions, and are important in biological control. Neutral molecular markers have become increasingly accessible tools, revealing previously unknown parasitoid diversity. Thus, insect communities are now seen as more speciose. They have also been found to be more complex, based on trophic links detected using bits of parasitoid DNA in hosts, and host DNA in adult parasitoids. At the population level molecular markers are used to determine the influence of factors such as host dynamics on parasitoid population structure. Finally, at the individual level, they are used to identify movement of individuals. Overall molecular markers greatly increase the value of parasitoid samples collected, for both basic and applied research, at all levels of study.

The cryptic sexual strategies of human fungal pathogens

Sexual reproduction is a pervasive attribute of eukaryotic species and is now recognized to occur in many clinically important human fungal pathogens. These fungi use sexual or parasexual strategies for various purposes that can have an impact on pathogenesis, such as the formation of drug-resistant isolates, the generation of strains with increased virulence or the modulation of interactions with host cells. In this Review, we examine the mechanisms regulating fungal sex and the consequences of these programmes for human disease.