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1. Speciation in kleptoparasites of oak gall wasps often correlates with shifts into new tree habitats, tree organs, or gall morphospace

   https://doi.org/10.1093/evolut/qpad202  

   Ward, Anna_K G; Zhang, Y Miles; Brown, Guerin E; Hippee, Alaine C; Prior, Kirsten M; Rollins, Shannon; Sierra, Nicolas; Sheikh, Sofia I; Tribull, Carly M; Forbes, Andrew A (November 2023, Evolution)

   Taylor, Scott; Zelditch, Miriam (Ed.)

   Abstract Host shifts to new plant species can drive speciation for plant-feeding insects, but how commonly do host shifts also drive diversification for the parasites of those same insects? Oak gall wasps induce galls on oak trees and shifts to novel tree hosts and new tree organs have been implicated as drivers of oak gall wasp speciation. Gall wasps are themselves attacked by many insect parasites, which must find their hosts on the correct tree species and organ, but also must navigate the morphologically variable galls with which they interact. Thus, we ask whether host shifts to new trees, organs, or gall morphologies correlate with gall parasite diversification. We delimit species and infer phylogenies for two genera of gall kleptoparasites, Synergus and Ceroptres, reared from a variety of North American oak galls. We find that most species were reared from galls induced by just one gall wasp species, and no parasite species was reared from galls of more than four species. Most kleptoparasite divergence events correlate with shifts to non-ancestral galls. These shifts often involved changes in tree habitat, gall location, and gall morphology. Host shifts are thus implicated in driving diversification for both oak gall wasps and their kleptoparasitic associates. 

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2. A role for the local environment in driving species‐specific parasitism in a multi‐host parasite system

   https://doi.org/10.1111/fwb.13961  

   Hasik, Adam Z.; Siepielski, Adam M. (June 2022, Freshwater Biology)

   Abstract The extent and magnitude of parasitism often vary among closely related host species and across populations within species. Determining the ecological basis for this species and population‐level variation in parasitism is critical for understanding infection dynamics in multi‐host–parasite systems. To investigate such ecological underpinnings of variation in parasitism, we studiedEnallagmadamselfly host species and their water mite (Arrenurusspp.) ectoparasites in lakes.We first evaluated how host identity and density could shape parasitism. To test the effects of con‐ and heterospecific host density on parasitism, we used a field experiment withEnallagma basidensandE. signatum. We found that parasitism did not vary with con‐ or heterospecific density and was determined by host identity alone, with no spillover effects.We also evaluated the potential role of local adaptation and resource availability in shaping parasitism. To do so, we usedE. signatumin a reciprocal transplant experiment crossed with a prey resource‐level manipulation. This experiment revealed that parasitism declined sharply for one host population in its non‐local lake, but not the other source population, with no effects of prey levels. This asymmetry implies that damselflies express enhanced defences against parasitism that are neither population‐specific nor dependent on resource abundance, or that mites developed heightened local host specificity.The results of multivariate modeling from an observational study generally supported these experimental findings: neither host density nor resource abundance strongly explained among‐population variation in parasitism. Instead, local abiotic conditions (pH) had the strongest relationship with parasitism, with minimal associations with predator density, temperature and a measure of immune function.Collectively, our findings suggest a crucial role for the local environment in shaping host–parasite interactions within multi‐host–parasite systems. More generally, these results show that research at the intersection of community ecology and disease ecology is critical for understanding host–parasite dynamics within natural communities. 

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3. Long-distance dispersal of pigeons and doves generated new ecological opportunities for host-switching and adaptive radiation by their parasites

   https://doi.org/10.1098/rspb.2022.0042  

   Boyd, Bret M.; Nguyen, Nam-Phuong; Allen, Julie M.; Waterhouse, Robert M.; Vo, Kyle B.; Sweet, Andrew D.; Clayton, Dale H.; Bush, Sarah E.; Shapiro, Michael D.; Johnson, Kevin P. (March 2022, Proceedings of the Royal Society B: Biological Sciences)

   Adaptive radiation is an important mechanism of organismal diversification and can be triggered by new ecological opportunities. Although poorly studied in this regard, parasites are an ideal group in which to study adaptive radiations because of their close associations with host species. Both experimental and comparative studies suggest that the ectoparasitic wing lice of pigeons and doves have adaptively radiated, leading to differences in body size and overall coloration. Here, we show that long-distance dispersal by dove hosts was central to parasite diversification because it provided new ecological opportunities for parasites to speciate after host-switching. We further show that among extant parasite lineages host-switching decreased over time, with cospeciation becoming the more dominant mode of parasite speciation. Taken together, our results suggest that host dispersal, followed by host-switching, provided novel ecological opportunities that facilitated adaptive radiation by parasites. 

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4. Parasite spatial distribution shapes parasite aggregation on host populations, but not at high parasite density

   https://doi.org/10.1002/ecs2.70324  

   Wojan, Chris; Shaw, Allison_K; Craft, Meggan_E (June 2025, Ecosphere)

   Abstract Spatial aggregation of environmental or trophically transmitted parasites has the potential to influence host–parasite interactions. The distribution of parasites on hosts is one result of those interactions, and the role of spatial aggregation is unclear. We use a spatially explicit agent‐based model to determine how spatial aggregation of parasites influences the distribution of parasite burdens across a range of parasite densities and host recovery rates. Our model simulates the random movement of hosts across landscapes with varying spatial configurations of areas occupied by environmental parasites, allowing hosts to acquire parasites they encounter and subsequently lose them. When parasites are more spatially aggregated in the environment, the aggregation of parasite burdens on hosts is higher (i.e., more hosts with few parasites, fewer hosts with many parasites), but the effect is less pronounced at high parasite density and fast host recovery rates. In addition, the correlation between individual hosts' final parasite burdens and their cumulative parasite burdens (including lost parasites) is greater at higher levels of spatial parasite aggregation. Our work suggests that fine‐scale spatial patterns of parasites can play a strong role in shaping how hosts are parasitized, particularly when parasite density is low‐to‐moderate and recovery rates are slow. 

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5. Into the danger zone: How the within‐host distribution of parasites controls virulence

   https://doi.org/10.1111/ele.14352  

   Johnson, Pieter T. J.; Stewart Merrill, Tara; Calhoun, Dana M.; McDevitt‐Galles, Travis; Hobart, Brendan (December 2023, Ecology Letters)

   Abstract Despite the importance of virulence in epidemiological theory, the relative contributions of host and parasite to virulence outcomes remain poorly understood. Here, we use reciprocal cross experiments to disentangle the influence of host and parasite on core virulence components—infection and pathology—and understand dramatic differences in parasite‐induced malformations in California amphibians. Surveys across 319 populations revealed that amphibians' malformation risk was 2.7× greater in low‐elevation ponds, even while controlling for trematode infection load. Factorial experiments revealed that parasites from low‐elevation sites induced higher per‐parasite pathology (reduced host survival and growth), whereas there were no effects of host source on resistance or tolerance. Parasite populations also exhibited marked differences in within‐host distribution: ~90% of low‐elevation cysts aggregated around the hind limbs, relative to <60% from high‐elevation. This offers a novel, mechanistic basis for regional variation in parasite‐induced malformations while promoting a framework for partitioning host and parasite contributions to virulence. 

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