Extrinsic and Intrinsic Predictors of Variation in Infection by Posthodiplostomum minimum MacCallum, 1921 (Trematoda) in Sunfishes ( Lepomis Rafinesque, 1819) from Eastern Ohio.

The trematode Posthodiplostomum minimum MacCallum, 1921 (white grub) is a common parasite of centrarchid fishes, frequently reaching high prevalence and intensity in local populations. However, significant variation in infection has been observed across Lepomis Rafinesque, 1819 sunfish species, which are common and abundant hosts for this parasite. Previous observations suggest both extrinsic factors (e.g., habitat-specific characteristics and environmental parameters) and intrinsic factors (e.g., host size, behavior, and susceptibility) as important predictors of infection in this parasite-host system. In the present study, we evaluated the prevalence and intensity of P. minimum in 6 sympatric species of Lepomis sunfish (total of 563 individuals) across 9 lakes in eastern Ohio, U.S.A., that range in surface area from 5.6 to 1,448.7 hectares, and assessed the importance of both extrinsic and intrinsic factors as predictors of infection. We found that collection site (extrinsic factor) and host body mass and species identity (intrinsic factors) were the strongest predictors of infection intensity. Specifically, infection intensity was negatively associated with lake surface area and positively associated host body mass. Lepomis macrochirus (bluegill sunfish) and hybrid sunfish displayed the highest infection intensities, whereas Lepomis cyanellus (green sunfish) and Lepomis gulosus (warmouth sunfish) were significantly less burdened. We were unable to conclude if the observed variation among host species was due to host ecology or susceptibility. These general findings were supported by classification and regression tree (CART) analysis, which optimally partitioned variation in individual host infection intensities by using lake size and host body mass (but not host species identity). Although infection intensity was negatively associated with lake surface area (even among host individuals of the same size and species), the causal mechanisms involved remain unresolved and should be the impetus of future work on this parasite-host system.

Habitat-based constraints on food web structure and parasite life cycles.

Habitat is frequently implicated as a powerful determinant of community structure and species distributions, but few studies explicitly evaluate the relationship between habitat-based patterns of species' distributions and the presence or absence of trophic interactions. The complex (multi-host) life cycles of parasites are directly affected by these factors, but almost no data exist on the role of habitat in constraining parasite-host interactions at the community level. In this study the relationship(s) between species abundances, distributions and trophic interactions (including parasitism) were evaluated in the context of habitat structure (classic geomorphic designations of pools, riffles and runs) in a riverine community (Raritan River, Hunterdon County, NJ, USA). We report 121 taxa collected over a 2-year period, and compare the observed food web patterns to null model expectations. The results show that top predators are constrained to particular habitat types, and that species' distributions are biased towards pool habitats. However, our null model (which incorporates cascade model assumptions) accurately predicts the observed patterns of trophic interactions. Thus, habitat strongly dictates species distributions, and patterns of trophic interactions arise as a consequence of these distributions. Additionally, we find that hosts utilized in parasite life cycles are more overlapping in their distributions, and this pattern is more pronounced among those involved in trophic transmission. We conclude that habitat structure may be a strong predictor of parasite transmission routes, particularly within communities that occupy heterogeneous habitats.

Current opinions: Zeros in host-parasite food webs: Are they real?

As the data have poured in, and the number of published food webs containing parasites has increased, questions have been raised as to why free-living species consistently outnumber parasites, even though most general reviews on the subject of host:parasite species richness suggest the contrary. Here, I describe this pattern as it exists in the literature, posit both real and artifactual sources of these findings, and suggest ways that we might interpret existing parasite-inclusive food webs. In large part, the reporting of free-living species devoid of any associated parasites (termed here in the coding of food web matrices as "zeros") is a consequence of either sampling issues or the intent of the study. However, there are also several powerful explanatory features that validate real cases of this phenomenon. Some hosts appear to authentically lack parasitism in portions of their geographic ranges, and parasites are often lost from systems that are either in early phases of community re-colonization or are compromised by environmental perturbation. Additionally, multi-stage parasite life cycles and broad host spectra allow some parasite species to partially saturate systems without providing a corresponding increase in parasite species richness, leading to low parasite species richness values relative to the free-living community. On the whole, the existing published food webs are sufficient to, at least in principle, determine basic patterns and pathways associated with parasite establishment and persistence in free-living communities because (1) for the purpose of those features, species rarity is roughly analogous to absence and (2) the existing data seem to suggest that the addition of more parasite taxa would reinforce the patterns already observed. This is particularly true for helminth parasites, in which our understanding and the resolution of our work is most robust.

Host quality and spatial patterning in infections of the Eastern mudsnail (Ilyanassa obsoleta) by two trematodes (Himasthla quissetensis and Zoogonus rubellus).

Several studies have suggested that the fitness of a parasite can be directly impacted by the quality of its host. In such cases, selective pressures could act to funnel parasites towards the highest-quality hosts in a population. The results of this study demonstrate that snail host quality is strongly correlated with spatial patterning in trematode infections and that habitat type is the underlying driver for both of these variables. Two trematodes (Himasthla quissetensis and Zoogonus rubellus) with very different life cycles assume the same spatial infection pattern in populations of the first intermediate host (Ilyanassa obsoleta) in coastal marsh habitats. Infected snails are disproportionately recovered from intertidal panne habitats, which offer more hospitable environs for snails than do adjacent habitats (intertidal creeks, coastal flats, and subtidal creeks), in terms of protection from turbulence and wave action, as well as the availability of food stuffs. Snails in intertidal panne habitats are of higher quality when assessed in terms of average size-specific mass, growth rate, and fecundity. In mark-recapture experiments, snails frequently dispersed into intertidal pannes but were never observed leaving them. In addition, field experiments demonstrate that snails confined to intertidal panne habitats are disproportionately infected by both trematode species, relative to conspecifics confined to adjacent habitats. Laboratory experiments show that infected snails suffer significant energetic losses and consume more than uninfected conspecifics, suggesting that infected snails in intertidal pannes may survive better than in adjacent habitats. We speculate that 1 possible mechanism for the observed patterns is that the life cycles of both trematode species allows them to contact the highest-quality snails in this marsh ecosystem.

Rapid evolution by positive selection and gene gain and loss: PLA(2) venom genes in closely related Sistrurus rattlesnakes with divergent diets.

Rapid evolution of snake venom genes by positive selection has been reported previously but key features of this process such as the targets of selection, rates of gene turnover, and functional diversity of toxins generated remain unclear. This is especially true for closely related species with divergent diets. We describe the evolution of PLA(2) gene sequences isolated from genomic DNA from four taxa of Sistrurus rattlesnakes which feed on different prey. We identified four to seven distinct PLA(2 )sequences in each taxon and phylogenetic analyses suggest that these sequences represent a rapidly evolving gene family consisting of both paralogous and homologous loci with high rates of gene gain and loss. Strong positive selection was implicated as a driving force in the evolution of these protein coding sequences. Exons coding for amino acids that make up mature proteins have levels of variation two to three times greater than those of the surrounding noncoding intronic sequences. Maximum likelihood models of coding sequence evolution reveal that a high proportion (approximately 30%) of all codons in the mature protein fall into a class of codons with an estimated d(N)/d(S) (omega) ratio of at least 2.8. An analysis of selection on individual codons identified nine residues as being under strong (p < 0.01) positive selection, with a disproportionately high proportion of these residues found in two functional regions of the PLA(2) protein (surface residues and putative anticoagulant region). This is direct evidence that diversifying selection has led to high levels of functional diversity due to structural differences in proteins among these snakes. Overall, our results demonstrate that both gene gain and loss and protein sequence evolution via positive selection are important evolutionary forces driving adaptive divergence in venom proteins in closely related species of venomous snakes.