Coevolution and compatibility in the snail–schistosome system

Abstract

In stark contrast to the huge body of theoretical work on the importance of hosts and parasites as selective agents acting on each other, until recently, little systematic empirical investigation of this issue has been attempted. Research on snail-schistosome interactions have, therefore, the potential for making an important contribution to the study of coevolution or reciprocal adaptation. This may be particularly pertinent since snail-schistosomes represent an indirectly transmitted macroparasite system, so often overlooked amongst both theoretical and empirical studies. Here we review ideas and experiments on snail-schistosome interactions, with particular emphasis on those that may have relevance to the potential coevolution between host resistance and parasite infectivity and virulence. We commence with an introduction and definition of the general concepts, before going into detail of some specific studies to illustrate these: evidence of snail-schistosome coevolutionary process in the field; evidence of coevolutionary processes in the laboratory; a general assessment of the applicability of coevolutionary models in snail-schistosome interactions; and finishing with a section on conclusions and areas for further study.

Detection of Schistosoma mansoni and Schistosoma haematobium DNA by Loop-Mediated Isothermal Amplification: Identification of Infected Snails from Early Prepatency

“…Given that major differences can exist in snail-schistosome compatibility in different areas, 39 snail prepatency rates (as determined by snail molecular monitoring) cannot be expected to serve as a direct indicator of snail-to-human transmission potential. However, changes in snail prepatency rates over time after an intervention can be expected to reflect changes in the force of transmission from humans to snails in the study area.…”

Is host-schistosome coevolution going anywhere?

“…Transmission between hosts occurs via free-swimming larval stages, miracidia (infective to the mollusc) and cercariae (infective to the mammal). Prevalence and transmission of schistosome infections in natural populations is highly variable across space and time [16,17]. Although the precise genetic or molecular mechanisms have not yet been identified, snail resistance and susceptibility, and schistosome virulence and infectivity, have each been demonstrated to have heritable, strain-specific bases [16,18], and variability for each trait maintained through a range of cost-benefit trade-offs [16,19].…”

“…Finally, the current study also incorporated cross-infections between selected parasite lines with novel host lines at each generation, aimed to disentangle any potential change in parasite characteristics independent of host factors and/or to detect the strain-specificity of any response [16]. For the former, one may predict any phenotype, such as a change in infection rates in co-selected snails over time, if displayed also in novel snails exposed to the same parasite, would indicate a predominantly parasite-focused trait change.…”

Infestation of Lymnaea stagnalis by digenean flukes in the Jeziorak Lake

“…The phenomenon known as fecundity compensation may counteract inhibition of snail reproduction caused by parasites. It results in a faster maturation of infested individuals and their intensive reproduction during the prepatent period of parasite development Webster and Davies 2001).…”