Host species that are particularly abundant, infectious, and/or infected tend to contribute disproportionately to symbiont (parasite or mutualist) maintenance in multi-host systems. Therefore, in a facultative multi-host system where two host species had high densities, high symbiont infestation intensities, and high infestation prevalence, we expected interspecific transmission rates to be high. Instead, we found that interspecific symbiont transmission rates to caged sentinel hosts were an order of magnitude lower than intraspecific transmission rates in the wild. Using laboratory experiments to decompose transmission rates, we found that opportunities for interspecific transmission were frequent, where interspecific and intraspecific contact rate functions were statistically indistinguishable. But most interspecific contacts did not lead to transmission events due to a previously unrecognized transmission barrier: strong host preferences. During laboratory choice experiments, the symbiont preferred staying on or dispersing to its current host species, even though the oligochaete symbiont is a globally distributed host generalist that can survive and reproduce on many snail species. These surprising results suggest that when managing symbiont transmission, identifying key host species is still important, but it may be equally important to identify and manage transmission barriers that keep potential superspreader host species in check.

Though Chaetogaster limnaei has a broad host distribution, two snail genera, Helisoma and Physa, have been the focus for most prior experiments and are especially abundant in ponds in North America. Therefore, for all field and laboratory studies, we used H. trivolvis snails as the focal host species (i.e., the species in which we quantified infestation rates) and P. gyrina as the alternative host species. We especially focused on interspecific transmission from P. gyrina to H. trivolvis (alternative to focal host transmission; hereafter alternative–focal), which complements prior work that quantified intraspecific transmission among H. trivolvis snails (focal to focal host transmission; hereafter focal–focal). We used a sentinel host approach to quantify the rate that uninfested focal hosts (H. trivolvis) that had been raised in the laboratory became infested when caged in the field due to contacts with wild H. trivolvis and P. gyrina through the mesh on the field cages. We then decomposed the estimated interspecific transmission rate using two laboratory experiments that quantified interspecific contact rates and interspecific transmission success. The three R script files correspond to the analyses of the field data, interspecific contact rate experiment, and interspecific transmission success experiment.

This repository contains a detailed metadata file that describes each script and dataset.