Genetic diversity and population structure of parasite infrapopulations within and across hosts for two trophically transmitted trematode parasites.

Complex parasite life cycles frequently require trophic transfer of parasites from an intermediate host prey to a definitive host predator. This results in aggregated distributions of parasites in predator host populations, which are subsequently expected to host more genetically diverse parasite infrapopulations than lower trophic level hosts. Host dispersal and seasonal population dynamics, particularly in the case of first-intermediate hosts, are also expected to drive population genetic patterns within and across populations. To examine how parasite life history and host ecology influence parasite genetic patterns, we characterized the genetic diversity of within-host infrapopulations, as well as overall population genetic structure, of sympatric tongueworm () and lungworm () freshwater trematode parasite populations. Parasites were collected across three host stages (snail, odonate insect, and frog) and sequenced at the cytochrome oxidase I (COI) mitochondrial region (519 bp for lungworms; 526 bp for tongueworms) to characterize genetic variation within and across hosts. Infection abundance per host and genetic diversity of within-host parasite infrapopulations generally increased with host trophic level, as expected. Additionally, tongueworm assemblages in odonate hosts were essentially equally as genetically diverse (depending on the index used) as those in definitive host frogs; tongueworms have an additional trophic transfer in their life cycle before the odonate stage, which highlights how trophic transmission and multi-host life cycle structure can benefit parasites by increasing genetic diversity of sexually reproducing adult assemblages. We also found that tongueworm populations, which infect a long-lived snail as a first-intermediate host, had higher population genetic diversity than lungworms, which infect a much shorter-lived snail with highly unstable population dynamics. Thus, we expect that first-intermediate host dynamics and dispersal ability played a large role in predicting population-level parasite genetic diversity and genetic structure in this system. This study investigates the effects of small- and large-scale processes on parasite genetic population structure and diversity and provides critical genetic data for future studies on these genera.