  Inferring parameters related to the aggregation pattern of parasites and to
their dispersal propensity is important for predicting their ecological
consequences and evolutionary potential. Nonetheless, it is notoriously
difficult to infer these parameters from wildlife parasites given the
difficulty in tracking these organisms. Molecular-based inferences constitute a
promising approach that has yet rarely been applied in the wild.Here, we
combinedseveral population genetic analyses including sibship reconstruction to
documentthe genetic structure, patterns of sibship aggregation and the
dispersal dynamics of a non-native parasite of fish, the freshwater copepod
ectoparasiteTracheliastespolycolpus. We collected parasites according to a
hierarchical sampling design,withthe sampling of all parasites from all host
individualscapturedineight sites spread along an upstream-downstream river
gradient. Individual multilocus genotypes were obtained from 14 microsatellite
markers, and used to assign parasites to full-sib families and to investigate
the genetic structure of T.polycolpus among both hosts and sampling sites. The
distribution of full-sibs obtainedamong the sampling sites was used to estimate
individual dispersal distances within families. Our results showed that T.
polycolpus sibs tend to be aggregated within sites but not withinhost
individuals. We detected important upstream-to-downstream dispersal events of
T.polycolpusbetween sites (modal distance: 25.4 km; 95% CI [22.9, 27.7]),
becoming scarcer as the geographic distance from their family core location
increases. Such a dispersal pattern likely contributes to the strong
isolation-by-distance observed at the river scale. We also detected some
downstream-to-upstream dispersal events (modal distance: 2.6 km; 95% CI
[2.2-23.3]) that likely result from movementsof infected hosts.Within each
site, the dispersal of free-living infective larvae among hosts likely
contributes to increasing genetic diversity on hosts, possibly fostering the
evolutionary potential of T. polycolpus.
