Genetic signatures in an invasive parasite of Anguilla anguilla correlate with differential stock management

Abstract

In this article, it is shown that available genetic tools for the omnipresent parasite Anguillicoloides crassus in European eels Anguilla anguilla are sensitive to different immigration rates into local A. anguilla stocks for two separated river systems. Relying on four highly polymorphic microsatellite markers, it was inferred that under natural recruitment, nematode samples meet Hardy-Weinberg expectations for a single panmictic population, while genetic signals show signs for a strong Wahlund effect most likely due to very recent population mixing under frequent restocking of young A. anguilla. This was indicated by a low but significant F(ST) value among within-host populations (infrapopulations) along with high inbreeding indices F(IS) consistent over all loci. The latter signal is shown to stem from high levels of admixture and the presence of first-generation migrants, and alternative explanations such as marker- and sex-specific biases in the nematode populations could be dismissed. Moreover, the slightly increased degree of relatedness within infrapopulations in the stocked river system cannot explain the excessive inbreeding values found and are most likely a direct consequence of recent influx of already infected fish harbouring parasites with different genetic signatures. Applying a simulation approach using known variables from the nematode's invasion history, only the artificial introduction of a Wahlund effect leads to a close match between simulated and real data, which is a strong argument for using the parasite as a biological tag for detecting and characterizing fish translocation.

The population genetics of parasitic nematodes of wild animals

“…Hence, the population genetics of A. crassus has revealed its introduction history. Infrapopulation differentiation in A. crassus was studied in two European rivers, one of which was regularly artificially restocked with eels from a variety of sources and one in which eels had arrived by natural dispersal [ 151 ]. This showed that in the restocked river, A. crassus had high genetic diversity among hosts and a substantial deviation from Hardy-Weinberg equilibrium (see Table 1 ), while in the river with natural recruitment, there was no among-host structuring or deviation from Hardy-Weinberg equilibrium.…”

“…This showed that in the restocked river, A. crassus had high genetic diversity among hosts and a substantial deviation from Hardy-Weinberg equilibrium (see Table 1 ), while in the river with natural recruitment, there was no among-host structuring or deviation from Hardy-Weinberg equilibrium. These contrasting patterns were thought to be because the introduced eels had retained A. crassus infrapopulations reflective of their genetically distinct source populations, while A. crassus in the river with natural recruitment are derived from a single population that was already at Hardy-Weinberg equilibrium [ 151 ].…”

Transcriptome analyses ofAnguillicola crassusfrom native and novel hosts

“…Populations from Taiwanese aquacultures might not be in Hardy–Weinberg Equillibrium, as this might only fail to be rejected in most markers due to the low sample sizes. It cannot be excluded that Taiwanese isolates are from populations experiencing a strong Wahlund effect, as observed in isolates from the River Rhine ( Wielgoss et al, 2010 ). A continued mixing of population could have resulted in the affiliation of our two different Taiwanese isolates over the European isolates.…”

Transcriptome analyses of Anguillicola crassus from native and novel hosts

“…It cannot be excluded that Taiwanese isolates are from populations experiencing a strong Wahlund effect, as observed in isolates from the River Rhine (Wielgoss et al, 2010 . As expected, genes with overall higher gene expression had a lower dn/ds, probably because genes with higher expression are under stronger purifying selection (Drummond et al, 2005).…”