Host Co-Occurrence and Population Size Explain Genetic Differentiation and Diversity in Seal Lice
Sromek L. Johnson K.P. Kunnasranta M. Rosing-Asvid A. Bäcklin B.-M. Tasmagambetova A. Tovassarov A. Rusinek O. Ziel H.L. Rubio-García A. Nyman T.
December 2025John Wiley and Sons Inc
Molecular Ecology
2025#34Issue 24
We studied the drivers of population-genetic structuring and genetic diversity in specialist parasites based on whole-genome resequencing data from 82 Echinophthirius horridus seal louse individuals sampled from 12 ecologically and behaviourally different phocine seal species, subspecies and populations across the Holarctic. We found that the main genetic disjunctions in E. horridus lice occur across seal host species and subspecies, with a further level of population subdivision emerging among host individuals within some populations. Endemic and relict landlocked seal (sub)species host the genetically most distinct louse populations, while lice associated with sympatric marine seals show signatures of occasional gene flow across hosts. Within the latter, the most extreme case is seen in the near-panmictic lice associated with northern European grey and harbour seals, which aggregate in shared rookeries and colonies. Although the louse and seal phylogenies were overall statistically significantly congruent, evidence for similar host shifts in the past is reflected in several conflicts in the phylogenetic trees of the lice and their hosts. Population-level mean heterozygosity and theta in seal lice varied considerably, and both measures of genetic variation were statistically significantly related to host population size. Taken together, our results support a non-adaptive model of parasite diversification, in which geographic and behavioural isolation among hosts drives parasite genetic differentiation, and genetic erosion in bottlenecked hosts cascades up to their specialist parasites. Our results provide new insights into processes that generate parasite diversity and trigger parallel losses of genetic diversity in endangered host–parasite systems.
coevolution , genetic diversity , host-associated genetic differentiation , host–parasite interactions , infrapopulation structure
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Department of Marine Ecosystems Functioning, Faculty of Oceanography and Geography, University of Gdansk, Gdynia, Poland
Illinois Natural History Survey, Prairie Research Institute, University of Illinois, Champaign, IL, United States
Department of Environmental and Biological Sciences, University of Eastern Finland, Joensuu, Finland
Natural Resources Institute Finland, Joensuu, Finland
Greenland Institute of Natural Resources, Nuuk, Greenland
Swedish Museum of Natural History, Stockholm, Sweden
Central Asian Institute for Ecological Research, Almaty, Kazakhstan
Baikal Museum of the Siberian Branch of the Russian Academy of Sciences, Listvyanka, Russian Federation
Marine Mammal Laboratory, Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration, Seattle, WA, United States
Veterinary, Care & Research Department, Seal Centre Pieterburen Foundation, Lauwersoog, Netherlands
Department of Ecosystems in the Barents Region, Svanhovd Research Station, Norwegian Institute of Bioeconomy Research, Svanvik, Norway
Department of Marine Ecosystems Functioning
Illinois Natural History Survey
Department of Environmental and Biological Sciences
Natural Resources Institute Finland
Greenland Institute of Natural Resources
Swedish Museum of Natural History
Central Asian Institute for Ecological Research
Baikal Museum of the Siberian Branch of the Russian Academy of Sciences
Marine Mammal Laboratory
Veterinary
Department of Ecosystems in the Barents Region
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