Article CC BY 4.0
refereed
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Evidence of hybridization between genetically distinct Baltic cod stocks during peak population abundance(s)

ORCID
0000-0002-4701-8641
Affiliation
Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, Oslo, Norway
Helmerson, Cecilia;
GND
1143146395
ORCID
0000-0002-3715-6936
Affiliation
Thünen Institute of Fisheries Ecology, Bremerhaven, Germany
Weist, Peggy;
ORCID
0000-0001-8601-2122
Affiliation
Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, Oslo, Norway
Brieuc, Marine Servane Ono;
ORCID
0000-0001-7894-1693
Affiliation
Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, Oslo, Norway
Maurstad, Marius F.;
GND
1051791782
ORCID
0000-0003-3655-1210
Affiliation
Thünen Institute of Baltic Sea Fisheries, Rostock, Germany
Schade, Franziska Maria;
GND
1011754908
Affiliation
GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
Dierking, Jan;
ORCID
0000-0002-6816-2087
Affiliation
Gemeinschaftsschule an der Schlei, Kappeln, Germany
Petereit, Christoph;
ORCID
0000-0002-7627-7634
Affiliation
Institute of Marine Research, Bergen, Norway
Knutsen, Halvor;
Affiliation
Centre for Environment Fisheries and Aquaculture Science, Lowestoft, United Kingdom
Metcalfe, Julian;
ORCID
0000-0001-8643-3672
Affiliation
Centre for Environment Fisheries and Aquaculture Science, Lowestoft, United Kingdom
Righton, David;
Affiliation
Department of Marine Sciences – Tjärnö, University of Gothenburg, Strömstad, Sweden
André, Carl;
GND
1018949593
ORCID
0000-0001-5426-1079
Affiliation
Thünen Institute of Baltic Sea Fisheries, Rostock, Germany
Krumme, Uwe;
GND
1117945502
Affiliation
Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, Oslo, Norway
Jentoft, Sissel;
GND
1011755254
Affiliation
Thünen Institute of Fisheries Ecology, Bremerhaven, Germany
Hanel, Reinhold

Range expansions can lead to increased contact of divergent populations, thus increasing the potential of hybridization events. Whether viable hybrids are produced will most likely depend on the level of genomic divergence and associated genomic incompatibilities between the different entities as well as environmental conditions. By taking advantage of historical Baltic cod (Gadus morhua) otolith samples combined with genotyping and whole genome sequencing, we here investigate the genetic impact of the increased spawning stock biomass of the eastern Baltic cod stock in the mid 1980s. The eastern Baltic cod is genetically highly differentiated from the adjacent western Baltic cod and locally adapted to the brackish environmental conditions in the deeper Eastern basins of the Baltic Sea unsuitable for its marine counterparts. Our genotyping results show an increased proportion of eastern Baltic cod in western Baltic areas (Mecklenburg Bay and Arkona Basin)—indicative of a range expansion westwards—during the peak population abundance in the 1980s. Additionally, we detect high frequencies of potential hybrids (including F1, F2 and backcrosses), verified by whole genome sequencing data for a subset of individuals. Analysis of mitochondrial genomes further indicates directional gene flow from eastern Baltic cod males to western Baltic cod females. Our findings unravel that increased overlap in distribution can promote hybridization between highly divergent populations and that the hybrids can be viable and survive under specific and favourable environmental conditions. However, the observed hybridization had seemingly no long-lasting impact on the continuous separation and genetic differentiation between the unique Baltic cod stocks.

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