Evidence of hybridization between genetically distinct Baltic cod stocks during peak population abundance(s).

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.

The complete mitochondrial genome of the Atlantic spiny lumpsucker Eumicrotremus spinosus (Fabricius, 1776).

The complete mitogenome of the Atlantic spiny lumpsucker (Eumicrotremus spinosus) was generated using the PacBio Sequel II HiFi sequencing platform. The mitogenome assembly has a length of 19,281 bp and contains 13 protein-coding sequences, 22 tRNA genes, 2 rRNA genes, one control region containing the D-loop (2383 bp) and a duplicate control region (1133 bp) Phylogenetic analysis using maximum likelihood revealed that E. spinosus is closely related to the Siberian lumpsucker (E. asperrimus). The mitogenome of the spiny lumpsucker will be useful in population genomics and systematic studies of Cyclopteridae, Liparidae, and Cottidae.