Transcriptome of the bivalve Limecola balthica L. from Western Pacific: A new resource for studies of European populations

Confirmation of the first intronic sequence in the bivalvian mitochondrial genome of Macoma balthica (Linnaeus, 1758)

In 2020, the first male-type mitochondrial genome from the clam Macoma balthica was published. Apart from the unusual doubly uniparental inheritance of mtDNA, scientists observed a unique (over 4k bp long) extension in the middle of the cox2 gene. We have attempted to replicate these data by NGS DNA sequencing and explore further the expression of the long cox2 gene. In our study, we report an even longer cox2 gene (over 5.5 kbp) with no stop codon separating conserved cox2 domains, as well as, based on the rtPCR, a lower relative gene expression pattern of the middle part of the gene (5' = 1; mid = 0.46; 3' = 0.89). Lastly, we sequenced the cox2 gene transcript proving the excision of the intronic sequence.

Cycles of trans-Arctic dispersal and vicariance, and diversification of the amphi-boreal marine fauna

The amphi-boreal faunal element comprises closely related species and conspecific populations with vicarious distributions in the North Atlantic and North Pacific basins. It originated from an initial trans-Arctic dispersal in the Pliocene after the first opening of the Bering Strait, and subsequent inter-oceanic vicariance through the Pleistocene when the passage through the Arctic was severed by glaciations and low sea levels. Opportunities for further trans-Arctic dispersal have risen at times, however, and molecular data now expose more complex patterns of inter-oceanic affinities and dispersal histories. For a general view on the trans-Arctic dynamics and of the roles of potential dispersal-vicariance cycles in generating systematic diversity, we produced new phylogeographic data sets for amphi-boreal taxa in 21 genera of invertebrates and vertebrates, and combined them with similar published data sets of mitochondrial coding gene variation, adding up to 89 inter-oceanic comparisons involving molluscs, crustaceans, echinoderms, polychaetes, fishes and mammals. Only 39% of the cases correspond to a simple history of Pliocene divergence; in most taxonomical groups, the range of divergence estimates implies connections through the entire Pliocene-Pleistocene-Holocene time frame. Repeated inter-oceanic exchange was inferred for 23 taxa, and the latest connection was usually post-glacial. Such repeated invasions have usually led to secondary contacts and occasionally to widespread hybridization between the different invasion waves. Late- or post-glacial exchange was inferred in 46% of the taxa, stressing the importance of the relatively recent invasions to the current diversity in the North Atlantic. Individual taxa also showed complex idiosyncratic patterns and histories, and several instances of cryptic speciation were recognized. In contrast to a simple inter-oceanic vicariance scenario underlying amphi-boreal speciation, the data expose complex patterns of reinvasion and reticulation that complicate the interpretation of taxon boundaries in the region.