Systematic headwater sampling of white-spotted charr reveals stream capture events across dynamic topography

Aim: River system rearrangements such as stream capture play a crucial role in determining range expansion and gene flow of stream organisms between different watersheds. However, it is challenging to clarify the role of stream capture in dynamic landscapes where frequent connections and disconnections occur in stream networks. We hypothesized that fine -scale sampling with genome- wide analysis could reveal the effect of stream capture on the range expansion of stream fish and detect unidentified river rearrangement of headwater streams in a highly heterogeneous landscape. Location: Honshu Island, Japan. Taxon: Salvelinus leucomaenis. Methods: To identify stream capture events from biological evidence, a sampling strategy was designed with fish collected from multiple pairs of adjacent headwater streams. This included a typical site' where the occurrence of a stream capture event has been confirmed with geological evidence. The genetic population structure was analysed using genome- wide single nucleotide polymorphism (SNP) data using MIGseq, followed by a Bayesian clustering approach. Results: Evidence was found for historical dispersal across watersheds, including a typical stream capture site. This cross-drainage dispersal promoted gene flow among the three genetic groups of focal fish inhabiting western Honshu Island. The likelihood of past stream capture increased substantially when the inter-headwater distance was less than 2000 m, or the altitudinal difference was less than 200 m in the current topography. Main Conclusions: In highly dynamic areas, such as the study site, which is surrounded by active faults, stream capture may be frequent, and aids inter-drainage dispersal of freshwater organisms, where large -scale range expansions would otherwise be impossible. Systematic pairwise headwater sampling, together with genome- wide SNP analysis, provides biological evidence for the history of water system rearrangements and enables the identification of geological events.

Automated summary

This study used genome-wide analysis and fine-scale sampling to reveal the important role of stream capture in the expansion and gene flow of stream organisms, as well as to detect previously unidentified river rearrangement events in a highly heterogeneous landscape.