| Summary: | During the past 50 years Fennoscandian populations of spring-spawning Baltic cisco ( Coregonus albula ), sympatric to common autumn-spawners, have declined or disappeared; e.g., three out of four known spring-spawning populations in Sweden are regarded as extinct. Over the same period, climate has changed and populations have been subject to other anthropogenic stressors. We compared historic (1960s) and recent (1990-2000s) morphological data from the still existent sympatric cisco populations in Lake Fegen, Sweden. Phenotypic changes were found for spring-spawners making them more similar to the sympatric autumn-spawners (that had remained virtually unchanged). Based on results for other salmoniform fishes, a phenotypically plastic response to increased temperature during early development appears unlikely. The recent material was also analysed with microsatellite markers; long-term effective population size in spring-spawners was estimated to be about 20 times lower than autumn-spawners, with signs of long-term gene flow in both directions and a recent genetic bottleneck in spring-spawners. The change towards a less distinct phenotype in spring-spawners is suggested to reflect a recent increase in gene flow from autumn-spawners. Time since divergence was estimated to only c. 1 900 years (95% CI: 400 – 5 900), but still the Fegen populations represent the most morphologically and genetically distinct sympatric populations studied. Consequently, we hypothesise that less distinct population pairs can be even younger, and that spring-spawning may have repeatedly evolved and disappeared in several lakes since the end of the last glaciation, concurrent with changed environmental conditions. The submitted dataset contains microsatellite genotypes (nine loci) from the individuals in Delling & Palm (Ecology and Evolution 2019;9:12727-12753). Please see the uploaded "Read me" file for details. Funding provided by: Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning Crossref Funder ...
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