Graylings (Thymallus) are among the less well-studied groups of salmonid fishes, especially across their Asian distribution range. Here we perform a comprehensive global review of their phylogeography, systematic diversity and range distributions, including biogeographic reconstruction and assessment of both conservation and taxonomic status of each species. Based on a mitogenomic phylogenetic analysis, three approaches to the delineation of molecular operational units, and evaluation of 15 a-priori defined species, we provide biological support for the recognition of 13 grayling species, plus two additional species tentatively. Several instances of paraphyly and its potential effect on systematic inferences are discussed. Overall, the genus displays increasing species diversity and decreasing range size from higher to lower latitudes and ancestral trait reconstruction supports an East Asian origin for extant diversity, most likely centred in the Amur River drainage. Europe’s colonization by Thymallus took place as early as the late Miocene, at least two colonisations of North America are supported, and multiple dispersal events likely took place into Western Siberia. The conservation status for the 15 taxa was estimated to be: 6 least concern, 1 near-threatened, 2 vulnerable, 3 endangered and 3 data deficient.

Large‐scale phenotypic and genetic studies of the salmonid genus Thymallus (grayling) in the Palaearctic suggest that most major phylogeographic lineages represent good biological species. Evaluating such a premise in areas involved in palaeo‐hydrological dynamics where multiple species are found in sympatry should serve to assess the level of reproductive isolation, the traditional sine qua non of species recognition. Molecular sequence (mtDNA) and microsatellite (nDNA, seven loci) analysis of grayling in the upper Bureya River watershed support the occurrence of three distinctive species of grayling living in sympatry in this large oligotrophic tributary of the Amur River. One of these lineages is primarily found throughout the Lena River basin and is recognized as Baikal‐Lena grayling Thymallus baicalolenensis; the second, the upper Amur grayling Thymallus grubii is found over large areas of the Amur catchment including the entire headwater region; and the third, the Bureya grayling Thymallus burejensis is endemic to the study area. A limited number of hybrids were identified, primarily between Baikal‐Lena grayling T. baicalolenensis and Bureya grayling T. burejensis with little to no signs of introgression among non‐hybrid individuals. Morphological distinctiveness among populations of these species living in sympatry was greater than between populations living in allopatry, suggesting character displacement. Divergence estimates among taxa range up to 6.2 MY, and allopatric origins for all three species’ is suggested. To our knowledge, this is the first data‐based confirmation of three species of grayling living in sympatry.

Among fishes, salmonids (family Salmonidae) have attracted a great deal of research attention focused on sexual dimorphism and associated selective forces. Most of this research has been directed toward anadromous and mostly semelparous salmon and trout (Oncorhynchus, Salmo), and comparatively little is known about intersexual variability in strictly iteroparous freshwater salmonids. We examined a comprehensive data set of 28 linear morphometric characters in 11 of 15 currently recognised species of grayling (Thymallinae, Thymallus), a genus consisting of iteroparous species only, to identify general patterns of intersexual morphological variability. Overall, we found that all grayling species show common sex-specific traits particularly relating to size dimensions of the dorsal, anal, pelvic and pectoral fins. Although the magnitude of sexual dimorphism differed among species, there was no significant phylogenetic signal associated with these differences across the genus. These results are discussed in terms of the assumed selection pressures driving sexual dimorphism in graylings and are compared to existing knowledge in Salmonidae as a whole where similarities and differences with both Salmoninae and Coregoninae exist. The present study provides the first detailed genus-wide comparison of sexually dimorphic phenotypic characters in graylings, and highlights the need for more large-scale comparative studies in multiple salmonid species to better understand general macroevolutionary trends among this important group of freshwater fishes.

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