In this work, we describe the transcriptional profiles of preadapted and non-adapted one-month-old juvenile Baikal whitefish after heat shock exposure. Preadapted fish were exposed to a repeated thermal rise of 6 °C above the control temperature every three days throughout their embryonic development. One month after hatching, preadapted and non-adapted larvae were either kept at control temperatures (12 °C) or exposed to an acute thermal stress (TS) of 12 °C above the control temperature. In response to this acute stress, an increase in HSP gene expression (HSP-30, HSP-40, HSP-47, HSP-70, and HSP-90) and TRIM16 was detected, independent of preadaptation. The expression levels of genes responsible for the response to oxygen levels, growth factors and the immune response, HBA, HBB, Myosin VI, Myosin VII, MHC, Plumieribetin, TnI, CYP450, and LDB3 were higher in individuals that had previously undergone adaptation. Genes responsible for the regulation of metabolism, MtCK, aFGF, ARF, CRYGB, and D-DT, however, increased their activity in non-adapted individuals. This information on transcriptional profiles will contribute to further understanding of the mechanisms of adaptation of whitefish to their environment.

Acclimation through the hormesis effect increases the plasticity of organisms, which has been shown for many ectothermic animals, including fish. We investigated the effect of temperature acclimation in Baikal whitefish Coregonus baicalensis (Dybowski, 1874). Telomere length, telomerase activity, and the expression of genes, whose products are involved in the regulation of telomere length and defense against reactive oxygen species, were selected to assess the state of the larvae. Acclimation and acute temperature stress (+12 °C) had no effect on telomere length, but altered telomerase activity (acclimation decreased it; stress increased it) and the levels of genes expression. Under stress, the expression of superoxide dismutase genes was increased in acclimated larvae and that of glutathione peroxidases in non-acclimated larvae, which may indicate lower reactive oxygen species formation and slower antioxidant responses in acclimated fish. The expression of some telomere-related genes was reduced under temperature stress, but the expression of the tzap and smg genes, whose products improve the control of telomere length by preventing them from lengthening or shortening, was increased in acclimated individuals. The data obtained indicate a positive effect of acclimation on the state of the Baikal whitefish larvae by remodulation of their telomerase activity and the transcriptional profile.

Using five microsatellite loci, Bwf1, Bom22b, Cocl23, Cam1, and Cam5, the polymorphism of nine populations of the all three Lake Baikal coregonid species (the omul, lacustrine whitefish and lacustrine-riverine whitefish), were studied. The total sample consisted of 244 individuals. The omul and lacustrine whitefish populations cluster into two separate groups with small pairwise genetic distances. The distance between the groups is also small, suggesting that the omul/lacustrine whitefish pair divergedin the recent geological past. The omul population from Lake Kulinda (basin of the Kichera River, the northern tributary of Lake Baikal) is located at a great distance from these groups, which testifies to its isolation from the omul/lacustrine whitefish species pair. The intermediate position of the omul population spawning in another northern tributary of Lake Baikal, the Upper Angara River, between the other omul populations and the Kulinda omul, indicates the presence of a gene flow from Lake Kulinda to the northern tributaries of Lake Baikal. The revealed genetic polymorphism pattern points to recent sympatric divergence of the omul, lacustrine whitefish, and their populations and supports the geographical isolation of one of the populations of the omul in Lake Kulinda, which is of glacial-tectonic origin. The population of the Upper Angara lacustrine-riverine whitefish, representing the third species of Lake Baikal coregonid fishes, is also at a considerable distance from all the other populations, despite the partial overlape in spawning time and grounds with the Upper Angara omul. This finding do not contradict the hypothesis on long allopatric divergence of the ancestor of the Baikal lacustrine-riverine whitefish outside the lake.

Understanding the molecular mechanisms that enable vertebrate adaptation to deep-water environments remains a central goal in evolutionary biology. This study used comparative transcriptomics of skeletal muscle tissue to investigate these mechanisms in four endemic sculpin fish species (Cottoidei) from Lake Baikal, which inhabit different depth niches ranging from pelagic to benthic-abyssal zones up to 1642 m. Pelagic species showed increased activity in genes involved in sarcomere structure, calcium handling, and motor proteins, indicating adaptations for sustained locomotion. In contrast, deep-benthic specialists showed enrichment in pathways for glycolytic metabolism, proteasome function, and ubiquitination, reflecting adaptations for energy efficiency and protein homeostasis in a high-pressure environment. We conclude that the colonization of the Baikal abyssal zone by sculpins relies on a suite of shared molecular mechanisms, with distinct ecological pressures driving specific transcriptional changes in motility, metabolic strategy, and cellular integrity. This study provides a systems-level model for deep-water adaptation in vertebrates.