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Species of littoral freshwater environments in regions with continental climate experience pronounced seasonal temperature changes. Coping with long cold winters and hot summers requires specific physiological and behavioral adaptations. Endemic amphipods of Lake Baikal, Eulimnogammarus verrucosus and Eulimnogammarus cyaneus, show high metabolic activity throughout the year; E. verrucosus even reproduces in winter. In contrast, the widespread Holarctic amphipod Gammarus lacustris overwinters in torpor. This study investigated the transcriptomic hallmarks of E. verrucosus, E. cyaneus and G. lacustris exposed to low water temperatures. Amphipods were exposed to 1.5 °C and 12 °C (corresponding to the mean winter and summer water temperatures, respectively, in the Baikal littoral) for one month. At 1.5 °C, G. lacustris showed up-regulation of ribosome biogenesis and mRNA processing genes, as well as down-regulation of genes related to growth, reproduction and locomotor activity, indicating enhanced energy allocation to somatic maintenance. Our results suggest that the mitogen-activated protein kinase (MAPK) signaling pathway is involved in the preparation for hibernation; down-regulation of the actin cytoskeleton pathway genes could relate to the observed low locomotor activity of G. lacustris at 1.5 °C. The differences between the transcriptomes of E. verrucosus and E. cyaneus from the 1.5 °C and 12 °C exposures were considerably smaller than for G. lacustris. In E. verrucosus, cold-exposure triggered reproductive activity was indicated by up-regulation of respective genes, whereas in E. cyaneus, genes related to mitochondria functioning were up-regulated, indicating cold compensation in this species. Our data elucidate the molecular characteristics behind the different adaptations of amphipod species from the Lake Baikal area to winter conditions.

Eulimnogammarus verrucosusis an amphipod endemic to the unique ecosystem of Lake Baikal and serves in particular as an emerging model in ecotoxicological studies. We report here on a survey sequencing of its genome as a first step to establish sequence resources for this species. From a sin gle lane of paired-end sequencing data we estimated the genome size as nearly 10 Gb and we obtained an overview of the repeat content. At least two thirds of the genome are non-unique DNA, and a third of the genomic DNA is composed of just five families of repetitive elements, including low-complexity sequences. Attempts to use off-the-shelf assembly tools failed on the available low-coverage data both before and after removal of highly repetitive components. Using a seed-based approach we nevertheless assembled short contigs covering 33 pre-microRNAs and the homeodomain-containing exon of nine Hox genes. The absence of clear evidence for paralogs implies that a genome duplication did not contribute to the large genome size. We furthermore report the assembly of the mitochondrial genome using a new, guided “crystallization” procedure. The initial results presented here set the stage for a more compl ete sequencing and analysis of this large genome.

We studied various aspects of heat-shock response with special emphasis on the expression of heat-shock protein 70 (hsp70) genes at various levels in two congener species of littoral endemic amphipods (Eulimnogammarus cyaneus and E. verrucosus) from Lake Baikal which show striking differences in their vertical distribution and thermal tolerance. Although both the species studied demonstrate high constitutive levels of Hsp70, the thermotolerant E. cyaneus exhibited a 5-fold higher basal level of Hsp70 proteins under normal physiological conditions (7 °C) and significantly lower induction of Hsp70 after temperature elevation compared with the more thermosensitive E. verrucosus. We isolated the hsp70 genes from both species and analysed their sequences. Two isoforms of the cytosolic Hsp70/Hsc70 proteins were detected in both species under normal physiological conditions and encoded by two distinct hsp/hsc70 family members. While both Hsp70 isoforms were synthesized without heat shock, only one of them was induced by temperature elevation. The observed differences in the Hsp70 expression patterns, including the dynamics of Hsp70 synthesis and threshold of induction, suggest that the increased thermotolerance in E. cyaneus (compared with E. verrucosus) is associated with a complex structural and functional rearrangement of the hsp70 gene family and favoured the involvement of Hsp70 in adaptation to fluctuating thermal conditions. This study provides insights into the molecular mechanisms underlying the thermal adaptation of Baikal amphipods and represents the first report describing the structure and function of the hsp70 genes of endemic Baikal species dwelling in thermally contrasting habitats.