Divergent ecological histories of two sister Antarctic krill species led to contrasted patterns of genetic diversity in their heat‐shock protein (hsp70) arsenal

Abstract

The Arctic and the Antarctic Peninsula are currently experiencing some of the most rapid rates of ocean warming on the planet. This raises the question of how the initial adaptation to extreme cold temperatures was put in place and whether or not directional selection has led to the loss of genetic variation at key adaptive systems, and thus polar species’ (re)adaptability to higher temperatures. In the Southern Ocean, krill represents the most abundant fauna and is a critical member at the base of the Antarctic food web. To better understand the role of selection in shaping current patterns of polymorphisms, we examined genetic diversity of the cox‐1 and hsp70 genes by comparing two closely related species of Euphausiid that differ in ecology. Results on mtcox‐1 agreed with previous studies, indicating high and similar effective population sizes. However, a coalescent‐based approach on hsp70 genes highlighted the role of positive selection and past demographic changes in their recent evolution. Firstly, some form of balancing selection was acting on the inducible isoform C, which reflected the maintenance of an ancestral adaptive polymorphism in both species. Secondly, E. crystallorophias seems to have lost most of its hsp70 diversity because of a population crash and/or directional selection to cold. Nonsynonymous diversities were always greater in E. superba, suggesting that it might have evolved under more heterogeneous conditions. This can be linked to species’ ecology with E. superba living in more variable pelagic conditions, while E. crystallorophias is strictly associated with continental shelves and sea ice.

Heat shock protein 70 from a thermotolerant Diptera species provides higher thermoresistance to Drosophila larvae than correspondent endogenous gene

“…hsp70 genes from D. melanogaster and S. singularior exhibit only 85% identity, and some of the observed differences in the sequence may have adaptive value. Previously, it was demonstrated in several organisms that polymorphism in the hsp70 gene itself can be a determinant of a species' heat tolerance (Bhat et al, 2016;Papot et al, 2016).…”

“…Along these lines, accumulating data from other organisms suggest that genetic polymorphism in the regulatory and/or coding regions of hsp70 may be associated with high tolerance to heat and other environmental challenges (Bettencourt et al, 1999;Bettencourt & Feder, 2001Bhat et al, 2016). These studies suggest that the hsp70 genes of organisms from thermally contrasting niches are under stringent evolutionary constraints, and that these constraints may modify the structure of these genes to enhance the chaperone capacities of hsps within the cell and provide normal functioning under extreme environmental conditions (Garbuz et al, 2011;Qi et al, 2014;Papot et al, 2016).…”

Status, Change, and Futures of Zooplankton in the Southern Ocean

“…Between 40 and 400 kya, E. crystallorophias underwent a population crash or directional selection to cold adaptation over time owing to periods of glaciation and deglaciation that resulted in the loss of the genetic diversity in its heat shock proteins. The diversity of heat shock proteins is thought to provide phenotypic plasticity to species to tolerate and adapt to changing environments (Cascella et al, 2015;Papot et al, 2016).…”

Description of strongly heat-inducible heat shock protein 70 transcripts from Baikal endemic amphipods

“…Thus, it is essential to identify the heat-inducible forms of hsp70 in amphipods. As demonstrated in the recent studies of krill, next-generation sequencing and whole-transcriptome differential gene expression analysis can serve as the best method to identify the most appropriate for biomonitoring hsp70 orthologs 18,25,26 .…”