Heat-shock protein adaptation in abyssal and hadal amphipods

Heat-shock proteins (HSPs) are a prominent family of cellular chaperones that are involved in the folding, assembly and degradation of cellular proteins, cell-cycling and signal transduction. HSPs are high conserved across taxa and form a key component of the stress response with signatures of molecular adaptation in some species exposed to extreme environmental stressors such as dehydration, heavy metal pollutants and arctic temperatures. Here we characterise two key heat-shock protein genes (hsp70 and hsp90) in deep-sea Lysianassoidea amphipods, with a focus on copy number variation and signatures of selection on the DNA sequences. Four phylogenetically distinct isoforms were resolved for both hsp70 and hsp90, with one isoform in each gene being exclusive to the hadal genus Hirondellea. Signatures of purifying selection were shown across hsp70 and hsp90 from dN:dS ratios. The GC content of each gene was lower, and the number of codons used was higher, than in shallow water amphipods suggesting a relaxation in codon usage bias. Such observations suggest that increased hydrostatic pressure is an important environmental stress that shapes the adaptation of heat-shock protein genes in deep-sea amphipods.