Genome-wide identification and evolutionary analysis of the Hsp70 gene family in mandarin fish (Siniperca chuatsi): Insights into their differential expression under thermal and bacterial stress.

The heat shock protein 70 (Hsp70) family is essential for maintaining protein homeostasis and mediating responses to environmental stresses; however, comprehensive characterizations of this family in the mandarin fish (Siniperca chuatsi) are presently absent. Through comparative genomic analysis, a total of 178 Hsp70 genes were identified across nine vertebrate species (human, mouse, and seven teleost fishes), including 17 unique members within the mandarin fish genome. Phylogenetic and gene structure analyses demonstrated that the mandarin fish Hsp70 family has remained highly conserved throughout teleost evolution, marked by lineage-specific expansions (notably in hspa1, hspa4, hspa8, and hspa12) and selective gene loss (e.g., the absence of hspa2 and hspa6). Specifically, tandem duplication was observed for hspa8.1 and hspa8.2, at the same time, two pairs of syntenic genes (hspa4a/hspa4b and hyou1/hspa8b) were found. Ka/Ks analysis further indicated that this gene family has mainly evolved under purifying selection. Transcriptomic profiling showed that hspa8.1 was constantly expressed across all examined tissues. In addition, under thermal stress and Aeromonas hydrophila infection, Hsp70 genes in mandarin fish exhibited divergent expression patterns: certain members contribute to basal homeostasis (e.g., hspa8.1), whereas others demonstrate specialized responses to heat/cold adaptation (e.g., hspa5) or pathogen infection (e.g., hspa1l). Respectively, these findings together provide a thorough understanding of the composition, evolutionary trajectory, and stress-responsive dynamics of the Hsp70 family, establishing a foundational molecular basis for understanding the environmental adaptation of mandarin fish.