Abstract
The molecular chaperone heat shock protein A2 (HSPA2), a member of the 70 kDa heat shock protein (HSP70) family, plays an important role in spermatogenesis and male fertility. Although HSPA2 is evolutionarily highly conserved across the metazoan lineages, the observation of striking differences in temperature-sensitive expressions, testicular physiology, spermatogenesis, as well as its role in male fertility indicates that avian and mammalian HSPA2 may exhibit distinct evolutionary trajectory. The present study reports that while mammalian HSPA2 is constrained by intense purifying selection, avian HSPA2 has been subjected to positive selection. The majority of the positively selected amino acid residues fall on the α-helix and β-sheets of the peptide-binding domain located at the carboxyl-terminal region of the avian HSPA2. The detection of positively selected sites at the helix and β-sheets, which are less tolerant to molecular adaptation, indicates an important functional consequence and contribution to the structural and functional diversification of the avian HSPA2. Collectively, avian HSPA2 may have an adaptive advantage over the mammals in response to heat stress, and therefore, mammals with testicular descent may be at a greater risk in the event of scrotal temperature rise.
Highlights
Spermatogenesis, the most fundamental biological process in male reproductive system, is strongly yet adversely affected by the increase in scrotal temperature in mammals with descended testicle[1,2,3,4,5,6,7,8,9]
While heat shock protein A2 (HSPA2) was reported to be down-regulated in mammalian male germ cells in response to heat stress[3,15,16,29], this gene was up-regulated in chicken[18]
By quantifying the ratio of the rates of non-synonymous and synonymous substitutions (ω = dN/dS), which has been widely used to detect the footprints of natural selection in the protein-coding genes[33], we seek to evaluate the pervasive role of positive selection in the evolution of avian and mammalian HSPA2
Summary
Phylogenetic analyses based on the amino acid sequences representing the members of HSP70 family revealed the orthology and monophyly of the avian and mammalian HSPA2 with strong nodal support (Fig. 1). Given the long evolutionary history of HSPA222,31, the observation of high sequence homologies (and low sequence divergence) of HSPA2 at the amino acid and nucleotide levels between and within the mammalian and avian groups (Table 1; Fig. S1) further indicates that this testis-enriched protein is evolutionarily conserved across the metazoan lineages. Based on this observation, one might speculate that HSPA2 is likely to have been subjected to intense purifying selection throughout the mammalian and avian evolution. Analyses using other methods provide evidence of positive selection on the avian
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