Geographical variation and differential selection modes of paralogs of chloroplast small heat shock protein genes in Machilus kusanoi (Lauraceae)

The hydrophobicity of amino acid residues in the α-crystallin domain (ACD) of small heat shock proteins (sHSPs) is thought to be important for polypeptide binding and play key roles in substrate protection. A molecular population approach was applied in an attempt to understand the geographic variations and selective forces exerted on the chloroplast sHSP gene duplicates (CPsHSP-1 and CPsHSP-2) of Machilus kusanoi. In total, 84 individuals from 17 sampling sites were used in this investigation. Five haplotypes, including synonymous and nonsynonymous substitutions, were found for CPsHSP-1. However, only one synonymous substitution was found for CPsHSP-2. The conservation of CPsHSP-2 might be related to the action of purifying selection. In contrast, the wide distribution of several CPsHSP-1 haplotypes in different populations and the significantly positive value of Tajima's D suggested that balancing selection has governed the evolution of CPsHSP-1. The functional novelty of the paralogs of CPsHSPs was inferred from the hydrophobicity profile analysis and functional divergence test. Changes in hydrophobicity profile are also observed for the allelic variants of CPsHSP-1, and the hydrophbicity shift might have played important roles in expanding substrate specificity of the CPsHSP-1 as molecular chaperones.