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Geographical Variation and Differential Selection Modes of Paralogs of Chloroplast Small Heat Shock Protein Genes in Machilus kusanoi (Lauraceae)



低分子量熱休克蛋白α-水晶體蛋白區域的胺基酸疏水性與多胜肽的鍵結有其重要性並扮演著保護受質蛋白的功能。我們針對大葉楠以族群遺傳的研究角度來瞭解葉綠體低分子量熱休克蛋白基因在地理分布的變異及探討選汰壓力所扮演的角色。由兩個大葉楠低分子量熱休克蛋白同源基因(CPsHSP-1及CPsHSP-2),我們發現CPsHSP-1因同義及非同義置換的突變而有五種單套型的出現;然而CPsHSP-2僅有兩個因同義置換而形成之兩個單套型。CPsHSP-2基因序列的保守性可能與淨化選汰力量有關。相對的,CPsHSP-1在不同族群有不同單套型的組合,其Tajima D的檢測為顯著正值,意味著CPsHSP-1受到均衡選汰力量的影響。大葉楠低分子量熱休克蛋白兩個同源基因CPsHSP-1及CPsHSP-2的疏水性的變異及功能差異的檢測顯示兩者間有功能上的分歧。CPsHSP-1不同單套型之胺基酸疏水性亦呈現變異而可能與其受質蛋白對象的擴充有關。

Parallel abstracts

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.