microRNA遗传鲁棒性与环境鲁棒性的一致进化
Congruent evolution of genetic and environmental robustness in microRNA
摘要：
遗传稳健性，即在突变面前保持最佳表型，对于理解进化至关重要，因为表型表达的遗传变异是自然选择的燃料。然而，遗传稳健性的起源，无论是通过自然选择直接进化还是其他表型性状的相关副产品，都没有得到解决。Borenstein和Ruppin（Borenstein et al.2006，PNAS 103：6593）检查了几种真核生物物种的微小RNA（miRNA）基因，表明与具有相同茎环结构的随机RNA序列样品相比，miRNA前体茎环的结构表现出显著增加的突变稳健性。观察到的鲁棒性被发现与传统的环境鲁棒性测量不相关-这意味着miRNA序列显示出遗传鲁棒性直接进化的证据。这些发现是令人惊讶的，因为理论结果表明稳健性的直接进化需要仅在RNA病毒中发现的高突变率和/或大的有效种群大小，而不是多细胞真核生物。引入基于miRNA前体序列的二级结构的平衡热力学系综的环境稳健性的新测量，我们证明RNA折叠的生物物理学诱导遗传（突变）稳健性和环境（热力学）稳健性之间的高水平相关性，如从Ancel和Fontana引入的质体发生一致性理论所预期的（Ancel等人，2000，J. Exp. Zool.288：242）。根据理论考虑，我们认为这种相关性强烈表明，在miRNA序列中观察到的遗传鲁棒性是环境鲁棒性选择的副产品。
英文摘要：
Genetic robustness, the preservation of an optimal phenotype in the face of mutations, is critical to the understanding of evolution as phenotypically expressed genetic variation is the fuel of natural selection. The origin of genetic robustness, whether it evolves directly by natural selection or it is a correlated byproduct of other phenotypic traits, is, however, unresolved. Examining microRNA (miRNA) genes of several eukaryotic species, Borenstein and Ruppin (Borenstein et al. 2006, PNAS 103: 6593), showed that the structure of miRNA precursor stem-loops exhibits significantly increased mutational robustness in comparison with a sample of random RNA sequences with the same stem-loop structure. The observed robustness was found to be uncorrelated with traditional measures of environmental robustness -- implying that miRNA sequences show evidence of the direct evolution of genetic robustness. These findings are surprising as theoretical results indicate that the direct evolution of robustness requires high mutation rates and/or large effective population sizes only found among RNA viruses, not multicellular eukaryotes. Introducing a novel measure of environmental robustness based on the equilibrium thermodynamic ensemble of secondary structures of the miRNA precursor sequences we demonstrate that the biophysics of RNA folding, induces a high level of correlation between genetic (mutational) and environmental (thermodynamic) robustness, as expected from the theory of plastogenetic congruence introduced by Ancel and Fontana (Ancel et al. 2000, J. Exp. Zool. 288: 242). In light of theoretical considerations we believe that this correlation strongly suggests that genetic robustness observed in miRNA sequences is the byproduct of selection for environmental robustness.
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