The present study describes the most comprehensive comparison of turtle mtD-loop regions to date. The primary structure was compared from DNA sequences accessed from GenBank from 48 species in 13 families of extant turtles, and secondary structures of the mtD-loop region were inferred from thermal stabilities, using the program Mfold, for each superfamiliy of turtles. Both primary and secondary structures were found to be highly variable across the order. The Cryptodira showed conservation in the primary structure at conserved sequence blocks (CSBs), but the Pleurodira displayed limited conservation of primary structural characters, other than the coreTAS, a binding site for the helicase TWINKLE, which was highly conserved in the Central and Right Domains across the order. No secondary structure was associated with a TAS, but an AT-rich fold (secondary structure) near the 3’ terminus of the mtD-loop region was detected in all turtle superfamilies. Mapping of character states of structural features of the mtD-loop region revealed that most character states were autapomorphies and inferred a number of homoplasies. The Left Domain of turtles, containing no highly conserved structural elements, likely does not serve a functional role; therefore, the Central Domain in turtles is likely equivalent to the Left Domain of mammals. The AT-rich secondary structural element near the 3’ terminus of the mtD-loop region may be conserved across turtles because of a functional role, perhaps containing the Light Strand Promotor, or perhaps interacting with the TWINKLE-coreTAS complex in the Central and Right Domains to regulate mtDNA replication and transcription.

本研究描述了迄今为止最全面的海龟mtD环区域比较。比较了从现存的13个海龟科的48个物种从GenBank获得的DNA序列的一级结构，并使用程序Mfold从热稳定性推断了每个海龟超家族的mtD-loop区二级结构。发现一级和二级结构在整个订单中变化很大。Cryptodira在保守序列区（CSB）的一级结构中显示出保守性，而Pleurodira在核心结构中显示了有限的一级结构性特征，而核心TAS是解旋酶TWINKLE的结合位点，在中央和右域中高度保守整个订单。没有二级结构与TAS相关联，但是在所有乌龟超家族中都检测到了mtD环区域3'末端附近的AT富集折叠（二级结构）。mtD环区域的结构特征的字符状态的映射显示，大多数字符状态是自发的，并推断出许多同质性。不含高度保守的结构元素的乌龟左域可能没有起到功能性作用；因此，海龟的中央区域很可能等同于哺乳动物的左侧区域。mtD环区域3'末端附近富含AT的二级结构元件可能在海龟中由于功能性作用而被保守，其中可能包含轻链启动子，或者可能与中部和右侧的TWINKLE-coreTAS复合体相互作用调节mtDNA复制和转录的域。