Shashikant Cooduvalli, Bolanowski Stacey A, Danke Joshua, Amemiya Chris T
Department of Dairy and Animal Science, College of Agricultural Sciences, The Pennsylvania State University, University Park, 324 Henning Building, University Park, PA 16802, USA.
J Exp Zool B Mol Dev Evol. 2004 Nov 15;302(6):557-63. doi: 10.1002/jez.b.21018.
Hoxc8 early enhancer controls the initiation and establishment phase of Hoxc8 expression in the mouse. Comparative studies indicate the presence of Hoxc8 early enhancer sequences in different vertebrate clades including mammals, birds and fish. Previous studies have shown differences between teleost and mammalian Hoxc8 early enhancers with respect to sequence and organization of protein binding elements. This raises the question of when the Hoxc8 early enhancer arose and how it has become modified in different vertebrate lineages. Here, we describe Hoxc8 early enhancer from the Indonesian coelacanth, Latimeria menadoensis. Coelacanths are the only extant lobefinned fish whose genome is tractable to genome analysis. The Latimeria Hoxc8 early enhancer sequence more closely resembles that of the mouse than that of Fugu or zebrafish. When assayed for enhancer activity by reporter gene analysis in transgenic mouse embryos, Latimeria Hoxc8 early enhancer directs expression to the posterior neural tube and mesoderm similar to that of the mouse enhancer. These observations support a close relationship between coelacanths and tetrapods and place the origin of a common Hoxc8 early enhancer sequence within the sarcopterygian lineage. The divergence of teleost (actinopterygii) Hoxc8 early enhancer may reflect a case of relaxed selection or other forms of instability induced by genome duplication events.
Hoxc8早期增强子控制小鼠中Hoxc8表达的起始和建立阶段。比较研究表明，在包括哺乳动物、鸟类和鱼类在内的不同脊椎动物类群中存在Hoxc8早期增强子序列。先前的研究已经表明，硬骨鱼和哺乳动物的Hoxc8早期增强子在蛋白质结合元件的序列和组织方面存在差异。这就提出了一个问题，即Hoxc8早期增强子是何时出现的，以及它在不同脊椎动物谱系中是如何被修饰的。在这里，我们描述了来自印度尼西亚腔棘鱼（Latimeria menadoensis）的Hoxc8早期增强子。腔棘鱼是现存唯一一种其基因组易于进行基因组分析的叶鳍鱼。腔棘鱼的Hoxc8早期增强子序列与小鼠的序列比与河豚或斑马鱼的序列更相似。当通过转基因小鼠胚胎中的报告基因分析来检测增强子活性时，腔棘鱼的Hoxc8早期增强子将表达导向后神经管和中胚层，类似于小鼠增强子的情况。这些观察结果支持了腔棘鱼和四足动物之间的密切关系，并将共同的Hoxc8早期增强子序列的起源置于肉鳍鱼谱系中。硬骨鱼（辐鳍鱼纲）Hoxc8早期增强子的分歧可能反映了一种选择放松的情况或由基因组复制事件诱导的其他形式的不稳定性。
