Kourakis M J, Master V A, Lokhorst D K, Nardelli-Haefliger D, Wedeen C J, Martindale M Q, Shankland M
Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637, USA.
Dev Biol. 1997 Oct 15;190(2):284-300. doi: 10.1006/dbio.1997.8689.
Molecular developmental studies of fly and mouse embryos have shown that the identity of individual body segments is controlled by a suite of homeobox-containing genes called the Hox cluster. To examine the conservation of this patterning mechanism in other segmented phyla, we here describe four Hox gene homologs isolated from glossiphoniid leeches of the genus Helobdella. Based on sequence similarity and phylogenetic analysis, the leech genes Lox7, Lox6, Lox20, and Lox5 are deemed to be orthologs of the Drosophila genes lab, Dfd, Scr, and Antp, respectively. Sequence similarities between Lox5 and Antp outside the homeodomain and phylogenetic reconstructions suggest that the Antennapedia family of Hox genes (as defined by Bürglin, 1994) had already expanded to include at least two discrete Antp and Ubx/abdA precursors prior to the annelid/arthropod divergence. In situ hybridization reveals that the four Lox genes described in this study are all expressed at high levels within the segmented portion of the central nervous system (CNS), with variable levels of expression in the segmental mesoderm. Little or no expression was seen in peripheral ectoderm or endoderm, or in the unsegmented head region (prostomium). Each Lox gene has a distinct anterior expression boundary within one of the four rostral segments, and the anterior-posterior (AP) order of these expression boundaries is identical to that reported for the orthologous Hox gene products in fly and mouse. This finding supports the idea that the process of AP axis differentiation is conserved among the higher metazoan phyla with respect to the regional expression of individual Hox genes along that axis. One unusual feature of leech Hox genes is the observation that some genes are only expressed during later development -- beginning at the time of terminal cell differentiation -- whereas others begin expression at a much earlier stage, and their RNA ceases to be detectable shortly after the onset of expression of the 'late' Hox genes. The functional significance of this temporal disparity is unknown, but it is noteworthy that only the two 'early' Hox genes display high levels of mesodermal expression.
对果蝇和小鼠胚胎的分子发育研究表明，个体身体节段的特征是由一组名为Hox簇的含同源异型框基因控制的。为了研究这种模式形成机制在其他分节动物门中的保守性，我们在此描述了从Helobdella属的舌蛭科水蛭中分离出的四个Hox基因同源物。基于序列相似性和系统发育分析，水蛭基因Lox7、Lox6、Lox20和Lox5分别被认为是果蝇基因lab、Dfd、Scr和Antp的直系同源物。Lox5和Antp在同源异型域外的序列相似性以及系统发育重建表明，在环节动物/节肢动物分化之前，Hox基因的触角足家族（如Bürglin在1994年所定义）已经扩展到包括至少两个离散的Antp和Ubx/abdA前体。原位杂交显示，本研究中描述的四个Lox基因在中枢神经系统（CNS）的分节部分均高水平表达，在节段中胚层中的表达水平各不相同。在外胚层或内胚层外周，或在不分节的头部区域（口前叶）几乎没有观察到表达。每个Lox基因在四个吻段之一内都有一个独特的前表达边界，这些表达边界的前后（AP）顺序与果蝇和小鼠中直系同源Hox基因产物的报道顺序相同。这一发现支持了这样一种观点，即就单个Hox基因沿该轴的区域表达而言，AP轴分化过程在高等后生动物门中是保守的。水蛭Hox基因的一个不寻常特征是观察到一些基因仅在后期发育期间表达——从终末细胞分化时开始——而其他基因则在更早的阶段开始表达，并且它们的RNA在“晚期”Hox基因表达开始后不久就不再能被检测到。这种时间差异的功能意义尚不清楚，但值得注意的是，只有两个“早期”Hox基因在中胚层中显示出高水平的表达。
