van Eeden F J, Holley S A, Haffter P, Nüsslein-Volhard C
Max Planck Institut für Entwicklungsbiologie, Abt. Genetik, Tübingen, Germany.
Dev Genet. 1998;23(1):65-76. doi: 10.1002/(SICI)1520-6408(1998)23:1<65::AID-DVG7>3.0.CO;2-4.
Segmentation in the vertebrate embryo is evident within the paraxial mesoderm in the form of somites, which are repeated structures that give rise to the vertebrae and muscle of the trunk and tail. In the zebrafish, our genetic screen identified two groups of mutants that affect somite formation and pattern. Mutations of one class, the fss-type mutants, disrupt the formation of the anterior-posterior somite boundaries during somitogenesis. However, segmentation within the paraxial mesoderm is not completely eliminated in these mutants. Irregular somite boundaries form later during embryogenesis and, strikingly, the vertebrae are not fused. Here, we show that formation of the irregular somite boundaries in these mutants is dependent upon the activity of a second group of genes, the you-type genes, which include sonic you, the zebrafish homologue of the Drosophila segment polarity gene, sonic hedgehog. Further to characterize the defects caused by the fss-type mutations, we examined their effects on the expression of her1, a zebrafish homologue of the Drosophila pair-rule gene hairy. In wild-type embryos, her1 is expressed in a dynamic, repeating pattern, remarkably similar to that of its Drosophila and Tribolium counterparts, suggesting that a pair-rule mechanism also functions in the segmentation of the vertebrate paraxial mesoderm. We have found that the fss-type mutants have abnormal pair-rule patterning. Although a her1 mutant could not be identified, analysis of a double mutant that abolishes most her1 expression suggests that a her1 mutant may not display a pair-rule phenotype analogous to the hairy phenotype observed in Drosophila. Cumulatively, our data indicate that zebrafish homologues of both the Drosophila segment polarity genes and pair-rule genes are involved in segmenting the paraxial mesoderm. However, both the relationship between these two groups of genes within the genetic heirarchy governing segmentation and the precise roles that they play during segmentation likely differ significantly between the two organisms.
脊椎动物胚胎中的体节形成在轴旁中胚层内表现为体节，体节是重复结构，可发育为躯干和尾部的椎骨及肌肉。在斑马鱼中，我们的基因筛选鉴定出两组影响体节形成和模式的突变体。一类突变体，即fss型突变体，在体节发生过程中破坏前后体节边界的形成。然而，这些突变体中轴旁中胚层内的分节并未完全消除。在胚胎发育后期会形成不规则的体节边界，而且引人注目的是，椎骨并未融合。在此，我们表明这些突变体中不规则体节边界的形成依赖于另一组基因即you型基因的活性，you型基因包括音猬因子you，它是果蝇节段极性基因音猬因子的斑马鱼同源物。为了进一步表征由fss型突变引起的缺陷，我们研究了它们对her1表达的影响，her1是果蝇成对规则基因hairy的斑马鱼同源物。在野生型胚胎中，her1以动态、重复的模式表达，与其果蝇和赤拟谷盗的对应物非常相似，这表明成对规则机制也在脊椎动物轴旁中胚层的分节中起作用。我们发现fss型突变体具有异常的成对规则模式。虽然未能鉴定出her1突变体，但对一个消除了大部分her1表达的双突变体的分析表明，her1突变体可能不会表现出与果蝇中观察到的hairy表型类似的成对规则表型。总体而言，我们的数据表明果蝇节段极性基因和成对规则基因的斑马鱼同源物都参与了轴旁中胚层的分节。然而，在控制分节的遗传层级中，这两组基因之间的关系以及它们在分节过程中所起的精确作用在这两种生物之间可能存在显著差异。
