Dettman R W, Turner F R, Raff E C
Department of Biology and Institute for Molecular and Cellular Biology, Indiana University, Bloomington, Indiana, 47405, USA.
Dev Biol. 1996 Jul 10;177(1):117-35. doi: 10.1006/dbio.1996.0150.
We have investigated the cellular basis for lethality of mutant alleles of the Drosophila melanogaster beta3-tubulin gene, betaTub60D. Lethal beta3 mutations can be grouped into two classes: the most severe mutations (Class I alleles) cause death during the first larval instar, while weaker alleles (Class II) cause death in later larval stages or in early pupal development. Since beta3 is not expressed during larval development, lethality of the Class I mutations must reflect essential functions of beta3 in embryogenesis. Beta3-tubulin is zygotically expressed during midembryogenesis in the developing mesoderm, and the major site of beta3 accumulation is in the developing muscles during myogenesis. We show that the embryonic pattern of beta3 expression, including accumulation in the developing musculature, is conserved in other Drosophila species. However, we found that loss of beta3 function does not cause discernible defects in either the ultrastructure or function of the larval muscle. Thus beta3-tubulin is dispensable in its highest site of accumulation. Rather, the essential site of function of beta3 in embryos is in cells of the visceral mesoderm. Lethality of Class I alleles is caused by defects in midgut morphogenesis and failure of gut function. Although the folding pattern is irregular and the gut is smaller than normal, a complete folded gut forms in mutant larvae, and the visceral muscle functions normally to move food through the gut. However, mutant larvae cannot absorb nutrients across the gut wall. Thus loss of beta3 function in the mesoderm results in defects in the underlying endodermally derived layer of the gut. Our data provide an assay for cellular interactions between mesoderm and endodermal tissues and reveal a role for the microtubule cytoskeleton of the visceral mesodermal cells in differentiation of the endodermal cell layer of the larval gut.
我们研究了果蝇β3 -微管蛋白基因（βTub60D）突变等位基因致死性的细胞基础。致死性β3突变可分为两类：最严重的突变（I类等位基因）导致一龄幼虫期死亡，而较弱的等位基因（II类）导致后期幼虫阶段或早期蛹发育阶段死亡。由于β3在幼虫发育期间不表达，I类突变的致死性必定反映了β3在胚胎发生中的基本功能。β3 -微管蛋白在胚胎发育中期的中胚层中合子表达，β3积累的主要部位是在肌肉发生期间的发育中的肌肉中。我们表明，β3表达的胚胎模式，包括在发育中的肌肉组织中的积累，在其他果蝇物种中是保守的。然而，我们发现β3功能丧失在幼虫肌肉的超微结构或功能上并未导致可察觉的缺陷。因此，β3 -微管蛋白在其最高积累部位是可有可无的。相反，β3在胚胎中的基本功能部位是在内脏中胚层的细胞中。I类等位基因的致死性是由中肠形态发生缺陷和肠道功能衰竭引起的。尽管折叠模式不规则且肠道比正常的小，但突变幼虫中形成了完整折叠的肠道，并且内脏肌肉正常运作以推动食物通过肠道。然而，突变幼虫无法通过肠壁吸收营养。因此，中胚层中β3功能的丧失导致肠道内胚层衍生层的潜在缺陷。我们的数据提供了一种中胚层与内胚层组织之间细胞相互作用的检测方法，并揭示了内脏中胚层细胞的微管细胞骨架在幼虫肠道内胚层细胞层分化中的作用。
