Glardon S, Holland L Z, Gehring W J, Holland N D
Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland.
Development. 1998 Jul;125(14):2701-10. doi: 10.1242/dev.125.14.2701.
Pax-6 genes have been identified from a broad range of invertebrate and vertebrate animals and shown to be always involved in early eye development. Therefore, it has been proposed that the various types of eyes evolved from a single eye prototype, by a Pax-6-dependent mechanism. Here we describe the characterization of a cephalochordate Pax-6 gene. The single amphioxus Pax-6 gene (AmphiPax-6) can produce several alternatively spliced transcripts, resulting in proteins with markedly different amino and carboxy termini. The amphioxus Pax-6 proteins are 92% identical to mammalian Pax-6 proteins in the paired domain and 100% identical in the homeodomain. Expression of AmphiPax-6 in the anterior epidermis of embryos may be related to development of an olfactory epithelium. Expression is also detectable in Hatschek's left diverticulum as it forms the preoral ciliated pit, part of which gives rise to the homolog of the vertebrate anterior pituitary. A zone of expression in the anterior neural plate of early embryos is carried into the cerebral vesicle (a probable diencephalic homolog) during neurulation. This zone includes cells that will differentiate into the lamellar body, a presumed homolog of the vertebrate pineal eye. In neurulae, AmphiPax-6 is also expressed in ventral cells at the anterior tip of the nerve cord; these cells are precursors of the photoreceptive neurons of the frontal eye, the presumed homolog of the vertebrate paired eyes. However, AmphiPax-6 expression was not detected in two additional types of photoreceptors, the Joseph cells or the organs of Hesse, which are evidently relatively recent adaptations (ganglionic photoreceptors) and appear to be rare exceptions to the general rule that animal photoreceptors develop from a genetic program triggered by Pax-6.
已从多种无脊椎动物和脊椎动物中鉴定出Pax-6基因，并表明其始终参与早期眼睛发育。因此，有人提出，各种类型的眼睛是通过一种依赖Pax-6的机制从单一的眼睛原型进化而来的。在此，我们描述了一种头索动物Pax-6基因的特征。文昌鱼单一的Pax-6基因（AmphiPax-6）可产生多种选择性剪接转录本，从而产生氨基酸和羧基末端明显不同的蛋白质。文昌鱼Pax-6蛋白在配对结构域中与哺乳动物Pax-6蛋白有92%的同一性，在同源结构域中则100%相同。AmphiPax-6在胚胎前表皮中的表达可能与嗅觉上皮的发育有关。在哈氏左侧憩室形成口前纤毛窝时也可检测到表达，其中一部分会发育成脊椎动物垂体前叶的同源物。早期胚胎前神经板中的一个表达区域在神经胚形成过程中会延伸至脑泡（可能是间脑的同源物）。这个区域包括将分化为板状体的细胞，板状体被认为是脊椎动物松果眼的同源物。在神经胚中，AmphiPax-6也在神经索前端的腹侧细胞中表达；这些细胞是额眼感光神经元的前体，额眼被认为是脊椎动物双眼的同源物。然而，在另外两种光感受器，即约瑟夫细胞或黑塞器官中未检测到AmphiPax-6的表达，这两种光感受器显然是相对较新的适应性结构（神经节光感受器），似乎是动物光感受器由Pax-6触发的遗传程序发育而来这一普遍规律的罕见例外。
