Pax 6: mastering eye morphogenesis and eye evolution.

Pax 6 genes from various animal phyla are capable of inducing ectopic eye development, indicating that Pax 6 is a master control gene for eye morphogenesis. It is proposed that the various eye-types found in metazoa are derived from a common prototype, monophyletically, by a mechanism called intercalary evolution.     

Pax genes and eye organogenesis

Pax6 is a highly conserved gene that controls eye development in all species where it has been tested. In spite of this common 'master control regulator', the eyes of different animals are morphologically very different and it is believed that they have evolved independently multiple times through evolution. Recent works looking at eye development in 'primitive' species offer some explanation as to the surprising amount of conservation in genetic and morphogenetic pathways involved in eye development. These studies not only implicate the Pax genes but also the So/Six gene family in playing a crucial ancestral role in visual system development.     

Pax6 and eye development in Arthropoda

The arthropod compound eye is one of the three main types of eyes observed in the animal kingdom. Comparison of the eyes seen in Insecta, Crustacea, Myriapoda and Chelicerata reveals considerable variation in terms of overall cell number, cell positioning, and photoreceptor rhabdomeres, yet, molecular data suggest there may be unexpected similarities. We review here the role of Pax6 in eye development and evolution and the relationship of Pax6 with other retinal determination genes and signaling pathways. We then discuss how the study of changes in Pax6 primary structure, in the gene networks controlled by Pax6 and in the relationship of Pax6 with signaling pathways may contribute to our insight into the relative role of conserved molecular-genetic mechanisms and emergence of evolutionary novelty in shaping the ommatidial eyes seen in the Arthropoda.     

Spatial specification of mammalian eye territories by reciprocal transcriptional repression of Pax2 and Pax6

We have studied the molecular basis of the Pax2 and Pax6 function in the establishment of visual system territories. Loss-of-function mutants have revealed crucial roles for Pax2 in the generation of the optic stalk and for Pax6 in the development of the optic cup. Ectopic expression of Pax6 in the optic stalk under control of Pax2 promoter elements resulted in a shift of the optic cup/optic stalk boundary indicated by the presence of retinal pigmented cells on the optic stalk. By studying mouse embryos at early developmental stages we detected an expansion of Pax2 expression domain in the Pax6(-/-) mutant and of Pax6 expression domain in the Pax2(-/-) embryo. These results suggest that the position of the optic cup/optic stalk boundary depends on Pax2 and Pax6 expression, hinting at a possible molecular interaction. Using gel shift experiments, we confirmed the presence of Pax2- and Pax6-binding sites on the retina enhancer of the Pax6 gene and on the Pax2 upstream control region, respectively. Co-transfection experiments revealed a reciprocal inhibition of Pax2 promoter/enhancer activity by Pax6 protein and vice versa. Based on our findings, we propose a model for Pax gene regulation that establishes the proper spatial regionalization of the mammalian visual system.     

Incorporation of a horizontally transferred gene into an operon during cnidarian evolution

Genome sequencing has revealed examples of horizontally transferred genes, but we still know little about how such genes are incorporated into their host genomes. We have previously reported the identification of a gene (flp) that appears to have entered the Hydra genome through horizontal transfer. Here we provide additional evidence in support of our original hypothesis that the transfer was from a unicellular organism, and we show that the transfer occurred in an ancestor of two medusozoan cnidarian species. In addition we show that the gene is part of a bicistronic operon in the Hydra genome. These findings identify a new animal phylum in which trans-spliced leader addition has led to the formation of operons, and define the requirements for evolution of an operon in Hydra. The identification of operons in Hydra also provides a tool that can be exploited in the construction of transgenic Hydra strains.     

Aphakia (ak), a mouse mutation affecting early eye development: Fine mapping,consideration of candidate genes and altered Pax6 and Six3 gene expression pattern

The homozygous mouse mutant aphakia (ak) has been characterized by bilaterally aphakic eyes without a pupil [Varnum DS, Stevens, LC (1968): J Hered 59:147–150]. The mutation was mapped to chromosome 19 [Varnum DS, Stevens, LC (1975): Mouse News Lett 53:35]. Our linkage studies yielded a precise localization of the ak gene 0.6 ± 0.3 cM proximal to the microsatellite marker D19Mit10 and 0.7 ± 0.4 cM distal to D19Mit4 and D19Mit91. No recombination was found with the marker D19Mit9 among 418 backcross offspring tested. The developmental control gene Pax2 mapped 11.0 ± 3.5 cM proximal to ak and is excluded as a candidate gene. Sequence analysis of Fgf8 and Chuk1, which are localized close to the marker D19Mit10, detected no mutations in the ak/ak mutants. Histological analysis of homozygous mutants suggested the arrest of lens development at the lens stalk stage, a transient morphological structure during the formation of the lens vesicle. In the lens remnants, Pax6 and Six3 are expressed, whereas in the persisting lens stalk only Pax6 was detected. The expression pattern of Pax2 appeared normal; Cryaa expression could not be detected. As a consequence of the arrested lens development, other ocular tissues that require for their development information from the intact lens, such as iris, ciliary muscle, retina, and vitreous body, are absent or formed abnormally. Dev. Genet. 23:299–316, 1998. © 1998 Wiley-Liss, Inc.     

Pax基因家族在果蝇发育过程中的调控作用

Pax是一个在进化上相当保守的基因家族, 它们编码的产物是一组极为重要的转录调控因子, 并存在于从果蝇到人类的各种生物体中, 参与细胞内信号传导通路的调控, 在胚胎发育过程中对细胞分化、更新、凋亡起重要的调控作用, 影响器官和组织的形成。果蝇中已发现10个Pax基因家族成员, 它们对果蝇胚胎发育及成虫组织器官的分化有非常重要的调控作用。文章结合最新的研究进展, 就果蝇中Pax基因的结构、表达模式和主要功能做一简要综述。     

Pax group III genes and the evolution of insect pair-rule patterning

Pair-rule genes were identified and named for their role in segmentation in embryos of the long germ insect Drosophila. Among short germ insects these genes exhibit variable expression patterns during segmentation and thus are likely to play divergent roles in this process. Understanding the details of this variation should shed light on the evolution of the genetic hierarchy responsible for segmentation in Drosophila and other insects. We have investigated the expression of homologs of the Drosophila Pax group III genes paired, gooseberry and gooseberry-neuro in short germ flour beetles and grasshoppers. During Drosophila embryogenesis, paired acts as one of several pair-rule genes that define the boundaries of future parasegments and segments, via the regulation of segment polarity genes such as gooseberry, which in turn regulates gooseberry-neuro, a gene expressed later in the developing nervous system. Using a crossreactive antibody, we show that the embryonic expression of Pax group III genes in both the flour beetle Tribolium and the grasshopper Schistocerca is remarkably similar to the pattern in Drosophila. We also show that two Pax group III genes, pairberry1 and pairberry2, are responsible for the observed protein pattern in grasshopper embryos. Both pairberry1 and pairberry2 are expressed in coincident stripes of a one-segment periodicity, in a manner reminiscent of Drosophila gooseberry and gooseberry-neuro. pairberry1, however, is also expressed in stripes of a two-segment periodicity before maturing into its segmental pattern. This early expression of pairberry1 is reminiscent of Drosophila paired and represents the first evidence for pair-rule patterning in short germ grasshoppers or any hemimetabolous insect.     

The Optimedin gene is a downstream target of Pax6

The Optimedin gene, also known as Olfactomedin 3, encodes an olfactomedin domain-containing protein. There are two major splice variants of the Optimedin mRNA, Optimedin A and Optimedin B, transcribed from different promoters. The expression pattern of the Optimedin A variant in the eye and brain overlaps with that for Pax6, which encodes a protein containing the paired and homeobox DNA-binding domains. The Pax6 gene plays a critical role for the development of eyes, central nervous system, and endocrine glands. The proximal promoter of the Optimedin A variant contains a putative Pax6 binding site in position -86/-70. Pax6 binds this site through the paired domain in vitro as judged by electrophoretic mobility shift assay. Mutations in this site eliminate Pax6 binding as well as stimulation of the Optimedin promoter activity by Pax6 in transfection experiments. Pax6 occupies the binding site in the proximal promoter in vivo as demonstrated by the chromatin immunoprecipitation assay. Altogether these results identify the Optimedin gene as a downstream target regulated by Pax6. Although the function of optimedin is still not clear, it is suggested to be involved in cell-cell adhesion and cell attachment to the extracellular matrix. Pax6 regulation of Optimedin in the eye and brain may directly affect multiple developmental processes, including cell migration and axon growth.