青岛文昌鱼hedgehog基因表达图式的研究

Ｈｅｄｇｅｈｏｇ（ｈｈ）家族基因编码一类分泌性信号分子,在果蝇的体节和成虫盘等的图式形成和脊椎动物脊索、神经管、体节和肢等的发育中起着关键作用．探讨原索动物文昌鱼ｈｈ基因的功能,对于研究脊椎动物发育机制的进化具有重要意义．本文报道了用整体原位杂交方法,研究不同发育时期文昌鱼胚胎和幼体ｈｈ基因表达图式的结果,并对文昌鱼ｈｈ基因的功能和ｈｈ家族同源基因功能的演化进行了讨论．文昌鱼ｈｈ基因在脊索和神经管中的表达图式与脊椎动物Ｓｈｈ基因相似,其功能可能是介导脊索的诱导．     

Protochordate amphioxus is an emerging model organism for comparative immunology

Protochordate amphioxus is an extant invertebrate regarded quite recently as a basal chordate. It has a vertebrate-like body plan including a circulation system with an organization similar to that of vertebrates. However, amphioxus is less complex than vertebrates for having a genome uncomplicated by extensive genomic duplication, and lacking lymphoid organs and free circulating blood cells. Recent studies on immunity have demonstrated the presence in amphioxus of both the constituent elements of key molecules involved in adaptive immunity such as proto-major histocompatibility complex (proto-MHC), V region-containing chitin-binding protein (VCBP) and V and C domain-bearing protein (VCP), and the complement system operating via the alternative and lectin pathways resembling those seen in vertebrates. In addition, the acute phase response profile in amphioxus has been shown to be similar to that observed in vertebrates. These findings together with the relative structural and genomic simplicity make amphioxus an ideal organism for gaining insights into the origin and evolution of the vertebrate immune system, especially adaptive immunity, and the composition and mechanisms of the vertebrate innate immunity.     

Evolutionary biology: lamprey Hox genes and the origin of jaws

The development of jaws was a critical event in vertebrate evolution because it ushered in a transition to a predatory lifestyle, but how this innovation came about has been a mystery. In the embryos of jawed vertebrates (gnathostomes), the jaw cartilage develops from the mandibular arch, where none of the Hox genes is expressed; if these are expressed ectopically, however, jaw development is inhibited. Here I show that in the lamprey, a primitively jawless (agnathan) fish that is a sister group to the gnathostomes, a Hox gene is expressed in the mandibular arch of developing embryos. This finding, together with outgroup comparisons, suggests that loss of Hox expression from the mandibular arch of gnathostomes may have facilitated the evolution of jaws.     

Hox cluster disintegration with persistent anteroposterior order of expression in Oikopleura dioica

Tunicate embryos and larvae have small cell numbers and simple anatomical features in comparison with other chordates, including vertebrates. Although they branch near the base of chordate phylogenetic trees, their degree of divergence from the common chordate ancestor remains difficult to evaluate. Here we show that the tunicate Oikopleura dioica has a complement of nine Hox genes in which all central genes are lacking but a full vertebrate-like set of posterior genes is present. In contrast to all bilaterians studied so far, Hox genes are not clustered in the Oikopleura genome. Their expression occurs mostly in the tail, with some tissue preference, and a strong partition of expression domains in the nerve cord, in the notochord and in the muscle. In each tissue of the tail, the anteroposterior order of Hox gene expression evokes spatial collinearity, with several alterations. We propose a relationship between the Hox cluster breakdown, the separation of Hox expression domains, and a transition to a determinative mode of development.     

Head and backbone of the Early Cambrian vertebrate Haikouichthys

Agnathan fish hold a key position in vertebrate evolution, especially regarding the origin of the head and neural-crest-derived tissue. In contrast to amphioxus, lampreys and other vertebrates possess a complex brain and placodes that contribute to well-developed eyes, as well as auditory and olfactory systems. These sensory sytems were arguably a trigger to subsequent vertebrate diversifications. However, although they are known from skeletal impressions in younger Palaeozoic agnathans, information about the earliest records of these systems has been largely wanting. Here we report numerous specimens of the Lower Cambrian vertebrate Haikouichthys ercaicunensis, until now only known from the holotype. Haikouichthys shows significant differences from other fossil agnathans: key features include a small lobate extension to the head, with eyes and possible nasal sacs, as well as what may be otic capsules. A notochord with separate vertebral elements is also identifiable. Phylogenetic analysis indicates that this fish lies within the stem-group craniates. Although Haikouichthys somewhat resembles the ammocoete larva of modern lampreys, this is because of shared general craniate characters; adult lampreys and hagfishes (the cyclostomes if monophyletic) are probably derived in many respects.     

Deformed autoregulatory element from Drosophila functions in a conserved manner in transgenic mice.

The striking similarities in the structure, organization and anterior-posterior expression patterns between the murine Hox gene system and the Drosophila homeotic gene complexes, called HOM-C (ref. 3), may point to highly conserved mechanisms for specifying positional identities (reviewed in ref. 4). Strong support for this concept lies in the observation of conserved colinearity between the genomic order of the Hox/HOM genes and their unique successive expression domains along the anterior-posterior axes of both mouse and fly embryos. These unique and precise expression patterns appear to be facilitated by multiple cis-regulatory elements (reviewed in ref. 5). One of the few elements characterized in detail is the autoregulatory enhancer of the homeotic gene Deformed (Dfd), which supports expression in subregions of posterior head segments of Drosophila embryos. Here we present evidence that this enhancer is capable of conferring reporter gene expression to a discrete subregion of the hindbrain in transgenic mouse embryos. Remarkably, this anterior-posterior subregion lies within the common anterior expression domain of the Dfd cognate Hox genes in the postotic hindbrain. Our results indicate that the Dfd autoregulatory enhancer is part of a highly conserved mechanism for establishing region-specific gene expression along the anterior-posterior axis of the embryo.     

Ancient deuterostome origins of vertebrate brain signalling centres

Neuroectodermal signalling centres induce and pattern many novel vertebrate brain structures but are absent, or divergent, in invertebrate chordates. This has led to the idea that signalling-centre genetic programs were first assembled in stem vertebrates and potentially drove morphological innovations of the brain. However, this scenario presumes that extant cephalochordates accurately represent ancestral chordate characters, which has not been tested using close chordate outgroups. Here we report that genetic programs homologous to three vertebrate signalling centres-the anterior neural ridge, zona limitans intrathalamica and isthmic organizer-are present in the hemichordate Saccoglossus kowalevskii. Fgf8/17/18 (a single gene homologous to vertebrate Fgf8, Fgf17 and Fgf18), sfrp1/5, hh and wnt1 are expressed in vertebrate-like arrangements in hemichordate ectoderm, and homologous genetic mechanisms regulate ectodermal patterning in both animals. We propose that these genetic programs were components of an unexpectedly complex, ancient genetic regulatory scaffold for deuterostome body patterning that degenerated in amphioxus and ascidians, but was retained to pattern divergent structures in hemichordates and vertebrates.     

青岛文昌鱼碱性肌球蛋白轻链MLC-alk基因片段的克隆

碱性肌球蛋白轻链是构成球蛋白头部的必需分子，采用简并引物PCR方法获得青岛文昌鱼碱性肌球轻链基因片段，并对其氨基酸序列与其他生物如鸡和人的胚胎裂、骨骼肌型、平滑肌型、心肌型、非肌型等多种碱性肌球蛋白轻链基因家庭成员相应片段进行了同源性分析，均显示较高的同源性，研究结果支持青岛文昌鱼具有1个，可能也仅有1个碱性肌球蛋白轻链基因，碱性肌球蛋白轻链基因原倍增可能发生在脊椎动物与文昌鱼在进化中分支之后。     

A distinct Hox code for the branchial region of the vertebrate head.

The branchial region of the vertebrate head forms through complex interactions involving rhombomeric segments, neural crest and branchial arches. It is though that aspects of their patterning mechanisms are linked and involve Hox-2 genes, whose overlapping and spatially restricted expression domains represent a combinatorial code for generating regional diversity. Vertebrates possess four Hox clusters of Antennapedia class homeobox genes, related to each other by duplication and divergence from a common ancestral complex. In consequence, at equivalent positions in different clusters there are highly related genes known as subfamilies or paralogous groups. As Hox-2 genes cannot fully account for patterning individual rhombomeres, we investigated whether offsets in expression limits of paralogous genes could account for the generation of regional diversity. We report here that, with the exception of the labial subfamily, paralogues show identical expression limits in rhombomeres, cranial ganglia and branchial arches, providing a combinatorial Hox code for the branchial region that seems to be different in organization to that of the trunk.     

Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements

Bilaterian animals have a Hox gene cluster essential for patterning the main body axis, and a ParaHox gene cluster. Comparison of Hox and ParaHox genes has led workers to postulate that both clusters originated from the duplication of an ancient cluster named ProtoHox, which contained up to four genes with at least the precursors of anterior and posterior Hox/ParaHox genes. However, the way in which genes diversified within the ProtoHox, Hox and ParaHox clusters remains unclear because no systematic study of non-bilaterian animals exists. Here we characterize the full Hox/ParaHox gene complements and genomic organization in two cnidarian species (Nematostella vectensis and Hydra magnipapillata), and suggest a ProtoHox cluster simpler than originally thought on the basis of three arguments. First, both species possess bilaterian-like anterior Hox genes, but their non-anterior genes do not appear as counterparts of either bilaterian central or posterior genes; second, two clustered ParaHox genes, Gsx and a gene related to Xlox and Cdx, are found in Nematostella vectensis; and third, we do not find clear phylogenetic support for a common origin of bilaterian Cdx and posterior genes, which might therefore have appeared after the ProtoHox cluster duplication. Consequently, the ProtoHox cluster might have consisted of only two anterior genes. Non-anterior genes could have appeared independently in the Hox and ParaHox clusters, possibly after the separation of bilaterians and cnidarians.     

Segmental expression of Hox-2 homoeobox-containing genes in the developing mouse hindbrain

The vertebrate hindbrain develops in a segmental pattern, with distinctive groups of neurons originating from different segments. We report here that members of the Hox-2 cluster of murine homoeobox genes are expressed in segment-specific patterns in the developing hindbrain, with successive genes having boundaries at two-segment intervals. These data indicate that Hox genes specify segment phenotype, a role analogous to that of their Drosophila homologues.     

Early developmental arrest of mammalian limbs lacking HoxA/HoxD gene function

Vertebrate HoxA and HoxD cluster genes are required for proper limb development. However, early lethality, compensation and redundancy have made a full assessment of their function difficult. Here we describe mice that are lacking all Hoxa and Hoxd functions in their forelimbs. We show that such limbs are arrested early in their developmental patterning and display severe truncations of distal elements, partly owing to the absence of Sonic hedgehog expression. These results indicate that the evolutionary recruitment of Hox gene function into growing appendages might have been crucial in implementing hedgehog signalling, subsequently leading to the distal extension of tetrapod appendages. Accordingly, these mutant limbs may be reminiscent of an ancestral trunk extension, related to that proposed for arthropods.     

Axial patterning in cephalochordates and the evolution of the organizer

The organizer of the vertebrate gastrula is an important signalling centre that induces and patterns dorsal axial structures. Although a topic of long-standing interest, the evolutionary origin of the organizer remains unclear. Here we show that the gastrula of the cephalochordate amphioxus expresses dorsal/ventral (D/V) patterning genes (for example, bone morphogenetic proteins (BMPs), Nodal and their antagonists) in patterns reminiscent of those of their vertebrate orthlogues, and that amphioxus embryos, like those of vertebrates, are ventralized by exogenous BMP protein. In addition, Wnt-antagonists (for example, Dkks and sFRP2-like) are expressed anteriorly, whereas Wnt genes themselves are expressed posteriorly, consistent with a role for Wnt signalling in anterior/posterior (A/P) patterning. These results suggest evolutionary conservation of the mechanisms for both D/V and A/P patterning of the early gastrula. In light of recent phylogenetic analyses placing cephalochordates basally in the chordate lineage, we propose that separate signalling centres for patterning the D/V and A/P axes may be an ancestral chordate character.     

Hagfish embryology with reference to the evolution of the neural crest

Hagfish, which lack both jaws and vertebrae, have long been the subject of intense interest owing to their position at a crucial point in the evolutionary transition to a truly vertebrate body plan. However, unlike the comparatively well characterized vertebrate agnathan lamprey, little is known about hagfish development. The inability to analyse hagfish at early embryonic stages has frustrated attempts to resolve questions with important phylogenetic implications, including fundamental ones relating to the emergence of the neural crest. Here we report the obtainment of multiple pharyngula-stage embryos of the hagfish species Eptatretus burgeri and our preliminary analyses of their early development. We present histological evidence of putative neural crest cells, which appear as delaminated cells that migrate along pathways corresponding to neural crest cells in fish and amphibians. Molecular cloning studies further revealed the expression of several regulatory genes, including cognates of Pax6, Pax3/7, SoxEa and Sox9, suggesting that the hagfish neural crest is specified by molecular mechanisms that are general to vertebrates. We propose that the neural crest emerged as a population of de-epithelialized migratory cells in a common vertebrate ancestor, and suggest that the possibility of classical and molecular embryology in hagfish opens up new approaches to clarifying the evolutionary history of vertebrates.     

A distal enhancer and an ultraconserved exon are derived from a novel retroposon

Hundreds of highly conserved distal cis-regulatory elements have been characterized so far in vertebrate genomes. Many thousands more are predicted on the basis of comparative genomics. However, in stark contrast to the genes that they regulate, in invertebrates virtually none of these regions can be traced by using sequence similarity, leaving their evolutionary origins obscure. Here we show that a class of conserved, primarily non-coding regions in tetrapods originated from a previously unknown short interspersed repetitive element (SINE) retroposon family that was active in the Sarcopterygii (lobe-finned fishes and terrestrial vertebrates) in the Silurian period at least 410 million years ago (ref. 4), and seems to be recently active in the 'living fossil' Indonesian coelacanth, Latimeria menadoensis. Using a mouse enhancer assay we show that one copy, 0.5 million bases from the neuro-developmental gene ISL1, is an enhancer that recapitulates multiple aspects of Isl1 expression patterns. Several other copies represent new, possibly regulatory, alternatively spliced exons in the middle of pre-existing Sarcopterygian genes. One of these, a more than 200-base-pair ultraconserved region, 100% identical in mammals, and 80% identical to the coelacanth SINE, contains a 31-amino-acid-residue alternatively spliced exon of the messenger RNA processing gene PCBP2 (ref. 6). These add to a growing list of examples in which relics of transposable elements have acquired a function that serves their host, a process termed 'exaptation', and provide an origin for at least some of the many highly conserved vertebrate-specific genomic sequences.     

Maintenance of functional equivalence during paralogous Hox gene evolution

Biological diversity is driven mainly by gene duplication followed by mutation and selection. This divergence in either regulatory or protein-coding sequences can result in quite different biological functions for even closely related genes. This concept is exemplified by the mammalian Hox gene complex, a group of 39 genes which are located on 4 linkage groups, dispersed on 4 chromosomes. The evolution of this complex began with amplification in cis of a primordial Hox gene to produce 13 members, followed by duplications in trans of much of the entire unit. As a consequence, Hox genes that occupy the same relative position along the 5' to 3' chromosomal coordinate (trans-paralogous genes) share more similarity in sequence and expression pattern than do adjacent Hox genes on the same chromosome. Studies in mice indicate that although individual family members may have unique biological roles, they also share overlapping functions with their paralogues. Here we show that the proteins encoded by the paralogous genes, Hoxa3 and Hoxd3, can carry out identical biological functions, and that the different roles attributed to these genes are the result of quantitative modulations in gene expression.