A Gbx homeobox gene in amphioxus: insights into ancestry of the ANTP class and evolution of the midbrain/hindbrain boundary

In the vertebrate central nervous system (CNS), mutual antagonism between posteriorly expressed Gbx2 and anteriorly expressed Otx2 positions the midbrain/hindbrain boundary (MHB), but does not induce MHB organizer genes such as En, Pax2/5/8 and Wnt1. In the CNS of the cephalochordate amphioxus, Otx is also expressed anteriorly, but En, Pax2/5/8 and Wnt1 are not expressed near the caudal limit of Otx, raising questions about the existence of an MHB organizer in amphioxus. To investigate the evolutionary origins of the MHB, we cloned the single amphioxus Gbx gene. Fluorescence in situ hybridization showed that, as in vertebrates, amphioxus Gbx and the Hox cluster are on the same chromosome. From analysis of linked genes, we argue that during evolution a single ancestral Gbx gene duplicated fourfold in vertebrates, with subsequent loss of two duplicates. Amphioxus Gbx is expressed in all germ layers in the posterior 75% of the embryo, and in the CNS, the Gbx and Otx domains abut at the boundary between the cerebral vesicle (forebrain/midbrain) and the hindbrain. Thus, the genetic machinery to position the MHB was present in the protochordate ancestors of the vertebrates, but is insufficient for induction of organizer genes. Comparison with hemichordates suggests that anterior Otx and posterior Gbx domains were probably overlapping in the ancestral deuterostome and came to abut at the MHB early in the chordate lineage before MHB organizer properties evolved.     

Three amphioxus Wnt genes (AmphiWnt3, AmphiWnt5, and AmphiWnt6) associated with the tail bud: the evolution of somitogenesis in chordates.

The amphioxus tail bud is similar to the amphibian tail bud in having an epithelial organization without a mesenchymal component. We characterize three amphioxus Wnt genes (AmphiWnt3, AmphiWnt5, and AmphiWnt6) and show that their early expression around the blastopore can subsequently be traced into the tail bud; in vertebrate embryos, there is a similar progression of expression domains for Wnt3, Wnt5, and Wnt6 genes from the blastopore lip (or its equivalent) to the tail bud. In amphioxus, AmphiWnt3, AmphiWnt5, and AmphiWnt6 are each expressed in a specific subregion of the tail bud, tentatively suggesting that a combinatorial code of developmental gene expression may help generate specific tissues during posterior elongation and somitogenesis. In spite of similarities within their tail buds, vertebrate and amphioxus embryos differ markedly in the relation between the tail bud and the nascent somites: vertebrates have a relatively extensive zone of unsegmented mesenchyme (i.e., presomitic mesoderm) intervening between the tail bud and the forming somites, whereas the amphioxus tail bud gives rise to new somites directly. It is likely that presomitic mesoderm is a vertebrate innovation made possible by developmental interconversions between epithelium and mesenchyme that first became prominent at the dawn of vertebrate evolution.     

Embryonic expression and evolutionary analysis of the amphioxus Dickkopf and Kremen family genes

The secreted Wnt signaling inhibitor Dickkopf1(Dkk1)plays key role in vertebrate head induction.Its receptor Kremen synergizes with Dkkl in Wnt inhibition.Here we have carried out expression and functional studies of the Dkk and Kremen genes in amphioxus(Branchiostoma belcheri).During embryonic and larval development,BbDkk1/2/4 is expressed in the posterior mesoendoderm,anterior somatic mesoderm and the pharyngeal regions.Its expression becomes restricted to the pharyngeal region on the left side at larval stages.In 45 h larvae,BbDkk1/2/4 is expressed specifically in the cerebral vesicle.BbDkk3 was only detected at larval stages in the mid-intestine region.Seven Kremen related genes were identified in the genome of the Florida amphioxus(Branchiostoma floridae),clustered in 4scaffolds,and are designated Kremen1-4 and Kremen-like 1-3,respectively.In B.belcheri,Kremenl is strongly expressed in the mesoendoderm during early development and Kremen3 is expressed asymmetrically in spots in the larval pharyngeal region.In luciferase reporter assays,BbDkk1/2/4 can strongly inhibit Writ signaling,while BbDkk3,BbKremen1 and BbKremen3 can not.No co-operative effect was observed between amphioxus Dkk1/2/4 and Kremens,suggesting that the interaction between Dkk and Kremen likely originated later during evolution.     

Expression of somite segmentation genes in amphioxus: a clock without a wavefront?

In the basal chordate amphioxus (Branchiostoma), somites extend the full length of the body. The anteriormost somites segment during the gastrula and neurula stages from dorsolateral grooves of the archenteron. The remaining ones pinch off, one at a time, from the tail bud. These posterior somites appear to be homologous to those of vertebrates, even though the latter pinch off from the anterior end of bands of presomitic mesoderm rather than directly from the tail bud. To gain insights into the evolution of mesodermal segmentation in chordates, we determined the expression of ten genes in nascent amphioxus somites. Five (Uncx4.1, NeuroD/atonal-related, IrxA, Pcdhdelta2-17/18, and Hey1) are expressed in stripes in the dorsolateral mesoderm at the gastrula stage and in the tail bud while three (Paraxis, Lcx, and Axin) are expressed in the posterior mesendoderm at the gastrula and neurula stages and in the tail bud at later stages. Expression of two genes (Pbx and OligA) suggests roles in the anterior somites that may be unrelated to initial segmentation. Together with previous data, our results indicate that, with the exception that Engrailed is only segmentally expressed in the anterior somites, the genetic mechanisms controlling formation of both the anterior and posterior somites are probably largely identical. Thus, the fundamental pathways for mesodermal segmentation involving Notch-Delta, Wnt/beta-catenin, and Fgf signaling were already in place in the common ancestor of amphioxus and vertebrates although budding of somites from bands of presomitic mesoderm exhibiting waves of expression of Notch, Wnt, and Fgf target genes was likely a vertebrate novelty. Given the conservation of segmentation gene expression between amphioxus and vertebrate somites, we propose that the clock mechanism may have been established in the basal chordate, while the wavefront evolved later in the vertebrate lineage.     

An amphioxus snail gene: Expression in paraxial mesoderm and neural plate suggests a conserved role in patterning the chordate embryo

 Homologs of the Drosophila snail gene have been characterized in several vertebrates. In addition to being expressed in mesoderm during gastrulation, vertebrate snail genes are also expressed in presumptive neural crest and/or its derivatives. Given that neural crest is unique to vertebrates and is considered to be of fundamental importance in their evolution, we have cloned and characterized the expression of a snail gene from amphioxus, a cephalochordate widely accepted as the sister group of the vertebrates. We show that, at the amino acid sequence level, the amphioxus snail gene is a clear phylogenetic outgroup to all the characterized vertebrate snail genes. During embryogenesis snail expression initially becomes restricted to the paraxial or presomitic mesoderm of amphioxus. Later, snail is expressed at high levels in the lateral neural plate, where it persists during neurulation. Our results indicate that an ancestral function of snail genes in the lineage leading to vertebrates is to define the paraxial mesoderm. Furthermore, our results indicate that a cell population homologous to the vertebrate neural crest may be present in amphioxus, thus providing an important link in the evolution of this key vertebrate tissue. Received: 11 May 1998 / Accepted: 2 August 1998     

The single amphioxus Mox gene: insights into the functional evolution of Mox genes,somites, and the asymmetry of amphioxus somitogenesis

Mox genes are members of the "extended" Hox-cluster group of Antennapedia-like homeobox genes. Homologues have been cloned from both invertebrate and vertebrate species, and are expressed in mesodermal tissues. In vertebrates, Mox1 and Mox2 are distinctly expressed during the formation of somites and differentiation of their derivatives. Somites are a distinguishing feature uniquely shared by cephalochordates and vertebrates. Here, we report the cloning and expression of the single amphioxus Mox gene. AmphiMox is expressed in the presomitic mesoderm (PSM) during early amphioxus somitogenesis and in nascent somites from the tail bud during the late phase. Once a somite is completely formed, AmphiMox is rapidly downregulated. We discuss the presence and extent of the PSM in both phases of amphioxus somitogenesis. We also propose a scenario for the functional evolution of Mox genes within chordates, in which Mox was co-opted for somite formation before the cephalochordate-vertebrate split. Novel expression sites found in vertebrates after somite formation postdated Mox duplication in the vertebrate stem lineage, and may be linked to the increase in complexity of vertebrate somites and their derivatives, e.g., the vertebrae. Furthermore, AmphiMox expression adds new data into a long-standing debate on the extent of the asymmetry of amphioxus somitogenesis.     

Characterization of Amphioxus AmphiVent, an evolutionarily conserved marker for chordate ventral mesoderm

Structure and developmental expression are described for amphioxus AmphiVent, a homolog of vertebrate Vent genes. In amphioxus, AmphiVent-expressing ventral mesoderm arises at midneurula by outgrowth from the paraxial mesoderm, but in vertebrates, Vent-expressing ventral mesoderm originates earlier, at the gastrula stage. In other embryonic tissues (nascent paraxial mesoderm, neural plate, endoderm, and tailbud), AmphiVent and its vertebrate homologs are expressed in similar spatiotemporal domains, indicating conservation of many Vent gene functions during chordate evolution. The ventral mesoderm evidently develops precociously in vertebrates because their relatively large embryos probably require an early and extensive deployment of the mesoderm-derived circulatory system. The vertebrate ventral mesoderm, in spite of its strikingly early advent, still resembles the nascent ventral mesoderm of amphioxus in expressing Vent homologs. This coincidence may indicate that Vent homologs in vertebrates and amphioxus play comparable roles in ventral mesoderm specification.     

文昌鱼tropomyosin基因的克隆、进化分析及其胚胎发育与成体中的表达模式

Tropomyosin是一种分布广泛而且在进化上十分保守的蛋白,是肌肉形成和收缩过程中重要的调节蛋白质。通过RT-PCR和RACE技术得到文昌鱼tropomyosin基因全长,编码一个含284个氨基酸残基的蛋白质,将文昌鱼Tropomyosin和在其他物种中的同源物进行比对建树,发现其在功能域上高度保守并且只有一个拷贝,符合动物分类学中各物种的进化地位。胚胎整体原位杂交实验得知,tropomyosin在文昌鱼早期发育的表达,最早从原肠胚末期神经胚早期开始,定位于分化中的中内胚层。到神经胚期,tropomyosin的表达出现在发育中的体节和脊索中。随着发育的进行,tropomyosin的表达稳定地集中在体节、脊索处。到72h幼虫阶段,tropomyosin的表达仍然在肌节内。成体的切片原位杂交结果显示,tropomyosin在肌节中的表达大幅度下调,而在神经管细胞、脊索和腮区腮瓣处仍然可以检测到明显的表达,在外胚层和表皮内没有发现杂交信号。研究结果表明,tropomyosin的表达与文昌鱼肌节、肌肉以及神经索的发生相关,参与文昌鱼胚胎躯体模式的构建,而且在成体的生命活动中发挥重要作用。     

Characterization and expression of amphioxus ApoD gene encoding an archetype of vertebrate ApoD proteins

Here we report a homologue of the apolipoprotein D gene (AmphiApoD) in amphioxus, Branchiostoma belcheri tsingtauense, the first such finding in a basal chordate cephalochordate. The main features of the protein predicted from AmphiApoD are characteristic of the apolipoprotein D. Phylogenetic analysis places AmphiApoD at the base of the phylogenetic tree, suggesting that AmphiApoD is the archetype of the vertebrate ApoD genes. Both whole mount in situ hybridization and Northern blotting and RT-PCR as well as in situ hybridization histochemistry reveal that AmphiApoD is expressed in tissues derived from mesoderm and endoderm including notochord and hind-gut, which contrasts with the strong expression patterns of ApoD genes in the ectodermal derivatives in mammals and birds. The expression profiles of the ApoD gene may have been changed to be expressed in the endo-mesodermal derivatives in amphioxus after the vertebrate and cephalochordate lineages diverged; alternatively, the ApoD gene may first have been expressed in the endo-mesoderm during embryogenesis in the last common ancestor of all chordates, and subsequently came to be expressed in the ectodermal derivatives of vertebrates including mammals and birds.     

Retinoic acid signaling acts via Hox1 to establish the posterior limit of the pharynx in the chordate amphioxus

In the invertebrate chordate amphioxus, as in vertebrates, retinoic acid (RA) specifies position along the anterior/posterior axis with elevated RA signaling in the middle third of the endoderm setting the posterior limit of the pharynx. Here we show that AmphiHox1 is also expressed in the middle third of the developing amphioxus endoderm and is activated by RA signaling. Knockdown of AmphiHox1 function with an antisense morpholino oligonucleotide shows that AmphiHox1 mediates the role of RA signaling in setting the posterior limit of the pharynx by repressing expression of pharyngeal markers in the posterior foregut/midgut endoderm. The spatiotemporal expression of these endodermal genes in embryos treated with RA or the RA antagonist BMS009 indicates that Pax1/9, Pitx and Notch are probably more upstream than Otx and Nodal in the hierarchy of genes repressed by RA signaling. This work highlights the potential of amphioxus, a genomically simple, vertebrate-like invertebrate chordate, as a paradigm for understanding gene hierarchies similar to the more complex ones of vertebrates.     

Characterization of amphioxus AmphiWnt8: insights into the evolution of patterning of the embryonic dorsoventral axis

SUMMARY The full-length sequence and developmental expression of an amphioxus Wnt gene ( AmphiWnt8 ) are described. In amphioxus embryos, the expression patterns of AmphiWnt8 suggest patterning roles in the forebrain, in the hindgut, and in the paraxial mesoderm that gives rise to the muscular somites. Phylogenetic analysis indicates that a single Wnt8 subfamily gene in an ancestral chordate duplicated early in vertebrate evolution into a Wnt8 clade and a Wnt8b clade. Coincident with this gene duplication, the functions of the ancestral AmphiWnt8 -like gene appear to have been divided between vertebrate Wnt8b (exclusively neurogenic, especially in the forebrain) and vertebrate Wnt8 (miscellaneous, especially in early somitogenesis). Amphioxus AmphiWnt8 and its vertebrate Wnt8 homologs probably play comparable roles in the early dorsoventral patterning of the embryonic body axis.     

Early expressed genes showing a dichotomous developing pattern in the lancelet embryo

Lancelets (amphioxus), although showing the most similar anatomical features to vertebrates, never develop a vertebrate-like head but rather several structures specific to this animal. The lancelet anatomical specificity seems to be traceable to early developmental stages, such as the vertebrate dorsal and anterior-posterior determinations. The BMP and Wnt proteins play important roles in establishing the early basis of the dorsal structures and the head in vertebrates. The early behavior of BMP and Wnt may be also related to the specific body structures of lancelets. The expression patterns of a dpp-related gene, Bbbmp2/4, and two wnt-related genes, Bbwnt7 and Bbwnt8, have been studied in comparison with those of brachyury and Hnf-3beta class genes. The temporal expression patterns of these genes are similar to those of vertebrates; Bbbmp2/4 and Bbwnt8 are first expressed in the invaginating primitive gut and the equatorial region, respectively, at the initial gastrula stage. However, spatial expression pattern of Bbbmp2/4 differs significantly from the vertebrate cognates. It is expressed in the mid-dorsal inner layer of gastrulae and widely in the anterior region, in which vertebrates block BMP signaling. The present study suggests that the lancelet embryo may have two distinct developmental domains from the gastrula stage, the domains of which coincide later with the lateral diverticular and the somitocoelomic regions. The embryonic origin of the anterior-specific structures in lancelets corresponds to the anterior domain where Bbbmp2/4 is continuously expressed.