Expression pattern of the Brachyury gene in the arrow worm paraspadella gotoi (chaetognatha)

Arrow worms (the phylum Chaetognatha), which are among the major marine planktonic animals, are direct developers and exhibit features characteristic of both deuterostomes and protostomes. In particular, the embryonic development of arrow worms appears to be of the deuterostome type. Brachyury functions critically in the formation of the notochord in chordates, whereas the gene is expressed in both the blastopore and stomodeum invagination regions in embryos of hemichordates and echinoderms. Here we analyzed the expression of Brachyury (Pg-Bra) in the arrow worm Paraspadella gotoi and showed that Pg-Bra is expressed in the blastopore region and the stomodeum region in the embryo and then around the mouth opening region at the time of hatching. The expression of Pg-Bra in the embryo resembles that of Brachyury in embryos of hemichordates and echinoderms, whereas that in the mouth opening region in the hatchling appears to be novel.     

Vizualizing the morphogen adsorption gradient in the <Emphasis Type="Italic">Xenopus laevis</Emphasis> embryo using fluorescently labeled heparin-binding motif of BMP4 morphogen

Spatial distribution of heparan sulfates in the embryonic extracellular matrix at midgastrula stage has been demonstrated in the Xenopus laevis embryo model. Towards this end, fluorescently labeled fusion protein EGFP-hbmBMP4 made up by green fluorescent protein (EGFP) and heparin-binding motif of Bone Morphogenetic Protein 4 (BMP4) was produced in the E. coli expression system. Xenopus laevis embryos at midgastrula stage (stage 11) were fixed and cut along the anteroposterior axis and then incubated with EGFP-hbmBMP4. The fluorescently labeled samples were analyzed in fluorescence microscope. The spatial distribution of fluorescence intensity reflecting BMP4 adsorption on the embryonic extracellular matrix proved to be similar to the corresponding distribution pattern for the Noggin1 heparin-binding motif obtained previously. The highest intensity zone was detected around the dorsal blastopore lip; another high intensity zone, although slightly less prominent, was observed in the ventral blastopore lip region. Since on one hand, heparin-binding sites significantly differ in their organization in BMP4 and Noggin1 proteins and, on the other hand, spatial adsorption distribution patterns for these proteins coincide in the embryo, it appears that all secreted morphogens containing a heparin-binding site share a single adsorption gradient in the embryonic extracellular space.     

千年笛鲷早期发育及视蛋白基因表达规律分析

初步观察千年笛鲷早期发育各个时期的形态特征, 同时使用实时荧光定量PCR方法对4种视蛋白基因在早期发育中的表达规律进行分析。研究观察到千年笛鲷卵为圆球形, 属浮性卵, 中心有一明显的油球。在水温(24.50.5)℃的条件下, 千年笛鲷胚胎发育共经历6个发育阶段18个时期, 从受精卵到孵化一共经历24h。仔鱼经历1215d发育为稚鱼, 30d35d发育为幼鱼。同时对7个胚胎发育时期和2个仔鱼发育时期4种视蛋白(LWS、SWS1、SWS2、RH)基因的表达情况进行检测, 在下包1/2、胚孔封闭、视囊这3个时期有显著性表达(P0.05), 尤其在胚孔封闭时期, 表达量达到最高。其余时期4种基因的表达水平显著下降, 但在2个仔鱼时期表达量比孵化期略有增加。结果表明千年笛鲷4种视蛋白基因在早期表达过程中与神经胚的形成有密切的联系。       

Blastopore formation and dorsal mesoderm induction are independent events in early Cynops embryogenesis

The independent roles of blastopore formation and dorsal mesoderm induction in dorsal axis formation of the Cynops pyrrhogaster embryo were attempted to be clarified. The blastopore-forming (bottle) cells originated mainly from the progeny of the mid-dorsal C and/or D blastomeres of the 32-cell embryo, but were not defined to a fixed blastomere. It was confirmed that the isolated dorsal C and D blastomeres autonomously formed a blastopore. Ultraviolet-irradiated eggs formed an abnormal blastopore and then did not form a dorsal axis, although the lower dorsal marginal zone (LDMZ) still had dorsal mesoderm-inducing activity. Involution of the dorsal marginal zone was disturbed by the abnormal blastopore. These embryos were rescued by artificially facilitating involution of the dorsal marginal zone. Suramin-injected and nocodazole-treated blastulae did not have involution of the dorsal marginal zone, although the blastopore was formed. Neither embryos formed the dorsal axis. The dorsal mesoderm-inducing activity of the LDMZ in the nocodazole-treated gastrulae was still active. In contrast, the LDMZ of the suramin-injected embryos lost its dorsal mesoderm-inducing activity. bra expression was activated in the nocodazole-treated embryos but not in the suramin-injected embryos. The present study suggested that (i) the dorsal determinants consist of blastopore-forming and dorsal mesoderm-inducing factors, which are not always mutually dependent; (ii) both factors are activated during the late blastula stage; (iii) the dorsal marginal zone cannot specify to an organized notochord and muscle without the involution that blastopore formation leads to; and (iv) the localization of both factors in the same place is prerequisite for dorsal axis formation.     

Origin of bilateral-symmetrical animals (Bilateria)

The paper is an attempt to attack the old problem of the origin of Bilateria by the methods of evolutionary tetrad (i.e. combination of comparative anatomy, comparative embryology, paleontology, and molecular biology). Three groups of theories of classical comparative anatomy (planulod-turbellarian, archicoelomate, and metameric) are discussed. Comparative embryology brings out clearly that the ventral side of embryo comes from the blastoporal region in all groups of Bilateria (except Chordata, where the blastoporal region corresponds to the dorsal side that is come out of the upside-down morphology of chordates) and mouth and anus comes from the anterior and posterior ends of elongated blastopore. From the point of view of paleontology, some of vendian metazoans demonstrate transitional conditions between the Radiata and Bilateria. Vendian bilaterians are metameric organisms with normal or asymmetric position of segments and could be pictured as "bilateral coelenterates" creeping on the oral surface. In Cnidaria, the expression of homologues of "Brachyury", "goosecoid", and "fork head" genes are revealed in the circular region around the mouth. In Bilateria, these genes are expressed along the elongated blastopore and around the mouth and anus. These results support the old conception on the amphistomic origin of mouth and anus as well as the homology between the oral disc of cnidarians and ventral side of Bilateria. The combination of four mentioned approaches enables us to propose the conception of the origin of Bilateria from vendian bilateral coelenterates with numerous metameric pouches of gastral cavity. Bilaterian ancestors crawled on the oral disc (= ventral side). These ancestors gave rise to both phanerosoic cnidarians and triploblastic bilaterians. Cnidarian ancestors attached to bottom by the aboral pole with the resulting degradation of aboral nerve ganglion. Bilateral symmetry of anthozoans is considered to be primitive feature for cnidarians. In case of triploblastic Bilateria, the elongated blastopore closed in the middle and subdivided into mouth and anus (amphistomy) and gastral pouches separated from the central part of gastral cavity and transformed to metameric coelomic chambers. The primary bilaterians are supposed to be complicated organisms having coelom and segmentation. The complexity of primary Bilateria provides an explanation for the abundance of highly organized organisms (arthropods, mollusks etc.) in Cambrian time. It is postulated that Ctenophora is the only group recent eumetazoans with primary axial symmetry.     

Development of blastomere clones in the Ilyanassa embryo: transformation of the spiralian blastula into the larval body plan

Spiralian embryogenesis is deeply conserved and seems to have been in place in the last common ancestor of the large assemblage of protostome phyla known as the Lophotrochozoa. While the blastula fate maps of several spiralian embryos have been determined, little is known about the events that link the early embryo and the larva. For all cells in the Ilyanassa blastula, we determined the clonal morphology at four time points between the blastula and veliger stages. We found that ectomesoderm comes mostly from 3a and 3b, but also from 2c and 2b. We also observed the ingression and early proliferation of 3a- and 3b-derived ectomesoderm. We found cells in the 2b clone that marked the anterior edge of the blastopore and later the mouth and cells in the 3c/3d clones that marked the posterior edges of these structures. This demonstrates directly that the mouth forms in the same location as the blastopore. In the development of the shell field, we observed dramatic cell migration events that invert the positions of the 2b and 2d clones that contribute to the shell. Using time-lapse imaging, we followed and described the cleavage pattern of the conserved endomesodermal blast cell, 4d, up to 4d?+?45 h, when there were 52 cells in the clone. Our results show the growth and movement of clones derived from cells of the spiralian blastula as they transform into the trochophore-like and veliger stages. They have implications for the evolution of the shell in gastropods, the origins of mesoderm in spiralians, and the evolution of mouth formation in metazoans.     

Very early and transient vegetal-plate expression of SpKrox1, a Krüppel/Krox gene from Strongylocentrotus purpuratus

All endodermal and mesenchymal cells of the sea urchin embryo descend from the vegetal plate, a thickened epithelium of approximately 50 cells arising at the early blastula stage. Cell types that derive from the vegetal plate are specified conditionally by inductive interactions with underlying micromeres, but the molecular details of vegetal-plate specification remain unresolved. In a search for regulatory proteins that have roles in vegetal-plate specification, a screen was performed to clone Krüppel/Krox-related genes from a Strongylocentrotus purpuratus embryo cDNA library. One newly identified clone, named SpKrox1, contained four zinc fingers and a leucine zipper domain. SpKrox1 expression was low in unfertilized eggs, increased severalfold to the early blastula stage and decreased between the early gastrula and pluteus stages. SpKrox1 mRNA was first seen in macromeres of 16-cell stage embryos and was restricted to cells of the developing vegetal plate thereafter. Vegetal-plate expression corresponded to a ring of cells around the blastopore and overlapped the expression patterns of other genes with potential roles in vegetal plate-specification. As the vegetal-plate cells invaginated into the blastopore, SpKrox1 expression was lost, suggesting that its role was not in endoderm differentiation per se but rather in the initial establishment of the vegetal plate.     

Evolution of deuterostomy – and origin of the chordates

The chordates are usually characterized as bilaterians showing deuterostomy, i.e. the mouth developing as a new opening between the archenteron and the ectoderm, serial gill pores/slits, and the complex of chorda and neural tube. Both numerous molecular studies and studies of morphology and embryology demonstrate that the neural tube must be considered homologous to the ventral nerve cord(s) of the protostomes, but the origin of the ‘new’ mouth of the deuterostomes has remained enigmatic. However, deuterostomy is known to occur in several protostomian groups, such as the chaetognaths and representatives of annelids, molluscs, arthropods and priapulans. This raises the question whether the deuterostomian mouth is in fact homologous with that of the protostomes, viz. the anterior opening of the ancestral blastopore divided through lateral blastopore fusion, i.e. amphistomy. A few studies of gene expression show identical expression patterns around mouth and anus in protostomes and deuterostomes. Closer studies of the embryology of ascidians and vertebrates show that the mouth/stomodaeum differentiates from the anterior edge of the neural plate. Together this indicates that the chordate mouth has moved to the anterior edge of the blastopore, so that the anterior loop of the ancestral circumblastoporal nerve cord, which is narrow in the protostomes, has become indistinguishable. In the vertebrates, the mouth has moved further around the anterior pole to the ‘ventral’ side. The conclusion must be that the chordate mouth (and that of the deuterostomes in general) is homologous to the protostomian mouth and that the latest common ancestor of protostomes and deuterostomes developed through amphistomy, as suggested by the trochaea theory.     

Constitutive and stress‐inducible expression of the endoplasmic reticulum heat shock protein 70 gene family member,immunoglobulin‐binding protein (BiP), during Xenopus laevis early development

We have characterized the constitutive and stress‐inducible pattern of immunoglobulin‐binding protein (BiP) gene expression during Xenopus early development. Whole mount in situ hybridization analysis revealed that BiP mRNA was detected in unfertilized eggs, cleavage and blastula stage embryos. In gastrulae, BiP mRNA was present across the surface of the embryo, while in neurulae BiP mRNA was enriched in the neural plate, neural fold, and around the blastopore. In early and late tailbud embryos, BiP mRNA was found primarily in the dorsal region. Tunicamycin and A23187, the calcium ionophore, enhanced BiP mRNA accumulation first at the neurula stage, while heat shock induced BiP mRNA accumulation first at the gastrula stage. Compared to control, A23187‐ and heat shock‐treated neurulae displayed relatively high levels of BiP mRNA in selected tissues, including the neural plate, neural folds, around the blastopore, and ectoderm. At the early tailbud stage, A23187 and heat shock enhanced BiP mRNA accumulation primarily in the head, somites, tail, and along the spinal cord. A similar situation was found with A23187‐ and heat shock‐treated late tailbud embryos, except that heat‐shocked embryos also displayed enhanced BiP mRNA accumulation in the epidermis. These studies demonstrate a preferential accumulation of BiP mRNA in selected tissues during development and in response to stress. Dev. Genet. 25:31–39, 1999. © 1999 Wiley‐Liss, Inc.     

Larval and adult brains

Apical organs are a well-known structure in almost all ciliated eumetazoan larvae, although their function is poorly known. A review of the literature indicates that this small ganglion is the "brain" of the early larva, and it seems probable that it represents the brain of the ancestral, holopelagic ancestor of all eumetazoans, the gastraea. This early brain is lost before or at metamorphosis in all groups. Protostomes (excluding phoronids and brachiopods) appear to have brains of dual origin. Their larvae develop a pair of cephalic ganglia at the episphere lateral to the apical organ, and these two ganglia become an important part of the adult brain. The episphere and the cerebral ganglia show Otx expression, whereas Hox gene expression has not been seen in this part of the brain. A ventral nervous system develops around the blastopore, which becomes divided into mouth and anus by fusion of the lateral blastopore lips. The circumblastoporal nerve ring becomes differentiated into a nerve ring around the mouth, becoming part of the adult brain, a pair of ventral nerve cords, in some cases differentiated into a chain of ganglia, and a ring around the anus. This part of the nervous system appears to be homologous with the oral nerve ring of cnidarians. This interpretation is supported by the expression of Hox genes around the cnidarian mouth and in the ventral nervous system of the protostomes. The development of phoronids, brachiopods, echinoderms, and enteropneusts does not lead to the formation of an episphere or to differentiation of cerebral ganglia. In general, a well-defined brain is lacking, and Hox genes are generally not expressed in the larval organs, although this has not been well studied.     

Embryonic development of Trichocephalus trichiurus and Eucoleus oesophagicola (Nematoda, Trichocephalida)

The embryonal development of Trichocephalus trichiurus and Eucoleus oesophagicola is described. Clevage of both species is full, bilaterally-symmetrical, determined. At the two blastomere stage the epidermis material is localized in the anterior one. The development of both species lacks the rhomb figure characteristic of nematodes. The mouth opening breaks independent of blastopore. The stylet anlage formation was noted for T. trichiurus.     

Experimental reversal of the normal dorsal-ventral timing of blastopore formation does not reverse axis polarity in Xenopus laevis embryos

During gastrulation in Xenopus laevis, the dorsal lip of the blastopore normally appears before the ventral lip. Metabolic gradient models propose that the dorsal lip develops from the region of highest metabolic activity and somehow dominates other regions to prevent them from becoming dorsal. To test these ideas, I applied a temperature gradient of 12 degrees C across the embryo. Localized heating of the prospective ventral vegetal region from early in the first cleavage period until gastrulation causes the blastopore lip to form first by 2 hr at the prospective ventral meridian rather than at the prospective dorsal meridian. Despite this reversal of the timing of blastopore formation, gastrulation is completed, and the neural plate forms at its usual position on the prospective dorsal meridian. This demonstrates that the earliest gastrulating regions of the blastopore do not necessarily become dorsal, nor do they inhibit dorsal development by other regions. It is unlikely that axis polarity is based on regional differences in energy metabolism.     

Origin of the chordate central nervous system – and the origin of chordates

 Contrary to traditional views, molecular evidence indicates that the protostomian ventral nerve cord plus apical brain is homologous with the vertebrates’ dorsal spinal cord plus brain. The origin of the protostomian central nervous system from a larval apical organ plus longitudinal areas along the fused blastopore lips has been documented in many species. The origin of the chordate central nervous system is more enigmatic. About a century ago, Garstang proposed that the ciliary band of a dipleurula-type larva resembling an echinoderm larva should have moved dorsally and fused to form the neural tube of the ancestral chordate. This idea is in contrast to a number of morphological observations, and it is here proposed that the neural tube evolved through lateral fusion of a ventral, postoral loop of the ciliary band in a dipleurula larva; the stomodaeum should move from the ventral side via the anterior end to the dorsal side, which faces the substratum in cephalo- chordates and vertebrates. This is in accordance with the embryological observations and with the molecular data on the dorsoventral orientation. The molecular observations further indicate that the anterior part of the insect brain is homologous with the anterior parts of the vertebrate brain. This leads to the hypothesis that the two organs evolved from the same area in the latest common bilaterian ancestor, just anterior to the blastopore, with the protostome brain developing from the anterior rim of the blastopore (i.e. in front of the protostome mouth) and the chordate brain from an area in front of the blastopore, but behind the mouth (i.e. behind the deuterostome mouth). Received: 28 August 1998 / Accepted: 14 November 1998     

Appearance and Distribution of RNA-Rich Cytoplasms in the Embryo of Xenopus laevis during Early Development

The occurrence of the dorsal yolk-free cytoplasm in the fertilized egg of Xenopus was re-examined, and the appearance and the distribution of RNA-rich cytoplasms in Xenopus embryos during early development were examined with their paraffin sections. The results show that the dorsal yolk-free cytoplasm does not occur solely in the dorsal part of the embryo but is continuous to similar cytoplasmic mass in the central and the ventral part. The whole mass of this continuous cytoplasm is denoted here as the mesoplasm. The locations of the mesoplasm in the embryo can be traced by its high RNA content during cleavage and blastulation. The cells endowed with the mesoplasm constitute a broad band about the equator of the blastula. At the lower edge of this band, the blastopore lip is formed during gastrulation. Another mass of yolk-poor and RNA-rich cytoplasm becomes distinct around every nucleus in the stage 4 embryo and is denoted here as the nucleophilic plasm. This plasm is diminished at every nuclear division and disappears in the stage 10 embryo. Origins and roles of the mesoplasm and the nucleophilic plasm were discussed and a mechanism of blastulation was suggested.     

Animal phylogeny in the light of the trochaea theory

Ultrastructural similarities unite Choanoflagellata and Metazoa as the Kingdom Animalia. Mctazoa (Porifera + Placozoa + Gastraeozoa) are characterized by the presence of collagen, septate/tight junctions and spermatozoa. Porifera and Placozoa lack basal lamina, nerve cells and synapses, which characterize Gastraeozoa (Cnidaria + Trochaeozoa). Gnidaria have cnidoblasts and lack the multiciliate cells found in almost all Trochaeozoa (Gastroneuralia + Protornaeozoa). Gastroneuralia (Spiralia + Aschelminthes) have an apical brain and a pair of ventral nerves, a blastopore which becomes mouth and anus, a mouth surrounded by a downstream collecting system of compound cilia, and a mesoderm formed from the blastopore lips. Spiralia (Articulata + Parenchymia + Bryozoa) have spiral cleavage and 4d-cell mesoderm, whereas these characters are lacking in Aschelminthes, which all lack primary larvae. Protornaeozoa (Ctenophora + Notoneuralia) have mesoderm from vegetal cells. Ctenophores have colloblasts. Notoneuralia have a dorsal nervous system behind the apical area and form a new mouth surrounded by an upstream collecting system of single cilia on monociliate cells; the blastopore becomes the anus surrounded by a ring of compound cilia.
These features fit the trochaea theory, which proposes that Gastroneuralia and Notoneuralia evolved independently from the trochaea, a blastaea with the blastopore surrounded by a ring of compound cilia, which were both locomotory and particle collecting.     

Deuterostomic Development in the Protostome Priapulus caudatus

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Highlights? In the protostome P. caudatus, the blastopore forms the anus (deuterostomy) ? The hindgut markers brachyury and cdx are expressed in the blastopore and in the anus ? The foregut markers foxA and gsc are detected in the mouth, but not in the blastopore ? Results favor deuterostomy in ecdysozoan, protostome, and bilaterian ancestors     

A revised fate map for amphioxus and the evolution of axial patterning in chordates

The chordates include vertebrates plus two groups of invertebrates(the cephalochordates and tunicates). Previous embryonic fatemaps of the cephalochordate amphioxus (Branchiostoma) were influencedby preconceptions that early development in amphioxus and ascidiantunicates should be fundamentally the same and that the earlyamphioxus embryo, like that of amphibians, should have ventralmesoderm. Although detailed cell lineage tracing in amphioxushas not been done because of limited availability of the embryosand because cleavage is radial and holoblastic with the blastomeresnearly equal in size and not tightly adherent until the mid-blastulastage, a compilation of data from gene expression and function,blastomere isolation and dye labeling allows a more realisticfate map to be drawn. The revised fate map is substantiallydifferent from that of ascidians. It shows (1) that the anteriorpole of the amphioxus embryo is offset dorsally from the animalpole only by about 20°, (2) that the ectoderm/mesendodermboundary (the future rim of the blastopore) is at the equatorof the blastula, which approximately coincides with the 3rdcleavage plane, and (3) that there is no ventral mesoderm duringthe gastrula stage. Involution or ingression of cells over theblastopore lip is negligible, and the blastopore, which is posterior,closes centripetally as if by a purse string. During the gastrulastage, the animal pole shifts ventrally, coming to lie about20° ventral to the anterior tip of the late gastrula/earlyneurula. Comparisons of the embryos of amphioxus and vertebratesindicate that in spite of large differences in the mechanicsof cleavage and gastrulation, anterior/posterior and dorsal/ventralpatterning occur by homologous genetic mechanisms. Therefore,the small, nonyolky embryo of amphioxus is probably a reasonableapproximation of the basal chordate embryo before the evolutionof determinate cleavage in the tunicates and the evolution largeamounts of yolk in basal vertebrates.     

Spemann's organizer--it's origin and derivatives (cellular-tissue and molecular-genetic aspects)

In 1924 H. Spemann and H. Mangold discovered that a piece of the dorsal lip of a blastopore from Triturus cristatus, after transplantation to the ventral side of another embryo, was able to cause the neighbouring tissues to change their fate and participate in the formation of a new embryo. The dorsal lip was termed "the organizer". Since then, for as long as 75 years, attempts have been made to establish the intimate mechanisms of the organizer activity. However, no real advance was achieved in their understanding. Within the last 15 years, genetic and molecular techniques have been vastly improved, to help in tracing the fate of many cell lineages, and in compiling more exactly the fate maps for different parts of the embryo. Using these data, I have attempted to trace the fate of Spemann's organizer after the early gastrula stage. Analysis of data on inductive abilities of the organizer cells, on the use of markers, and on the observation of expression of specific genes allowed to conclude that Spemann's organizer in amphibia and its homologues in other vertebrates too are heterogeneous: they are composed of distinct cell populations able to induce primarity the development of either the head or trunk parts of the embryo. These population, determined to become the head of the trunk organizers still at the blastula stage, may be located either in the single continuous cell layer (as in amphibia and birds) or separated among different tissue germs (as in mammals). When the dorsal-ventral orientation of the embryo is established and the organizer is switched on the very early invaginating cells of the dorsal blastopore lip (in the case of amphibia) move in advance of the entire invaginating mesoderm and by the end of gastrulation occupy the place just in front of the notochord. It is supposed that the early dorsal lip and the prechordal mesoderm (PCM) are one and the same cell population, i.e. during gastrulation Spemann's organizer transfers from the lip of blastopore to the prechordal zone. The PCM seems to play an exclusive role in the formation of a head in vertebrate, because some mutations in genes expressed in the PCM result in the entire head deletion. It is supposed that spreading of differentiating signals from the PCM occurs along the main body axis in both caudal and rostral directions. After the main body plan formation the PCM is replaced by adenohypophysis. This conclusion is drawn not only from the same topology of both these structures, but also from the similarities of a set of specific genetical markers expressed in these, that makes it possible to suppose the existence of deep connections and succession between them. The adenohypophysis seems to arise directly from the PCM, or cells of the ectoderm influenced by the PCM may be subsequently transformed into humoral cells of adenohypophysis. In this interpretation, adenohypophysis and the much earlier established PCM may be considered as derivatives of Spemann's organizer. This inference is supported by the fact that all the three above structures first originate in vertebrates only.     

Molecular studies of hemichordate development: a key to understanding the evolution of bilateral animals and chordates

SUMMARY Using the Hawaiian acorn worm, Ptychodera flava, we began molecular studies on the development of hemichordates, a phylum previously unstudied at this level. Here we review results garnered from the examination of a few specific genes selected to help understand the evolution of vertebrate structures. These studies suggest new ideas about the evolution of developmental mechanisms in the deuterostomes. In a seminal observation, we noted an unexpected zone of expression of the Brachyury gene in the early anterior embryonic ectoderm where the mouth will form. Typically, the Brachyury gene is closely linked to development of the notochord and is expressed around the blastopore and in the posterior mesoderm in most animals. This first expression of Brachyury at the blastopore may represent a regulatory program associated with organizing the original animal head and gut opening, as suggested by the expression of Brachyury during hypostome formation in hydra. We believe that the anterior expression of Brachyury in deuterostomes represents the cooption of the program for organizing the original animal gut opening to form the deuterostome mouth. Recent data from the trochophore larva of a polychaete show that an anterior zone of expression of Brachyury is produced in this protostome by splitting of the Brachyury field during the formation of a gut with a mouth and anus by the lateral fusion of the sides of the blastopore. The ability to initiate independently a secondary regulatory program to organize the new mouth leading to an anterior field of Brachyury expression may be a signal event in the evolution of the deuterostomes. We also noted that the P. flava homolog of T‐brain/Eomes, a gene closely related by sequence and expression around the blastopore to Brachyury and associated with development of the vertebrate brain, also exhibits early posterior expression around the blastopore and a field of de novo anterior ectoderm expression during later embryogenesis. The tissue in the zone of de novo anterior ectoderm expression of Pf‐Tbrain produces the apical organ, a larval neural structure that has been touted as an evolutionary precursor of the chordate dorsal brain. The gene regulatory mechanisms responsible for initiating the anterior zone of de novo expression of T‐brain may represent a cooption to specify early neuroectoderm of the regulatory program evolved first to drive anterior Brachyury expression for deuterostome mouth formation. It will be interesting to examine the possibilities that an ability to initiate the de novo anterior expression of the program that includes T‐brain may be a key event in the evolution of the developmental mechanisms leading to the chordate dorsal nervous system.     

Discovery of the Corallivorous Polyclad Flatworm, Amakusaplana acroporae, on the Great Barrier Reef, Australia - the First Report from the Wild

The role of corallivory is becoming increasingly recognised as an important factor in coral health at a time when coral reefs around the world face a number of other stressors. The polyclad flatworm, Amakusaplana acroporae, is a voracious predator of Indo-Pacific acroporid corals in captivity, and its inadvertent introduction into aquaria has lead to the death of entire coral colonies. While this flatworm has been a pest to the coral aquaculture community for over a decade, it has only been found in aquaria and has never been described from the wild. Understanding its biology and ecology in its natural environment is crucial for identifying viable biological controls for more successful rearing of Acropora colonies in aquaria, and for our understanding of what biotic interactions are important to coral growth and fitness on reefs. Using morphological, histological and molecular techniques we determine that a polyclad found on Acropora valida from Lizard Island, Australia is A. acroporae. The presence of extracellular Symbiodinium in the gut and parenchyma and spirocysts in the gut indicates that it is a corallivore in the wild. The examination of a size-range of individuals shows maturation of the sexual apparatus and increases in the number of eyes with increased body length. Conservative estimates of abundance show that A. acroporae occurred on 7 of the 10 coral colonies collected, with an average of 2.6±0.65 (mean ±SE) animals per colony. This represents the first report of A. acroporae in the wild, and sets the stage for future studies of A. acroporae ecology and life history in its natural habitat.