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Holland, ND, Holland LZ.  2017.  The ups and downs of amphioxus biology: a history. International Journal of Developmental Biology. 61:575-583.   10.1387/ijdb.160395LH   AbstractWebsite

Humans (at least a select few) have long known about the cephalochordate amphioxus, first as something to eat and later as a subject for scientific study. The rate of publication on these animals has waxed and waned several times. The first big surge, in the late nineteenth century, was stimulated by Darwin's evolutionary ideas and by Kowalevsky's embryologic findings suggesting that an amphioxus-like creature might have bridged the gap between the invertebrates and the vertebrates. Interest declined sharply in the early twentieth century and remained low for the next 50 years. An important contributing factor (in addition to inhibition by two world wars and the Great Depression) was the indifference of the new evolutionary synthesis toward broad phylogenetic problems like the origin of the vertebrates. Then, during the 1960s and 1970s, interest in amphioxus resurged, driven especially by increased government support for basic science as well as opportunities presented by electron microscopy. After faltering briefly in the 1980s (electron microscopists were running out of amphioxus tissues to study), a third and still-continuing period of intensive amphioxus research began in the early 1990s, stimulated by the advent of evolutionary developmental biology (evo-devo) and genomics. The volume of studies peaked in 2008 with the publication of the genome of the Florida amphioxus. Since then, although the number of papers per year has dropped somewhat, sequencing of additional genomes and transcriptomes of several species of amphioxus (both in the genus Branchiostoma and in a second genus, Asymmetron) is providing the raw material for addressing the major unanswered question of the relationship between genotype and phenotype.

Holland, LZ.  2015.  Genomics, evolution and development of amphioxus and tunicates: The Goldilocks principle. Journal of Experimental Zoology Part B-Molecular and Developmental Evolution. 324:342-352.   10.1002/jez.b.22569   AbstractWebsite

Morphological comparisons among extant animals have long been used to infer their long-extinct ancestors for which the fossil record is poor or non-existent. For evolution of the vertebrates, the comparison has typically involved amphioxus and vertebrates. Both groups are evolving relatively slowly, and their genomes share a high level of synteny. Both vertebrates and amphioxus have regulative development in which cell fates become fixed only gradually during embryogenesis. Thus, their development fits a modified hourglass model in which constraints are greatest at the phylotypic stage (i.e., the late neurula/early larva), but are somewhat greater on earlier development than on later development. In contrast, the third group of chordates, the tunicates, which are sister group to vertebrates, are evolving rapidly. Constraints on evolution of tunicate genomes are relaxed, and they have discarded key developmental genes and organized much of their coding sequences into operons, which are transcribed as a single mRNA that undergoes trans-splicing. This contrasts with vertebrates and amphioxus, whose genomes are not organized into operons. Concomitantly, tunicates have switched to determinant development with very early fixation of cell fates. Thus, tunicate development more closely fits a progressive divergence model (shaped more like a wine glass than an hourglass) in which the constraints on the zygote and very early development are greatest. This model can help explain why tunicate body plans are so very diverse. The relaxed constraints on development after early cleavage stages are correlated with relaxed constraints on genome evolution. The question remains: which came first? J. Exp. Zool. (Mol. Dev. Evol.) 324B: 342-352, 2015. (c) 2014 Wiley Periodicals, Inc.

Yue, JX, Yu JK, Putnam NH, Holland LZ.  2014.  The transcriptome of an Amphioxus, Asymmetron lucayanum, from the Bahamas: A window into chordate evolution. Genome Biology and Evolution. 6:2681-2696.   10.1093/gbe/evu212   AbstractWebsite

Cephalochordates, the sister group of tunicates plus vertebrates, have been called "living fossils" due to their resemblance to fossil chordates from Cambrian strata. The genome of the cephalochordate Branchiostoma floridae shares remarkable synteny with vertebrates and is free from whole-genome duplication. We performed RNA sequencing from larvae and adults of Asymmetron lucayanum, a cephalochordate distantly related to B. floridae. Comparisons of about 430 orthologous gene groups among both cephalochordates and 10 vertebrates using an echinoderm, a hemichordate, and a mollusk as outgroups showed that cephalochordates are evolving more slowly than the slowest evolving vertebrate known (the elephant shark), with A. lucayanum evolving even more slowly than B. floridae. Against this background of slow evolution, some genes, notably several involved in innate immunity, stand out as evolving relatively quickly. This may be due to the lack of an adaptive immune system and the relatively high levels of bacteria in the inshore waters cephalochordates inhabit. Molecular dating analysis including several time constraints revealed a divergence time of similar to 120 Ma for A. lucayanum and B. floridae. The divisions between cephalochordates and vertebrates, and that between chordates and the hemichordate plus echinoderm clade likely occurred before the Cambrian.

Koop, D, Holland ND, Semon M, Alvarez S, de Lera AR, Laudet V, Holland LZ, Schubert M.  2010.  Retinoic acid signaling targets Hox genes during the amphioxus gastrula stage: Insights into early anterior-posterior patterning of the chordate body plan. Developmental Biology. 338:98-106.   10.1016/j.ydbio.2009.11.016   AbstractWebsite

Previous studies of vertebrate development have shown that retinoic acid (RA) signaling at the gastrula stage strongly influences anterior-posterior (A-P) patterning of the neurula and later stages. However, much less is known about the more immediate effects of RA signaling on gene transcription and developmental patterning at the gastrula stage. To investigate the targets of RA signaling during the gastrula stage, we used the basal chordate amphioxus, in which gastrulation involves very minimal tissue movements. First, we determined the effect of altered RA signaling on expression of 42 genes (encoding transcription factors and components of major signaling cascades) known to be expressed in restricted domains along the A-P axis during the gastrula and early neurula stage. Of these 42 genes, the expression domains during gastrulation of only four (Hox1, Hox3, HNF3-1 and Wnt3) were spatially altered by exposure of the embryos to excess RA or to the RA antagonist BMS009. Moreover, blocking protein synthesis with puromycin before adding RA or BMS009 showed that only three of these genes (Hox1, Hox3 and HNF3-1) are direct RA targets at the gastrula stage. From these results we conclude that in the amphioxus gastrula RA signaling primarily acts via regulation of Hox transcription to establish positional identities along the A-P axis and that Hox1, Hox3, HNF3-1 and Wnt3 constitute a basal module of RA action during chordate gastrulation. (C) 2009 Elsevier Inc. All rights reserved.

Onai, T, Yu JK, Blitz IL, Cho KWY, Holland LZ.  2010.  Opposing Nodal/Vg1 and BMP signals mediate axial patterning in embryos of the basal chordate amphioxus. Developmental Biology. 344:377-389.   10.1016/j.ydbio.2010.05.016   AbstractWebsite

The basal chordate amphioxus resembles vertebrates in having a dorsal, hollow nerve cord, a notochord and somites However, it lacks extensive gene duplications, and its embryos are small and gastrulate by simple invagination Here we demonstrate that Nodal/Vg1 signaling acts from early cleavage through the gastrula stage to specify and maintain dorsal/anterior development while, starting at the early gastrula stage, BMP signaling promotes ventral/posterior identity Knockdown and gain-of-function experiments show that these pathways act in opposition to one another Signaling by these pathways is modulated by dorsally and/or anteriorly expressed genes including Chordin, Cerberus, and Blimp1. Overexpression and/or reporter assays in Xenopus demonstrate that the functions of these proteins are conserved between amphioxus and vertebrates. Thus, a fundamental genetic mechanism for axial patterning involving opposing Nodal and BMP signaling is present in amphioxus and probably also in the common ancestor of amphioxus and vertebrates or even earlier in deuterostome evolution (C) 2010 Elsevier Inc. All rights reserved

Onai, T, Lin HC, Schubert M, Koop D, Osborne PW, Alvarez S, Alvarez R, Holland ND, Holland LZ.  2009.  Retinoic acid and Wnt/beta-catenin have complementary roles in anterior/posterior patterning embryos of the basal chordate amphioxus. Developmental Biology. 332:223-233.   10.1016/j.ydbio.2009.05.571   AbstractWebsite

A role for Wnt/beta-catenin signaling in axial patterning has been demonstrated in animals as basal as cnidarians, while roles in axial patterning for retinoic acid (RA) probably evolved in the deuterostomes and may be chordate-specific. In vertebrates, these two pathways interact both directly and indirectly. To investigate the evolutionary origins of interactions between these two pathways, we manipulated Wnt/beta-catenin and RA signaling in the basal chordate amphioxus during the gastrula stage, which is the RA-sensitive period for anterior/posterior (A/P) patterning. The results show that Wnt/beta-catenin and RA signaling have distinctly different roles in patterning the A/P axis of the amphioxus gastrula. Wnt/beta-catenin specifies the identity of the ends of the embryo (high Wnt = posterior; low Wnt = anterior) but not intervening positions. Thus, Upregulation of Wnt/beta-catenin signaling induces ectopic expression of posterior markers at the anterior tip of the embryo. In contrast, RA specifies position along the A/P axis, but not the identity of the ends of the embryo-increased RA signaling strongly affects the domains of Hox expression along the A/P axis but has little or no effect on the expression of either anterior or posterior markers. Although the two pathways may both influence such things as specification of neuronal identity, interactions between them in A/P patterning appear to be minimal. (C) 2009 Elsevier Inc. All rights reserved.

Holland, LZ, Holland ND, Gilland E.  2008.  Amphioxus and the evolution of head segmentation. Integrative and Comparative Biology. 48:630-646.   10.1093/icb/icn060   AbstractWebsite

Whether or not the vertebrate head is fundamentally segmented has been controversial for over 150 years. Beginning in the late 19th century, segmentalist theories proposed that the vertebrate head evolved from an amphioxus-like ancestor in which mesodermal somites extended the full length of the body with remnants of segmentation persisting as the mesodermal head cavities of sharks and lampreys. Antisegmentalists generally argued either that the vertebrate ancestors never had any mesodermal segmentation anteriorly or that they lost it before the origin of the vertebrates; in either case, the earliest vertebrates had an unsegmented head and the embryonic cranial mesoderm of vertebrates is at best pseudo-segmented, evolving independently of any pre-vertebrate segmental pattern. Recent morphologic studies have generally confirmed the accuracy of the major classical studies of head development in lampreys and sharks, yet disagree with their theoretical conclusions regarding the evolution of head segmentation. Studies of developmental genes in amphioxus and vertebrates, which have demonstrated conservation of the mechanisms of anteriorposterior patterning in the two groups, have shed new light on this controversy. Most pertinently, some homologs of genes expressed in the anterior amphioxus somites, which form as outpocketings of the gut, are also expressed in the walls of the head cavities of lampreys, which form similarly, and in their major derivatives (the velar muscles) as well as in the eye and jaw muscles of bony gnathostomes, which derive from unsegmented head mesoderm. These muscles share gene expression with the corresponding muscles of the shark, which derive from the walls of head cavities that form, not as outpocketings of the gut, but as secondary cavities within solid blocks of tissue. While molecular data that can be compared across all the relevant taxa remain limited, they are consistent with an evolutionary scenario in which the cranial paraxial mesoderm of the lamprey and shark evolved from the anterior somites of an amphioxus-like ancestor. Although, bony vertebrates have lost the mesodermal head segments present in the shark and lamprey, their remnants persist in the muscles of the eye and jaw.

Holland, LZ, Short S.  2008.  Gene Duplication, Co-Option and Recruitment during the Origin of the Vertebrate Brain from the Invertebrate Chordate Brain. Brain Behavior and Evolution. 72:91-105.   10.1159/000151470   AbstractWebsite

The brain of the basal chordate amphioxus has been compared to the vertebrate diencephalic forebrain, midbrain, hindbrain and spinal cord on the basis of the cell architecture from serial electron micrographs and patterns of developmental gene expression. In addition, genes specifying the neural plate and neural plate border as well as Gbx and Otx, that position the midbrain/hindbrain boundary (MHB), are expressed in comparable patterns in amphioxus and vertebrates. However, migratory neural crest is lacking in amphioxus, and although it has homologs of the genes that specify neural crest, they are not expressed at the edges of the amphioxus neural plate. Similarly, amphioxus has the genes that specify organizer properties of the MHB, but they are not expressed at the Gbx/Otx boundary as in vertebrates. Thus, the genetic machinery that created migratory neural crest and an MHB organizer was present in the ancestral chordate, but only co-opted for these new roles in vertebrates. Analyses with the amphioxus genome project strongly support the idea of two rounds of whole genome duplication with subsequent gene losses in the vertebrate lineage. Duplicates of developmental genes were preferentially retained. Although some genes apparently acquired roles in neural crest prior to these genome duplications, other key genes (e. g., FoxD3 in neural crest and Wnt1 at the MHB) were recruited into the respective gene networks after one or both genome duplications, suggesting that such an expansion of the genetic toolkit was critical for the evolution of these structures. The toolkit has also increased by alternative splicing. Contrary to the general rule, for at least one gene family with key roles in neural crest and the MHB, namely Pax genes, alternative splicing has not decreased subsequent to gene duplication. Thus, vertebrates have a much larger number of proteins available for mediating new functions in these tissues. The creation of new splice forms typically changes protein structure more than evolution of the protein after gene duplication. The functions of particular isoforms of key proteins expressed at the MHB and in neural crest have only just begun to be studied. Their roles in modulating gene networks may turn out to rival gene duplication for facilitating the evolution of structures such as neural crest and the MHB. Copyright (c) 2008 S. Karger AG, Basel

Kozmik, Z, Holland ND, Kreslova J, Oliveri D, Schubert M, Jonasova K, Holland LZ, Pestarino M, Benes V, Candiani S.  2007.  Pax-Six-Eya-Dach network during amphioxus development: Conservation in vitro but context specificity in vivo. Developmental Biology. 306:143-159.   10.1016/j.ydbio.2007.03.009   AbstractWebsite

The Drosophila retinal determination gene network occurs in animals generally as a Pax-Six-Eyes absent-Dachshund network (PSEDN). For amphioxus, we describe the complete network of nine PSEDN genes, four of which-AmphiSix1/2, AmphiSix4/5, AmphiSix3/6, and AmphiEya-are characterized here for the first time. For amphioxus, in vitro interactions among the genes and proteins of the network resemble those of other animals, except for the absence of Dach-Eya binding. Amphioxus PSEDN genes are expressed in highly stage- and tissue-specific patterns (sometimes conspicuously correlated with the local intensity of cell proliferation) in the gastrular organizer, notochord, somites, anterior central nervous system, peripheral nervous system, pharyngeal endoderm, and the likely homolog of the vertebrate adenohypophysis. In this last tissue, the anterior region expresses all three amphioxus Six genes and is a zone of active cell proliferation, while the posterior region expresses only AmphiPax6 and is non-proliferative. In summary, the topologies of animal PSEDNs, although considerably more variable than originally proposed, are conserved enough to be recognizable among species and among developing tissues; this conservation may reflect indispensable involvement of PSEDNs during the critically important early phases of embryology (e.g. in the control of mitosis, apoptosis, and cell/tissue motility). (C) 2007 Elsevier Inc. All rights reserved.

Castro, LFC, Rasmussen SLK, Holland PWH, Holland ND, Holland LZ.  2006.  A Gbx homeobox gene in amphioxus: Insights into ancestry of the ANTP class and evolution of the midbrain/hindbrain boundary. Developmental Biology. 295:40-51.   10.1016/j.ydbio.2006.03.003   AbstractWebsite

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. (c) 2006 Elsevier Inc. All rights reserved.

Holland, LZ.  2005.  Non-neural ectoderm is really neural: Evolution of developmental patterning mechanisms in the non-neural ectoderm of chordates and the problem of sensory cell homologies. Journal of Experimental Zoology Part B-Molecular and Developmental Evolution. 304B:304-323.   10.1002/jez.21038   AbstractWebsite

In chordates, the ectoderm is divided into the neuroectoderm and the so-called non-neural ectoderm. In spite of its name, however, the non-neural ectoderm contains. numerous sensory cells. Therefore, the term "non-neural" ectoderm should be replaced by "general ectoderm." At least in amphioxus and tunicates and possibly in vertebrates as well, both the neuroectoderm and the general ectoderm are patterned anterior/posteriorly by mechanisms involving retinoic acid and Hox genes. In amphioxus and tunicates the ectodermal sensory cells, which have a wide range of ciliary and microvillar configurations, are mostly primary neurons sending axons to the CNS, although a minority lack axons. In contrast, vertebrate mechanosensory cells, called hair cells, are all secondary neurons that lack axons and have a characteristic eccentric cilium adjacent to a group of microvilli of graded lengths. It has been highly controversial whether the ectodermal sensory cells in the oral siphons of adult tunicates are homologous to vertebrate hair cells. In some species of tunicates, these cells appear to be secondary neurons, and microvillar and ciliary configurations of some of these cells approach those of vertebrate hair cells. However, none of the tunicate cells has all the characteristics of a hair cell, and there is a high degree of variation among ectodermal sensory cells within and between different species. Thus, similarities between the ectodermal sensory cells of any one species of tunicate and craniate hair cells may well represent convergent evolution rather than homology.

Schubert, M, Yu JK, Holland ND, Escriva H, Laudet V, Holland LZ.  2005.  Retinoic acid signaling acts via Hox1 to establish the posterior limit of the pharynx in the chordate amphioxus. Development. 132:61-73.   10.1242/dev.01554   AbstractWebsite

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.

Schubert, M, Holland ND, Escriva H, Holland LZ, Laudet V.  2004.  Retinoic acid influences anteroposterior positioning of epidermal sensory neurons and their gene expression in a developing chordate (amphioxus). Proceedings of the National Academy of Sciences of the United States of America. 101:10320-10325.   10.1073/pnas.0403216101   AbstractWebsite

In developing chordates, retinoic acid (RA) signaling patterns the rostrocaudal body axis globally and affects gene expression locally in some differentiating cell populations. Here we focus on development of epidermal sensory neurons in an invertebrate chordate (amphioxus) to determine how RA signaling influences their rostrocaudal distribution and gene expression (for AmphiCoe, a neural precursor gene; for amphioxus islet and AmphiERR, two neural differentiation genes; and for AmphiHox1,-3, -4, and -6). Treatments with RA or an RA antagonist (BMS009) shift the distribution of developing epidermal neurons anteriorly or posteriorly, respectively. These treatments also affect gene expression patterns in the epidermal neurons, suggesting that RA levels may influence specification of neuronal subtypes. Although colinear expression of Hox genes is well known for the amphioxus central nervous system,we find an unexpected comparable colinearity for AmphiHox1, -3, -4, and -6 in the developing epidermis; moreover, RA levels affect the anteroposterior extent of these Hox expression domains, suggesting that RA signaling controls a colinear Hox code for anteroposterior patterning of the amphioxus epidermis. Thus, in amphioxus, the developing peripheral nervous system appears to be structured by mechanisms parallel to those that structure the central nervous system. One can speculate that, during evolution, an ancestral deuterostome that structured its panepidermal nervous system with an RA-influenced Hox code gave rise to chordates in which this patterning mechanism persisted within the epidermal elements of the peripheral nervous system and was transferred to the neuroectoderm as the central nervous system condensed dorsally.

Holland, LZ.  2002.  Heads or tails? Amphioxus and the evolution of anterior-posterior patterning in deuterostomes Developmental Biology. 241:209-228.   10.1006/dbio.2001.0503   AbstractWebsite

In Xenopus, the canonical Wnt-signaling pathway acting through (beta-catenin functions both in establishing the dorso-ventral axis and in patterning the anterior-posterior axis. This pathway also acts in patterning the animal-vegetal axis in sea urchins. However, because sea urchin development is typically indirect, and adult sea urchins have pentamerous symmetry and lack a longitudinal nerve cord, it has not been clear how the roles of the canonical Wnt-signaling pathway in axial patterning in sea urchins and vertebrates are evolutionarily related. The developmental expression patterns of Notch, brachyury, caudal, and eight Wnt genes have now been determined for the invertebrate chordate amphioxus, which, like sea urchins, has an early embryo that gastrulates by invagination, but like vertebrates, has a later embryo with a dorsal hollow nerve cord that elongates posteriorly from a tail bud. Comparisons of amphioxus with other deuterostomes suggest that patterning of the ancestral deuterostome embryo along its anterior-posterior axis during the late blastula and subsequent stages involved a posterior signaling center including Writs, Notch, and transcription factors such as brachyury and caudal. In tunicate embryos, in which cell numbers are reduced and cell fates largely determined during cleavage stages, only vestiges of this signaling center are still apparent; these include localization of Wnt-5 mRNA to the posterior cytoplasm shortly after fertilization and localization of beta-catenin to vegetal nuclei during cleavage stages. Neither in tunicates nor in amphioxus is there any evidence that the canonical Wnt-signaling pathway functions in establishment of the dorso-ventral axis. Thus, roles for Wnt-signaling in dorso-ventral patterning of embryos may be a vertebrate innovation that arose in connection with the evolution of yolky eggs and gastrulation by extensive involution. (C) 2001 Elsevier Science.

Holland, LZ, Rached LA, Tamme R, Holland ND, Inoko H, Shiina T, Burgtorf C, Lardelli M.  2001.  Characterization and developmental expression of the amphioxus homolog of notch (AmphiNotch): Evolutionary conservation of multiple expression domains in amphioxus and vertebrates. Developmental Biology. 232:493-507.   10.1006/dbio.2001.0160   AbstractWebsite

Notch encodes a transmembrane protein that functions in intercellular signaling. Although there is one Notch gene in Drosophila, vertebrates have three or more with overlapping patterns of embryonic expression. We cloned the entire 7575-bp coding region of an amphioxus Notch gene (AmphiNotch), encoding 2524 amino acids, and obtained the exon/intron organization from a genomic cosmid clone. Southern blot and PCR data indicate that AmphiNotch is the only Notch gene in amphioxus. AmphiNotch, like Drosophila Notch and vertebrate Notch1 and Notch2, has 36 EGF repeats, 3 Notch/lin-12 repeats, a transmembrane region, and B ankyrin repeats. Phylogenetic analysis places it at the base of all the vertebrate genes, suggesting it is similar to the ancestral gene from which the vertebrate Notch family genes evolved. AmphiNotch is expressed in all three embryonic perm layers in spatiotemporal patterns strikingly similar to those of all the vertebrate homologs combined. In the developing nerve cord, AmphiNotch is first expressed in the posteriormost part of the neural plate, then it becomes more broadly expressed and later is localized dorsally in the anteriormost part of the nerve cord corresponding to the diencephalon. In late embryos and larvae, AmphiNotch is also expressed in parts of the pharyngeal endoderm, ill the anterior gut diverticulum, and, like AmphiPax2/5/8, in the rudiment of Hatschek's kidney. A comparison with Notch1 and Pax5 and Pax8 expression in the embryonic mouse kidney helps support homology of the amphioxus and vertebrate kidneys. AmphiNotch is also an early marker for presumptive mesoderm, transcripts first being detectable at the gastrula stage in a ring of mesendoderm just inside the blastopore and subsequently in the posterior mesoderm, notochord, and somites. As in sea urchins and vertebrates, these domains of AmphiNotch expression overlap with those of several Wnt genes and brachyury. These relationships suggest that amphioxus shares with other deuterostomes a common mechanism for patterning along the anterior/posterior axis involving a posterior signaling center in which the Notch and Wnt pathways and brachyury interact. (C) 2001 Academic Press.

Langlois, MC, Vanacker JM, Holland ND, Escriva H, Queva C, Laudet V, Holland LZ.  2000.  Amphicoup-TF, a nuclear orphan receptor of the lancelet Branchiostoma floridae, is implicated in retinoic acid signalling pathways. Development Genes and Evolution. 210:471-482.   10.1007/s004270000087   AbstractWebsite

In vertebrates, the orphan nuclear receptors of the COUP-TF group function as negative transcriptional regulators that inhibit the hormonal induction of target genes mediated by classical members of the nuclear hormone superfamily, such as the retinoic acid receptors (RARs) or the thyroid hormone receptors (TRs). To investigate the evolutionary conservation of the roles of COUP-TF receptors as negative regulators in the retinoid and thyroid hormone pathways, we have characterized AmphiCOUP-TF, the homologue of COUP-TFI and COUP-TFII, in the chordate amphioxus (Branchiostoma floridae), the closest living invertebrate relative of the vertebrates. Electrophoretic mobility shift assays (EMSA) showed that AmphiCOUP-TF binds to a wide variety of response elements, as do its vertebrate homologues. Furthermore, AmphiCOUP-TF is a transcriptional repressor that strongly inhibits retinoic acid-mediated transactivation. In situ hybridizations revealed expression of AmphiCOUP-TF in the nerve cord of late larvae, in a region corresponding to hindbrain and probably anterior spinal cord. Although the amphioxus nerve cord appears unsegmented at the gross anatomical level, this pattern reflects segmentation at the cellular level with stripes of expressing cells occurring adjacent to the ends and the centers of each myotomal segment, which may include visceral motor neurons and somatic motor neurons respectively, among other cells. A comparison of the expression pattern of AmphiCOUP-TF with those of its vertebrate homologues, suggests that the roles of COUP-TF in patterning of the nerve cord evolved prior to the split between the amphioxus and vertebrate lineages. Furthermore, in vitro data also suggest that AmphiCOUP-TF acts as a negative regulator of signalling by other nuclear receptors such as RAR, TR or ER.

Holland, LZ, Schubert M, Holland ND, Neuman T.  2000.  Evolutionary conservation of the presumptive neural plate markers AmphiSox1/2/3 and AmphiNeurogenin in the invertebrate chordate amphioxus. Developmental Biology. 226:18-33.   10.1006/dbio.2000.9810   AbstractWebsite

Amphioxus, as the closest living invertebrate relative of the vertebrates, can give insights into the evolutionary origin of the vertebrate body plan. Therefore, to investigate the evolution of genetic mechanisms for establishing and patterning the neuroectoderm, we cloned and determined the embryonic expression of two amphioxus transcription factors, AmphiSox1/2/3 and AmphiNeurogenin. These genes are the earliest known markers for presumptive neuroectoderm in amphioxus. By the early neurula stage, AmphiNeurogenin expression becomes restricted to two bilateral columns of segmentally arranged neural plate cells, which probably include precursors of motor neurons. This is the earliest indication of segmentation in the amphioxus nerve cord, Later, expression extends to dorsal cells in the nerve cord, which may include precursors of sensory neurons. By the midneurula, AmphiSox1/2/3 expression becomes limited to the dorsal part of the forming neural tube. These patterns resemble those of their vertebrate and Drosophila homologs. Taken together with the evolutionarily conserved expression of the dorsoventral patterning genes, BLP2/4 and chordin, in nonneural and neural ectoderm, respectively, of chordates and Drosophila, our results are consistent with the evolution of the chordate dorsal nerve cord and the insect ventral nerve cord from a longitudinal nerve cord in a common bilaterian ancestor. However, AmphiSox1/2/3 differs from its vertebrate homologs in not being expressed outside the CNS, suggesting that additional roles for this gene have evolved in connection with gene duplication in the vertebrate lineage. In contrast, expression in the midgut of AmphiNeurogenin together with the gene encoding the insulin-like peptide suggests that amphioxus may have homologs of vertebrate pancreatic islet cells, which express neurogenin3. In addition, AmphiNeurogenin, like its vertebrate and Drosophila homologs, is expressed in apparent precursors of epidermal chemosensory and possibly mechanosensory cells, suggesting a common origin for protostome and deuterostome epidermal sensory cells in the ancestral bilaterian. (C) 2000 Academic Press.

Venkatesh, TV, Holland ND, Holland LZ, Su MT, Bodmer R.  1999.  Sequence and developmental expression of amphioxus AmphiNk2-1: insights into the evolutionary origin of the vertebrate thyroid gland and forebrain. Development Genes and Evolution. 209:254-259. AbstractWebsite

We characterized an amphioxus NK-2 homeobox gene (AmphiNk2-1), a homologue of vertebrate Nkx2-1, which is involved in the development of the central nervous system and thyroid gland. At the early neurula stage of amphioxus, AmphiNk2-1 expression is first detected medially in the neural plate. By the mid-neurula stage, expression is localized ventrally in the nerve cord and also begins in the endoderm. During the late neurula stage, the ventral neural expression becomes transiently segmented posteriorly and is then down-regulated except in the cerebral vesicle at the anterior end of the central nervous system. Within the cerebral vesicle AmphiNk2-1 is expressed in a broad ventral domain, probably comprising both the floor plate and basal plate regions: this pattern is comparable to Nkx2-1 expression in the mouse diencephalon. In the anterior part of the gut, expression becomes intense in the endostyle (the right wall of the pharynx), which is the presumed homologue of the vertebrate thyroid gland. More posteriorly, there is transitory expression in the midgut and hindgut. In sum, the present results help to support homologies (1) between the amphioxus endostyle and the vertebrate thyroid gland and (2) between the amphioxus cerebral vesicle and the vertebrate diencephalic forebrain.

Holland, LZ, Holland ND.  1998.  Developmental gene expression in amphioxus: New insights into the evolutionary origin of vertebrate brain regions, neural crest, and rostrocaudal segmentation. American Zoologist. 38:647-658. AbstractWebsite

Amphioxus is widely held to be the closest invertebrate relative of the vertebrates and the best available stand-in for the proximate ancestor of the vertebrates. The spatiotemporal expression patterns of developmental genes can help suggest body part homologies between vertebrates and amphioxus, This approach is illustrated using five homeobox genes (AmphiHox1, AmphiHox2, AmphiOtx, AmphiDll, and AmphiEn) to pro,ide insights into the evolutionary origins of three important vertebrate features: the major brain regions, the neural crest, and rostrocaudal segmentation. During amphioxus development, the neural expression patterns of these genes are consistent with the presence of a forebrain (detailed neuroanatomy indicates that the forebrain is all diencephalon without any telencephalon) and an extensive hindbrain; the possible presence of a midbrain requires additional study. Further, during neurulation, the expression pattern of AmphiDll as web as migratory cell behavior suggest that the epidermal cells bordering the neural plate may represent a phylogenetic precursor of the vertebrate neural crest. Finally, when the paraxial mesoderm begins to segment, the earliest expression of AmphiEn is detected in the posterior part of each nascent and newly formed somite, This pattern recalls the expression of the segment-polarity gene engrailed during establishment of the segments of metameric protostomes. Thus, during animal evolution, the role of engrailed in establishing and maintaining metameric body plans may have arisen in a common segmented ancestor of both the protostomes and deuterostomes.

Lacalli, TC, Holland LZ.  1998.  The developing dorsal ganglion of the salp Thalia democratica, and the nature of the ancestral chordate brain. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences. 353:1943-1967. AbstractWebsite

The development of the dorsal ganglion of the salp, Thalia democratica, is described from electron microscope reconstructions up to the stage of central neuropile formation. The central nervous system (CNS) rudiment is initially tubular with an open central canal. Early developmental events include: (i) the formation of a thick dorsal mantle of neuroblasts from which paired dorsal paraxial neuropiles arise; (ii) the differentiation of clusters of primary motor neurons along the ventral margin of the mantle; and (iii) the development from the latter of a series of peripheral nerves. The dorsal paraxial neuropiles ultimately connect to the large central neuropile, which develops later. Direct contact between neuroblasts and muscle appears to be involved in the development of some anterior nerves. The caudal nerves responsible for innervating more distant targets in the posterior part of the body develop without such contacts, which suggests that a different patterning mechanism may be employed in this part of the neuromuscular system. The results are compared with patterns of brain organization in other chordates. Because the salp CNS is symmetrical and generally less reduced than that of ascidian larvae, it is more easily compared with the CNS of amphioxus and vertebrates. The dorsal paraxial centres in the salp resemble the dorsolateral tectal centres in amphioxus in both position and organization; the central neuropile in salps likewise resembles the translumenal system in amphioxus. The neurons themselves are similar in that many of their neurites appear to be derived from the apical surface instead of the basal surface of the cell. Such neurons, with extensively developed apical neurites, may represent a new cell type that evolved in the earliest chordates in conjunction with the formation of translumenal or intralumenal integrative centres. In comparing the salp ganglion with vertebrates, we suggest that the main core of the ganglion is most like the mes-metencephalic region of the vertebrate brain, i.e. the zone occupied by the midbrain, isthmus, and anterior hindbrain. Counterparts of more anterior regions (forebrain) and posterior ones (segmented hindbrain) appear to be absent in salps, but are found in other tunicates, suggesting that evolution has acted quite differently on the main subdivisions of the CNS in different types of tunicates.

Holland, LZ, Venkatesh TV, Gorlin A, Bodmer R, Holland ND.  1998.  Characterization and developmental expression of AmphiNk2-2, an NK2 class homeobox gene from amphioxus (Phylum Chordata; Subphylum Cephalochordata). Development Genes and Evolution. 208:100-105.   10.1007/s004270050159   AbstractWebsite

The genome of amphioxus includes AmphiNk2-2, the first gene of the NK2 homeobox class to be demonstrated in any invertebrate deuterostome. AmphiNk2-2 encodes a protein with a TN domain, homeodomain, and NK2-specific domain; on the basis of amino acid identities in these conserved regions, AmphiNk2-2 is a homolog of Drosophila vnd and vertebrate Nkx2-2. During amphioxus development, expression of AmphiNk2-2 is first detected ventrally in the endoderm of late gastrulae. In neurulae, endodermal expression divides into three domains (the pharynx, midgut, and hindgut), and neural expression commences in two longitudinal bands of cells in the anterior neural tube. These neural tube cells occupy a ventrolateral position on either side of the cerebral vesicle (the probable homolog of the vertebrate diencephalic forebrain). The dynamic expression patterns of AmphiNFkx2-2 suggest successive roles, first in regionalizing the endoderm and nervous system and later during differentiation of specific cell types in the gut (possibly peptide endocrine cells) and brain (possibly including axon outgrowth and guidance).