Morphological characterization of the asexual reproduction in the acorn worm Balanoglossus simodensis

The acorn worm Balanoglossus simodensis reproduces asexually by fragmentation and subsequent regeneration from the body fragments. We examined the morphogenesis of its asexual reproduction. At first, we collected asexually reproducing specimens and observed their morphogenesis. Then, we succeeded in inducing the asexual reproduction artificially by cutting the worm at the end of the genital region. The process of morphogenesis is completely the same between naturally collected and artificially induced specimens. The stages during morphogenesis were established on the basis of the external features of the asexually reproducing fragments. The internal features of the fragments were also examined at each stage. In a separate phase of the study, the capacity for regeneration of some body parts was also examined by dividing intact worms into about 10 fragments. Although the capacity for regeneration varied among the different body parts, some fragments regenerated into complete individuals in 1 month. The process of regeneration was the same as that in the asexually produced fragments.     

The evolution of nervous system patterning: insights from sea urchin development

Recent studies of the sea urchin embryo have elucidated the mechanisms that localize and pattern its nervous system. These studies have revealed the presence of two overlapping regions of neurogenic potential at the beginning of embryogenesis, each of which becomes progressively restricted by separate, yet linked, signals, including Wnt and subsequently Nodal and BMP. These signals act to specify and localize the embryonic neural fields - the anterior neuroectoderm and the more posterior ciliary band neuroectoderm - during development. Here, we review these conserved nervous system patterning signals and consider how the relationships between them might have changed during deuterostome evolution.     

Organohalogen constituents of the acorn worm Ptychodera bahamensis

• 1.1. Twenty organohalogen compounds, primarily phenols, were detected in the volatile extracts of the acorn worm Ptychodera bahamensis.
• 2.2. Five chlorinated compounds, previously undescribed from acorn worms, were identified.
• 3.3. Enteropneusts can be significant contributors of halogenated organics to the marine environment.

     

Regeneration of the acorn worm pygochord with the implication for its convergent evolution with the notochord

The origin of the notochord is a central issue in chordate evolution. This study examined the development of the acorn worm pygochord, a putative homologue of the notochord. Because the pygochord differentiates only after metamorphosis, the developmental was followed process by inducing regeneration after artificial amputation in Ptychodera flava. It was found that although the regeneration of the posterior part of the body did not proceed via formation of an obvious regeneration bud, pygochord regeneration was observed within a few weeks, possibly via trans-differentiation of endoderm cells. The expression of the fibrillary collagen gene (Fcol) and elav in the pygochord during regeneration was detected. This indicates that pygochord cells are not part of gut epithelial cells, but that they differentiated as a distinct cell type. Our gene expression analyses do not provide supporting evidence for the homology between the pygochord and notochord, but rather favored the convergent evolution between them.     

New insights into molecular structure and function of ecto-nucleotidases in the nervous system

Nucleotides can be released as signaling substances in the nervous system from both neural and glial cells. Their function is terminated by ecto-nucleotidases and sequential extracellular metabolism to the nucleoside. Recently considerable progress has been made in unraveling the molecular structure of an ecto-ATPase and an ecto-ATP diphosphohydrolase, two closely related ecto-enzymes. Molecular structure, tissue distribution and functional properties of the ecto-nucleotidases are discussed with particular emphasis on the nervous system. © 1998 Elsevier Science Ltd. All rights reserved.     

The origin and evolution of the surfactant system in fish: insights into the evolution of lungs and swim bladders

Several times throughout their radiation fish have evolved either lungs or swim bladders as gas-holding structures. Lungs and swim bladders have different ontogenetic origins and can be used either for buoyancy or as an accessory respiratory organ. Therefore, the presence of air-filled bladders or lungs in different groups of fishes is an example of convergent evolution. We propose that air breathing could not occur without the presence of a surfactant system and suggest that this system may have originated in epithelial cells lining the pharynx. Here we present new data on the surfactant system in swim bladders of three teleost fish (the air-breathing pirarucu Arapaima gigas and tarpon Megalops cyprinoides and the non-air-breathing New Zealand snapper Pagrus auratus). We determined the presence of surfactant using biochemical, biophysical, and morphological analyses and determined homology using immunohistochemical analysis of the surfactant proteins (SPs). We relate the presence and structure of the surfactant system to those previously described in the swim bladders of another teleost, the goldfish, and those of the air-breathing organs of the other members of the Osteichthyes, the more primitive air-breathing Actinopterygii and the Sarcopterygii. Snapper and tarpon swim bladders are lined with squamous and cuboidal epithelial cells, respectively, containing membrane-bound lamellar bodies. Phosphatidylcholine dominates the phospholipid (PL) profile of lavage material from all fish analyzed to date. The presence of the characteristic surfactant lipids in pirarucu and tarpon, lamellar bodies in tarpon and snapper, SP-B in tarpon and pirarucu lavage, and SPs (A, B, and D) in swim bladder tissue of the tarpon provide strong evidence that the surfactant system of teleosts is homologous with that of other fish and of tetrapods. This study is the first demonstration of the presence of SP-D in the air-breathing organs of nonmammalian species and SP-B in actinopterygian fishes. The extremely high cholesterol/disaturated PL and cholesterol/PL ratios of surfactant extracted from tarpon and pirarucu bladders and the poor surface activity of tarpon surfactant are characteristics of the surfactant system in other fishes. Despite the paraphyletic phylogeny of the Osteichthyes, their surfactant is uniform in composition and may represent the vertebrate protosurfactant.     

Microglia - insights into immune system structure, function, and reactivity in the central nervous system

Microglia are essential cellular components of a well-functioning central nervous system (CNS). The development and establishment of the microglial population differs from the other major cell populations in the CNS i.e. neurons and macroglia (astrocytes and oligodendrocytes). This different ontogeny gives microglia unique properties. In recent years detailed studies of the microglial population have been greatly facilitated by the use of bone marrow (BM) chimeric animals. Experimental BM transplants have provided the opportunity to trace and investigate how BM cells migrate into the CNS and settle to become microglia. Furthermore various functional properties of microglia in the normal and pathological CNS are now being revealed because of combinations of BM transplantations and experimental disease models. Here, we describe some of the latest findings in microglial biology and discuss the potential for using microglia in therapeutic interventions.     

Formation of the ascidian epidermal sensory neurons: insights into the origin of the chordate peripheral nervous system

The vertebrate peripheral nervous system (PNS) originates from neural crest and placodes. While its developmental origin is the object of intense studies, little is known concerning its evolutionary history. To address this question, we analyzed the formation of the larval tail PNS in the ascidian Ciona intestinalis. The tail PNS of Ciona is made of sensory neurons located within the epidermis midlines and extending processes in the overlying tunic median fin. We show that each midline corresponds to a single longitudinal row of epidermal cells and neurons sharing common progenitors. This simple organization is observed throughout the tail epidermis, which is made of only eight single-cell rows, each expressing a specific genetic program. We next demonstrate that the epidermal neurons are specified in two consecutive steps. During cleavage and gastrula stages, the dorsal and ventral midlines are independently induced by FGF9/16/20 and the BMP ligand ADMP, respectively. Subsequently, Delta/Notch–mediated lateral inhibition controls the number of neurons formed within these neurogenic regions. These results provide a comprehensive overview of PNS formation in ascidian and uncover surprising similarities between the fate maps and embryological mechanisms underlying formation of ascidian neurogenic epidermis midlines and the vertebrate median fin.     

Spliceosomal introns: new insights into their evolution

A new genome-wide analysis of spliceosomal introns indicates massive loss and gain of introns has taken place in many eukaryotic lineages. Only a small subset of the analyzed introns was present in the common ancestor of plants, fungi, animals and Plasmodium.     

Mathematical and experimental insights into the development of the enteric nervous system and Hirschsprung's disease

The vertebrate enteric nervous system is formed by a rostro-caudally directed invasion of the embryonic gastrointestinal mesenchyme by neural crest cells. Failure to complete this invasion results in the distal intestine lacking intrinsic neurons. This potentially fatal condition is called Hirschsprung's Disease. A mathematical model of cell invasion incorporating cell motility and proliferation of neural crest cells to a carrying capacity predicted invasion outcomes to imagined manipulations, and these manipulations were tested experimentally. Mathematical and experimental results agreed. The results show that the directional invasion is chiefly driven by neural crest cell proliferation. Moreover, this proliferation occurs in a small region at the wavefront of the invading population. These results provide an understanding of why many genes implicated in Hirschsprung's Disease influence neural crest population size. In addition, during in vivo development the underlying gut tissues are growing simultaneously as the neural crest cell invasion proceeds. The interactions between proliferation, motility and gut growth dictate whether or not complete colonization is successful. Mathematical modeling provides insights into the conditions required for complete colonization or a Hirschsprung's-like deficiency. Experimental evidence supports the hypotheses suggested by the modeling.     

Morphological evolution in sea urchin development: hybrids provide insights into the pace of evolution

Hybridisations between related species with divergent ontogenies can provide insights into the bases for evolutionary change in development. One example of such hybridisations involves sea urchin species that exhibit either standard larval (pluteal) stages or those that develop directly from embryo to adult without an intervening feeding larval stage. In such crosses, pluteal features were found to be restored in fertilisations of the eggs of some direct developing sea urchins (Heliocidaris erythrogramma) with the sperm of closely (Heliocidaris tuberculata) and distantly (Pseudoboletia maculata) related species with feeding larvae. Such results can be argued to support the punctuated equilibrium model—conservation in pluteal regulatory systems and a comparatively rapid switch to direct development in evolution. 1 , 1 Generation of hybrids between distantly related direct developers may, however, indicate evolutionary convergence. The ‘rescue’ of pluteal features by paternal genomes may require maternal factors from H. erythrogramma because the larva of this species has pluteal features. In contrast, pluteal features were not restored in hybridisations with the eggs of Holopneustes purpurescens, which lacks pluteal features. How much of pluteal development can be lost before it cannot be rescued in such crosses? The answer awaits hybridisations among indirect and direct developing sea urchins differing in developmental phenotype, in parallel with investigations of the genetic programs involved. BioEssays 26:343–347, 2004. © 2004 Wiley Periodicals, Inc.     

New insights into the evolution of chromosome 1

A complex low-repetitive human DNA probe (BAC RP11-35B4) together with two microdissection-derived region-specific probes of the multicolor banding (MCB) probe-set for chromosome 1 were used to re-analyze the evolution of human chromosome 1 in comparison to four ape species. BAC RP11-35B4 derives from 1q21 and contains 143 kb of non-repetitive DNA; however, it produces three specific FISH signals in 1q21, 1p12 and 1p36.1 of Homo sapiens (HSA). Human chromosome 1 was studied in comparison to its homologues in Hylobates lar (HLA), Pongo pygmaeus (PPY), Gorilla gorilla (GGO) and Pan troglodytes (PTR). A duplication of sequences homologous to human 1p36.1 could be detected in PPY plus an additional signal on PPY 16q. The region homologous to HSA 1p36.1 is also duplicated in HLA, and split onto chromosomes 7q and 9p; the region homologous to HSA 1q21/1p12 is present as one region on 5q. Additionally, the breakpoint of a small pericentric inversion in the evolution of human chromosome 1 compared to other great ape species could be refined. In summary, the results obtained here are in concordance with previous reports; however, there is evidence for a deletion of regions homologous to human 1p34.2-->p34.1 during evolution in the Pongidae branch after separation of PPY.     

Molecular insights into the evolution of crop plants

The domestication and improvement of crop plants have long fascinated evolutionary biologists, geneticists, and anthropologists. In recent years, the development of increasingly powerful molecular and statistical tools has reinvigorated this now fast-paced field of research. In this paper, we provide an overview of how such tools have been applied to the study of crop evolution. We also highlight lessons that have been learned in light of a few long-standing and interrelated hypotheses concerning the origins of crop plants and the nature of the genetic changes underlying their evolution. We conclude by discussing compelling evolutionary genomic approaches that make possible the efficient and unbiased identification of genes controlling crop-related traits and provide further insight into the actual timing of selection on particular genomic regions.     

I-SceI Meganuclease-mediated transgenesis in the acorn worm, Saccoglossus kowalevskii

Hemichordates are a phylum of marine invertebrate deuterostomes that are closely related to chordates, and represent one of the most promising models to provide insights into early deuterostome evolution. The genome of the hemichordate, Saccoglossus kowalevskii, reveals an extensive set of non-coding elements conserved across all three deuterostome phyla. Functional characterization and cross-phyla comparisons of these putative regulatory elements will enable a better understanding of enhancer evolution, and subsequently how changes in gene regulation give rise to morphological innovation. Here, we describe an efficient method of transgenesis for the characterization of non-coding elements in S. kowalevskii. We first test the capacity of an I-SceI transgenesis system to drive ubiquitous or regionalized gene expression, and to label specific cell types. Finally, we identified a minimal promoter that can be used to test the capacity of putative enhancers in S. kowalevskii. This work demonstrates that this I-SceI transgenesis technique, when coupled with an understanding of chromatin accessibility, can be a powerful tool for studying how evolutionary changes in gene regulatory mechanisms contributed to the diversification of body plans in deuterostomes.     

Development of the nervous system in hatchlings of Spadella cephaloptera (Chaetognatha), and implications for nervous system evolution in Bilateria

Chaetognaths (arrow worms) play an important role as predators in planktonic food webs. Their phylogenetic position is unresolved, and among the numerous hypotheses, affinities to both protostomes and deuterostomes have been suggested. Many aspects of their life history, including ontogenesis, are poorly understood and, though some aspects of their embryonic and postembryonic development have been described, knowledge of early neural development is still limited. This study sets out to provide new insights into neurogenesis of newly hatched Spadella cephaloptera and their development during the following days, with attention to the two main nervous centers, the brain and the ventral nerve center. These were examined with immunohistological methods and confocal laser-scan microscopic analysis, using antibodies against tubulin, FMRFamide, and synapsin to trace the emergence of neuropils and the establishment of specific peptidergic subsystems. At hatching, the neuronal architecture of the ventral nerve center is already well established, whereas the brain and the associated vestibular ganglia are still rudimentary. The development of the brain proceeds rapidly over the next 6 days to a state that resembles the adult pattern. These data are discussed in relation to the larval life style and behaviors such as feeding. In addition, we compare the larval chaetognath nervous system and that of other bilaterian taxa in order to extract information with phylogenetic value. We conclude that larval neurogenesis in chaetognaths does not suggest an especially close relationship to either deuterostomes or protostomes, but instead displays many apomorphic features.     

Mitogenomics of Vetigastropoda: insights into the evolution of pallial symmetry

The nucleotide sequences of the complete or nearly complete mitochondrial (mt) genomes of seven vetigastropods were determined: Angaria neglecta (Angarioidea), Phasianella solida (Phasianelloidea), Granata lyrata (Seguenzioidea), Tegula lividomaculata and Bolma rugosa (Trochoidea), Diodora graeca (Fissurelloidea) and Lepetodrilus schrolli (Lepetodriloidea). While the mt genomes of the superfamilies Angarioidea, Phasianelloidea, Seguenzioidea and Trochoidea conform generally to the ancestral gene order of Vetigastropoda and Gastropoda, those of the superfamilies Fissurelloidea and Lepetodriloidea have suffered important rearrangements. The gene order of the mtDNA of Chrysomallon squamiferum, a representative of Neomphalina, was also analysed since it has been proposed to be closely related to Vetigastropoda, and showed a distinct arrangement. The reconstructed phylogenies recovered Neomphalina as a distinct gastropod lineage that is the sister group (only with moderate bootstrap support) of a clade including Vetigastropoda and Neritimorpha + Caeno‐gastropoda while the relative position of Heterobranchia and Patellogastropoda in the gastropod tree could not be determined definitively due to their long branches. Within the monophyletic Vetigastropoda, the superfamily Fissurelloidea was recovered as the sister group of two lineages, one including Lepetodriloidea as the sister group of Seguenzioidea + Halitoidea, the other including Phasianelloidea, Angarioidea and Trochoidea without resolved relationships. The long branches of Fissurelloidea were found to introduce significant tree instability in phylogenetic reconstruction. The new phylogeny supports that the loss of the right pallial gill occurred multiple times in vetigastropod evolution as previously suggested and that Phasianelloidea, Angarioidea and Trochoidea radiated from a common asymmetric (single‐gilled) ancestor that lived in the middle Palaeozoic.