DOCSLIB.ORG

The Embryonic Development of the Flatworm Macrostomum Sp

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Embryonic Development of the Flatworm Macrostomum Sp Dev Genes Evol (2004) 214:220–239 DOI 10.1007/s00427-004-0406-4 ORIGINAL ARTICLE Joshua Morris · Ramachandra Nallur · Peter Ladurner · Bernhard Egger · Reinhard Rieger · Volker Hartenstein The embryonic development of the flatworm Macrostomum sp. Received: 20 January 2004 / Accepted: 15 March 2004 / Published online: 9 April 2004 Springer-Verlag 2004 Abstract Macrostomid flatworms represent a group of zonula. Terminal web and zonula adherens are particu- basal bilaterians with primitive developmental and mor- larly well observed in the epidermis. During stage 6, the phological characteristics. The species Macrostomum sp., somatic primordium extends around the surface dorsally raised under laboratory conditions, has a short generation and ventrally to form a complete body wall. Muscle time of about 2–3 weeks and produces a large number of precursors extend myofilaments that are organized into a eggs year round. Using live observation, histology, elec- highly regular orthogonal network of circular, diagonal tron microscopy and immunohistochemistry we have and longitudinal fibers. Neurons of the brain primordium carried out a developmental analysis of Macrostomum sp. differentiate a commissural neuropile that extends a sin- Cleavage (stages 1–2) of this species follows a modified gle pair of ventro-lateral nerve trunks (the main longi- spiral pattern and results in a solid embryonic primordium tudinal cords) posteriorly. The primordial pharynx lu- surrounded by an external yolk layer. During stage 3, cells men fuses with the ventral epidermis anteriorly and the at the anterior and lateral periphery of the embryo evolve gut posteriorly, thereby generating a continuous digestive into the somatic primordium which gives rise to the body tract. The embryo adopts its final shape during stages 7 wall and nervous system. Cells in the center form the and 8, characterized by the morphallactic lengthening of large yolk-rich gut primordium. During stage 4, the brain the body into a U-shaped form and the condensation of primordium and the pharynx primordium appear as sym- the nervous system. metric densities anterior-ventrally within the somatic pri- mordium. Organ differentiation commences during stage Keywords Platyhelminth · Embryo · Morphogenesis · 5 when the neurons of the brain primordium extend axons Organogenesis · Differentiation that form a central neuropile, and the outer cell layer of the somatic primordium turns into a ciliated epidermal epithelium. Cilia also appear in the lumen of the pharynx Introduction primordium, in the protonephridial system and, slightly later, in the lumen of the gut. Ultrastructurally, these Flatworms represent a large and diverse taxon of simple differentiating cells show the hallmarks of platyhelminth invertebrate animals. Recently they have again attracted epithelia, with a pronounced apical assembly of micro- the attention of developmental biologists because, in re- filaments (terminal web) inserting at the zonula adherens, gard to numerous morphological criteria, all or at least and a wide band of septate junctions underneath the some of their taxa may have branched off the phyloge- netic tree near the root of the bilaterian stock (Rieger et al. 1991; Ax 1996; Tyler 2001; Jondelius et al. 2002). Edited by J. Campos-Ortega This makes them a highly relevant system in which to J. Morris · R. Nallur · V. Hartenstein ()) study basic bilaterian developmental processes, such as Department of Molecular, Cell and Developmental Biology, establishment of the body axes and organogenesis (e.g., University of California, Hartenstein and Ehlers 2000; Younossi-Hartenstein and Los Angeles, CA, 90095, USA Hartenstein 2000a, b; Hartenstein and Jones 2003). Flat- e-mail: [email protected] worms share a number of fundamental characters with Tel.: +1-310-2067523 Fax: +1-310-2063987 coelenterates, which evolved before the bilaterians. Most notable among these characters is a single gut opening P. Ladurner · B. Egger · R. Rieger that serves as both mouth and anus, and a ciliated epi- Institute of Zoology and Limnology, dermis used for locomotion (for recent comparative de- University of Innsbruck, scription of adult flatworm anatomy, see Ehlers 1985; Technikerstrasse 25, 6020 Innsbruck, Austria 221 Rieger et al. 1991; Ax 1996). Like higher animals, most simple pharynx shared by all macrostomids places this flatworms have a subepidermal muscular layer of circu- taxon near the base of the rhabditophoran platyhelminths lar, diagonal and longitudinal fibers (see Rieger et al. (see Doe 1981 and references cited above), a classifica- 1991, 1994; Hooge 2001), and a central nervous system tion that is supported by recent molecular data (Bagu et that comprises an anterior brain and a number of longi- al. 2001; Curini-Galletti 2001; Jondelius et al. 2001; tudinal nerve cords (Rieger et al. 1991; Reuter and Littlewood et al. 2001). Macrostomid development has Halton 2001). However, the central nervous system lacks been studied several times, but only very cursorily: his- compactness, being also penetrated by muscle and gland tological and much less in vivo by Seilern-Aspang (1957) cells. Nerve tracts issuing forth from the brain extend in M. appendiculatum, in vivo by Reisinger (1923) in M. dorsally, laterally and ventrally. These tracts are con- viride, by Papi (1953) in M. appendiculatum, by Bogo- nected to up to three subepidermal nerve plexus. The molow (1949, 1960) in M. viride and M. rossicum and connectivity between neurons and muscles in flatworms finally by Ax and Borkott (1968a, b) in M. romanicum, is different from higher animals, in that, frequently, long who provided the first film material about the embryonic muscle processes (sarco-neuronal processes) approach development, but still left open a number of important the nerve fibers, instead of the other way around (see questions. The formation of the body wall was addressed Rieger et al. 1991, similar to the well studied situation in in several more recent light and electron microscopic as nematodes). The internal structure of flatworms is usu- well as histochemical studies (Tyler 1981; Reiter et al. ally referred to as being very simple. Flatworms do not 1996; E. Robatscher and B. Egger, Innsbruck, unpub- possess a coelom. The interior of the body is filled with lished results). These studies show that Macrostomum sp. the gut tube and reproductive organs, with complex as well as other Macrostomum species of the M. hys- muscle and connective tissue cells. A specialized vas- tricinum species clade (see Rieger 1977) share with other cular system or respiratory system is absent in free-living primitive flatworm groups a spiral pattern of cleavage. platyhelminths. The only tubular structures are com- However, development starts to deviate after the first prised of protonephridia (see Rohde 2001). three to four rounds of divisions from the typical spiralian Embryonic development has been studied in a number mode, in agreement with the observations by Seilern- of different flatworm species, studying live embryos and Aspang (1957). Thus, blastomeres at the vegetal pole using classical techniques of histology and electron mi- seem to spread out at the surface of the embryo and form croscopy, as well as, to a very limited extent, cell type- an outer yolk mantle (Tyler 1981). This process was specific markers, including markers for muscle and nerve called “inverse epiboly” by Thomas (1986). The transi- cells (Thomas 1986; Bagu and Boyer 1990; Reiter et tion from this stage to the final generation of body wall al. 1996; Boyer et al. 1996; Ladurner and Rieger 2000; and internal organs has not been followed in any detail Younossi-Hartenstein et al. 2000; Younossi-Hartenstein satisfactorily. In this paper, we have undertaken a de- and Hartenstein 2000a, b, 2001; Hartenstein and Jones velopmental analysis of Macrostomum sp. on the basis of 2003). Molecular tools, both as markers and as agents to observation of live embryos, histological staining of disrupt function, have been developed to study regener- sectioned and whole-mount material, electron microscopy ation and cell differentiation in planarians, members of and immunohistochemistry. We define a series of mor- the flatworm clade Tricladida (Sanchez-Alvarado and phological stages that are in accordance with a system Newmark 1999; Sanchez-Alvarado et al. 2002; Pineda introduced for other platyhelminth taxa during recent et al. 2000, 2002; Salo et al. 2002; Cebria et al. 2002; years (Younossi-Hartenstein et al. 2000). Our work is Ogawa et al. 2002). In order to initiate a molecular aimed at providing a guide for the molecular-genetic analysis of embryonic development we have chosen the analysis of platyhelminth embryogenesis, in particular the microturbellarian Macrostomum sp. as a species that can interpretation of gene expression data from in situ hy- be easily raised in the laboratory, produces a multitude of bridization studies which are currently underway (V. eggs year round, and has a very short generation time of Hartenstein, UCLA, and P. Ladurner, Innsbruck, unpub- approximately 2–3 weeks. Macrostomum sp. is a new lished data). species that was first briefly characterized in Ladurner et al. (2000), and is now being described by Ladurner et al. (2004). The new species belongs to the clade Macrosto- Materials and methods mida within the Macrostomorpha. Based on morpholog- ical and molecular phylogenetic analysis, the latter oc- Animals cupy a position near the root of the rhabditophoran flat- Macrostomum sp. is
Load more

Recommended publications
  • Platyhelminthes, Nemertea, and "Aschelminthes" - A
    BIOLOGICAL SCIENCE FUNDAMENTALS AND SYSTEMATICS – Vol. III - Platyhelminthes, Nemertea, and "Aschelminthes" - A. Schmidt-Rhaesa PLATYHELMINTHES, NEMERTEA, AND “ASCHELMINTHES” A. Schmidt-Rhaesa University of Bielefeld, Germany Keywords: Platyhelminthes, Nemertea, Gnathifera, Gnathostomulida, Micrognathozoa, Rotifera, Acanthocephala, Cycliophora, Nemathelminthes, Gastrotricha, Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera Contents 1. Introduction 2. General Morphology 3. Platyhelminthes, the Flatworms 4. Nemertea (Nemertini), the Ribbon Worms 5. “Aschelminthes” 5.1. Gnathifera 5.1.1. Gnathostomulida 5.1.2. Micrognathozoa (Limnognathia maerski) 5.1.3. Rotifera 5.1.4. Acanthocephala 5.1.5. Cycliophora (Symbion pandora) 5.2. Nemathelminthes 5.2.1. Gastrotricha 5.2.2. Nematoda, the Roundworms 5.2.3. Nematomorpha, the Horsehair Worms 5.2.4. Priapulida 5.2.5. Kinorhyncha 5.2.6. Loricifera Acknowledgements Glossary Bibliography Biographical Sketch Summary UNESCO – EOLSS This chapter provides information on several basal bilaterian groups: flatworms, nemerteans, Gnathifera,SAMPLE and Nemathelminthes. CHAPTERS These include species-rich taxa such as Nematoda and Platyhelminthes, and as taxa with few or even only one species, such as Micrognathozoa (Limnognathia maerski) and Cycliophora (Symbion pandora). All Acanthocephala and subgroups of Platyhelminthes and Nematoda, are parasites that often exhibit complex life cycles. Most of the taxa described are marine, but some have also invaded freshwater or the terrestrial environment. “Aschelminthes” are not a natural group, instead, two taxa have been recognized that were earlier summarized under this name. Gnathifera include taxa with a conspicuous jaw apparatus such as Gnathostomulida, Micrognathozoa, and Rotifera. Although they do not possess a jaw apparatus, Acanthocephala also belong to Gnathifera due to their epidermal structure. ©Encyclopedia of Life Support Systems (EOLSS) BIOLOGICAL SCIENCE FUNDAMENTALS AND SYSTEMATICS – Vol.
    [Show full text]
  • Towards a Management Hierarchy (Classification) for the Catalogue of Life
    TOWARDS A MANAGEMENT HIERARCHY (CLASSIFICATION) FOR THE CATALOGUE OF LIFE Draft Discussion Document Rationale The Catalogue of Life partnership, comprising Species 2000 and ITIS (Integrated Taxonomic Information System), has the goal of achieving a comprehensive catalogue of all known species on Earth by the year 2011. The actual number of described species (after correction for synonyms) is not presently known but estimates suggest about 1.8 million species. The collaborative teams behind the Catalogue of Life need an agreed standard classification for these 1.8 million species, i.e. a working hierarchy for management purposes. This discussion document is intended to highlight some of the issues that need clarifying in order to achieve this goal beyond what we presently have. Concerning Classification Life’s diversity is classified into a hierarchy of categories. The best-known of these is the Kingdom. When Carl Linnaeus introduced his new “system of nature” in the 1750s ― Systema Naturae per Regna tria naturae, secundum Classes, Ordines, Genera, Species …) ― he recognised three kingdoms, viz Plantae, Animalia, and a third kingdom for minerals that has long since been abandoned. As is evident from the title of his work, he introduced lower-level taxonomic categories, each successively nested in the other, named Class, Order, Genus, and Species. The most useful and innovative aspect of his system (which gave rise to the scientific discipline of Systematics) was the use of the binominal, comprising genus and species, that uniquely identified each species of organism. Linnaeus’s system has proven to be robust for some 250 years. The starting point for botanical names is his Species Plantarum, published in 1753, and that for zoological names is the tenth edition of the Systema Naturae published in 1758.
    [Show full text]
  • The Interrelationships of Metazoan Parasites: a Review of Phylum- and Higher-Level Hypotheses from Recent Morphological and Molecular Phylogenetic Analyses
    FOLIA PARASITOLOGICA 48: 81-103, 2001 The interrelationships of metazoan parasites: a review of phylum- and higher-level hypotheses from recent morphological and molecular phylogenetic analyses Jan Zrzavý Department of Zoology, Faculty of Biological Sciences, University of South Bohemia, and Institute of Entomology, Academy of Sciences of the Czech Republic, Branišovská 31, 370 05 České Budějovice, Czech Republic Key words: phylogeny, parasitism, Myxozoa, Mesozoa, Neodermata, Myzostomida, Seisonida, Acanthocephala, Pentastomida, Nematomorpha, Nematoda Abstract. Phylogeny of seven groups of metazoan parasitic groups is reviewed, based on both morphological and molecular data. The Myxozoa (=Malacosporea + Myxosporea) are most probably related to the egg-parasitic cnidarian Polypodium (Hydrozoa?: Polypodiozoa); the other phylogenetic hypotheses are discussed and the possible non-monophyly of the Cnidaria (with the Polypodiozoa-Myxozoa clade closest to the Triploblastica) is suggested. The Mesozoa is a monophyletic group, possibly closely related to the (monophyletic) Acoelomorpha; whether the Acoelomorpha and Mesozoa represent the basalmost triploblast clade(s) or a derived platyhelminth subclade may depend on rooting the tree of the Triploblastica. Position of the monophyletic Neodermata (=Trematoda + Cercomeromorpha) within the rhabditophoran flatworms is discussed, with two major alternative hypotheses about the neodermatan sister-group relationships (viz., the “neoophoran” and “revertospermatan”). The Myzostomida are not annelids but belong among the Platyzoa, possibly to the clade of animals with anterior sperm flagella (=Prosomastigozoa). The Acanthocephala represent derived syndermates (“rotifers”), possibly related to Seison (the name Pararotatoria comb. n. is proposed for Seisonida + Acanthocephala). The crustacean origin of the Pentastomida based on spermatological and molecular evidence (Pentastomida + Branchiura = Ichthyostraca) is confronted with palaeontological views favouring the pre-arthropod derivation of the pentastomids.
    [Show full text]
  • Is the Turbellaria Polyphyletic?
    Is the Turbellaria polyphyletic? Julian P. S. Smith, III,' Seth Teyler' & Reinhard M . Rieger2 'Department of Zoology, University of Maine, Orono, ME 04469, USA 2Department of Biology, University of North Carolina, Chapel Hill, NC 27514, USA Keywords: Tirbellaria, phylogenetic systematics, Platyhelminthes, polyphyly, ultrastructure, epidermis, cilia Abstract Within the last two decades, syntheses of both light-microscopic and ultrastructural characters have shown that there are three well-defined monophyletic groups within the Platyhelminthes : 1) the Catenulidale, 2) the Nemertodermatida-Acoela, and 3) the Haplopharyngida-Macrostomida-Polycladida-Neoophora (+ parasit- ic platyhelminth classes) . However, the relationships among these three groups are problematic . The possible apomorphies that would unite them are either not true homologues (i.e. frontal organ), are mutually conflict- ing (i.e. 9+1 axoneme in spermatozoa vs . biflagellate spermatozoa, epidermal ciliary rootlet structure, and protonephridia), or are unrooted with any outgroup and hence untestable or uncertain as apomorphies (pro- tonephridia, mode of epidermal replacement, absence of accessory centrioles on cilia) . The chief obstacle to deciphering the relationships of these groups is the lack of information on them ; presently available infor- mation is insufficient to test potential synapomorphies and insufficient also to allow agreement upon a nar- rowly defined outgroup for the Turbellaria . A view consistent with the present evidence (and admittedly an unsatisfactory
    [Show full text]
  • Neodermata: Gyrocotylidea)
    FOLIA PARASITOLOGICA 57[3]: 173–184, 2010 © Institute of Parasitology, Biology Centre ASCR ISSN 0015-5683 (print), ISSN 1803-6465 (online) http://www.paru.cas.cz/folia/ Ultrastructure of the ovarian follicles, oviducts and oocytes of Gyrocotyle urna (Neodermata: Gyrocotylidea) Larisa G. Poddubnaya1, Roman Kuchta2, Tomáš Scholz2 and Willi E.R. Xylander3 1 Institute of Biology of Inland Waters, Russian Academy of Sciences, 152742 Borok, Yaroslavl Province, Russia; 2 Institute of Parasitology, Biology Centre of the Academy of Sciences of the Czech Republic, Branišovská 31, 370 05 České Budějovice, Czech Republic; 3 Senckenberg Museum für Naturkunde Görlitz, Postfach 300 154, 02806 Görlitz, Germany Abstract: An ultrastructural study of the ovarian follicles and their associated oviducts of the cestode Gyrocotyle urna Grube et Wagener, 1852, a parasite from the spiral valve of the rabbit fish,Chimaera monstrosa L., was undertaken. Each follicle gives rise to follicular oviduct, which opens into one of the five collecting ducts, through which pass mature oocytes. These collecting ducts open into an ovarian receptacle which, in turn, opens via a muscular sphincter (the oocapt) to the main oviduct. The maturation of oocytes surrounded by the syncytial interstitial cells within the ovarian follicles of G. urna follows a pattern similar to that in Eucestoda. The ooplasm of mature oocytes contain lipid droplets (2.0 × 1.8 µm) and cortical granules (0.26 × 0.19 µm). The cytoplasm of primary and secondary oocytes contains centrioles, indicating the presence of the so-called “centriole cycle” during ������������������������oocyte �����������������divisions. A mor- phological variation between different oviducts was observed. The luminal surface of the follicular and the collecting oviducts is smooth.
    [Show full text]
  • Animal Phylogeny and the Ancestry of Bilaterians: Inferences from Morphology and 18S Rdna Gene Sequences
    EVOLUTION & DEVELOPMENT 3:3, 170–205 (2001) Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences Kevin J. Peterson and Douglas J. Eernisse* Department of Biological Sciences, Dartmouth College, Hanover NH 03755, USA; and *Department of Biological Science, California State University, Fullerton CA 92834-6850, USA *Author for correspondence (email: [email protected]) SUMMARY Insight into the origin and early evolution of the and protostomes, with ctenophores the bilaterian sister- animal phyla requires an understanding of how animal group, whereas 18S rDNA suggests that the root is within the groups are related to one another. Thus, we set out to explore Lophotrochozoa with acoel flatworms and gnathostomulids animal phylogeny by analyzing with maximum parsimony 138 as basal bilaterians, and with cnidarians the bilaterian sister- morphological characters from 40 metazoan groups, and 304 group. We suggest that this basal position of acoels and gna- 18S rDNA sequences, both separately and together. Both thostomulids is artifactal because for 1000 replicate phyloge- types of data agree that arthropods are not closely related to netic analyses with one random sequence as outgroup, the annelids: the former group with nematodes and other molting majority root with an acoel flatworm or gnathostomulid as the animals (Ecdysozoa), and the latter group with molluscs and basal ingroup lineage. When these problematic taxa are elim- other taxa with spiral cleavage. Furthermore, neither brachi- inated from the matrix, the combined analysis suggests that opods nor chaetognaths group with deuterostomes; brachiopods the root lies between the deuterostomes and protostomes, are allied with the molluscs and annelids (Lophotrochozoa), and Ctenophora is the bilaterian sister-group.
    [Show full text]
  • Cellular Dynamics During Regeneration of the Flatworm Monocelis Sp. (Proseriata, Platyhelminthes) Girstmair Et Al
    Cellular dynamics during regeneration of the flatworm Monocelis sp. (Proseriata, Platyhelminthes) Girstmair et al. Girstmair et al. EvoDevo 2014, 5:37 http://www.evodevojournal.com/content/5/1/37 Girstmair et al. EvoDevo 2014, 5:37 http://www.evodevojournal.com/content/5/1/37 RESEARCH Open Access Cellular dynamics during regeneration of the flatworm Monocelis sp. (Proseriata, Platyhelminthes) Johannes Girstmair1,2, Raimund Schnegg1,3, Maximilian J Telford2 and Bernhard Egger1,2* Abstract Background: Proseriates (Proseriata, Platyhelminthes) are free-living, mostly marine, flatworms measuring at most a few millimetres. In common with many flatworms, they are known to be capable of regeneration; however, few studies have been done on the details of regeneration in proseriates, and none cover cellular dynamics. We have tested the regeneration capacity of the proseriate Monocelis sp. by pre-pharyngeal amputation and provide the first comprehensive picture of the F-actin musculature, serotonergic nervous system and proliferating cells (S-phase in pulse and pulse-chase experiments and mitoses) in control animals and in regenerates. Results: F-actin staining revealed a strong body wall, pharynx and dorsoventral musculature, while labelling of the serotonergic nervous system showed an orthogonal pattern and a well developed subepidermal plexus. Proliferating cells were distributed in two broad lateral bands along the anteroposterior axis and their anterior extension was delimited by the brain. No proliferating cells were detected in the pharynx or epidermis. Monocelis sp. was able to regenerate the pharynx and adhesive organs at the tip of the tail plate within 2 or 3 days of amputation, and genital organs within 8 to 10 days.
    [Show full text]
  • Platyhelminthes, Proseriata): Inferences from Rdna Sequences
    327 MOLECULAR PHYLOGENETICS AND EVOLUTION Vol. 6, No. 1, August, pp. 150-156, 1996 ARTICLE NO. 0067 A Reappraisal of the Systematics of the Monocelididae (Platyhelminthes, Proseriata): Inferences from rDNA Sequences M . K . L it v a it is , * M . C . C u r in i -G a l l e t t i ,+ P . M . M a r t e n s , * a n d T . D . K o c h e r * * Department o f Zoology, University o f New Hampshire, Durham, New Hampshire; t Istituto d i Zoología, Université degii Studi di Sassari, Sassari, Italy; and * Department SBM, Limburgs Universitair Centrum, Diepenbeek, Belgium Received August 11, 1995 on the presence (Minoninae) or the absence (Monoceli­ The current classification system for the Monocelidi­ dinae) of the accessory prostatoid organ, a musculo- dae which is based on the character “presence or ab­ glandular structure armed with a stylet (Fig. 1), and sence of an accessory prostatoid organ” divides the he attributed less phylogenetic weight to the structure family into two subfamilies, namely the Minoninae and of the male copulatory bulb. Within the Monocelididae, the Monocelidinae. However, other characters re­ the copulatory bulb is of the conjuncta type (Karling, lating to the structure of the male copulatory bulb and 1956), and two subtypes can be recognized. The sim­ to karyotypes do not support this division. Monoceli- plex-type consists of a single muscular wall, which de­ d id m ale c o p u la to ry b u lb s c an b e e ith e r o f th e sim plex rives from the last portion of the ductus ejaculatorius or the duplex-type, and if this character is mapped (Fig.
    [Show full text]
  • Phylum Platyhelminthes
    Author's personal copy Chapter 10 Phylum Platyhelminthes Carolina Noreña Departamento Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain Cristina Damborenea and Francisco Brusa División Zoología Invertebrados, Museo de La Plata, La Plata, Argentina Chapter Outline Introduction 181 Digestive Tract 192 General Systematic 181 Oral (Mouth Opening) 192 Phylogenetic Relationships 184 Intestine 193 Distribution and Diversity 184 Pharynx 193 Geographical Distribution 184 Osmoregulatory and Excretory Systems 194 Species Diversity and Abundance 186 Reproductive System and Development 194 General Biology 186 Reproductive Organs and Gametes 194 Body Wall, Epidermis, and Sensory Structures 186 Reproductive Types 196 External Epithelial, Basal Membrane, and Cell Development 196 Connections 186 General Ecology and Behavior 197 Cilia 187 Habitat Selection 197 Other Epidermal Structures 188 Food Web Role in the Ecosystem 197 Musculature 188 Ectosymbiosis 198 Parenchyma 188 Physiological Constraints 199 Organization and Structure of the Parenchyma 188 Collecting, Culturing, and Specimen Preparation 199 Cell Types and Musculature of the Parenchyma 189 Collecting 199 Functions of the Parenchyma 190 Culturing 200 Regeneration 190 Specimen Preparation 200 Neural System 191 Acknowledgment 200 Central Nervous System 191 References 200 Sensory Elements 192 INTRODUCTION by a peripheral syncytium with cytoplasmic elongations. Monogenea are normally ectoparasitic on aquatic verte- General Systematic brates, such as fishes,
    [Show full text]
  • Evolutionary History of the Tricladida and the Platyhelminthes: an Up-To
    Int. J. Dev. Biol. 56: 5-17 doi: 10.1387/ijdb.113441mr www.intjdevbiol.com Evolutionary history of the Tricladida and the Platyhelminthes: an up-to-date phylogenetic and systematic account MARTA RIUTORT*,1, MARTA ÁLVAREZ-PRESAS1, EVA LÁZARO1, EDUARD SOLÀ1 and JORDI PAPS2 1Institut de Recerca de la Biodiversitat (IRBio) i Departament de Genètica, Universitat de Barcelona, Spain and 2Department of Zoology, University of Oxford, UK ABSTRACT Within the free-living platyhelminths, the triclads, or planarians, are the best-known group, largely as a result of long-standing and intensive research on regeneration, pattern forma- tion and Hox gene expression. However, the group’s evolutionary history has been long debated, with controversies ranging from their phyletic structure and position within the Metazoa to the relationships among species within the Tricladida. Over the the last decade, with the advent of molecular phylogenies, some of these issues have begun to be resolved. Here, we present an up- to-date summary of the main phylogenetic changes and novelties with some comments on their evolutionary implications. The phylum has been split into two groups, and the position of the main group (the Rhabdithophora and the Catenulida), close to the Annelida and the Mollusca within the Lophotrochozoa, is now clear. Their internal relationships, although not totally resolved, have been clarified. Tricladida systematics has also experienced a revolution since the implementation of molecular data. The terrestrial planarians have been demonstrated to have emerged from one of the freshwater families, giving a different view of their evolution and greatly altering their classifi- cation. The use of molecular data is also facilitating the identification of Tricladida species by DNA barcoding, allowing better knowledge of their distribution and genetic diversity.
    [Show full text]
  • Developmental Diversity in Free-Living Flatworms Martín-Durán and Egger
    Developmental diversity in free-living flatworms Martín-Durán and Egger Martín-Durán and Egger EvoDevo 2012, 3:7 http://www.evodevojournal.com/content/3/1/7 (19 March 2012) Martín-Durán and Egger EvoDevo 2012, 3:7 http://www.evodevojournal.com/content/3/1/7 REVIEW Open Access Developmental diversity in free-living flatworms José María Martín-Durán1,2 and Bernhard Egger3,4* Abstract Flatworm embryology has attracted attention since the early beginnings of comparative evolutionary biology. Considered for a long time the most basal bilaterians, the Platyhelminthes (excluding Acoelomorpha) are now robustly placed within the Spiralia. Despite having lost their relevance to explain the transition from radially to bilaterally symmetrical animals, the study of flatworm embryology is still of great importance to understand the diversification of bilaterians and of developmental mechanisms. Flatworms are acoelomate organisms generally with a simple centralized nervous system, a blind gut, and lacking a circulatory organ, a skeleton and a respiratory system other than the epidermis. Regeneration and asexual reproduction, based on a totipotent neoblast stem cell system, are broadly present among different groups of flatworms. While some more basally branching groups - such as polyclad flatworms - retain the ancestral quartet spiral cleavage pattern, most flatworms have significantly diverged from this pattern and exhibit unique strategies to specify the common adult body plan. Most free-living flatworms (i.e. Platyhelminthes excluding the parasitic Neodermata) are directly developing, whereas in polyclads, also indirect developers with an intermediate free-living larval stage and subsequent metamorphosis are found. A comparative study of developmental diversity may help understanding major questions in evolutionary biology, such as the evolution of cleavage patterns, gastrulation and axial specification, the evolution of larval types, and the diversification and specialization of organ systems.
    [Show full text]
  • Developmental Diversity in Free-Living Flatworms Martín-Durán and Egger
    Developmental diversity in free-living flatworms Martín-Durán and Egger Martín-Durán and Egger EvoDevo 2012, 3:7 http://www.evodevojournal.com/content/3/1/7 (19 March 2012) Martín-Durán and Egger EvoDevo 2012, 3:7 http://www.evodevojournal.com/content/3/1/7 REVIEW Open Access Developmental diversity in free-living flatworms José María Martín-Durán1,2 and Bernhard Egger3,4* Abstract Flatworm embryology has attracted attention since the early beginnings of comparative evolutionary biology. Considered for a long time the most basal bilaterians, the Platyhelminthes (excluding Acoelomorpha) are now robustly placed within the Spiralia. Despite having lost their relevance to explain the transition from radially to bilaterally symmetrical animals, the study of flatworm embryology is still of great importance to understand the diversification of bilaterians and of developmental mechanisms. Flatworms are acoelomate organisms generally with a simple centralized nervous system, a blind gut, and lacking a circulatory organ, a skeleton and a respiratory system other than the epidermis. Regeneration and asexual reproduction, based on a totipotent neoblast stem cell system, are broadly present among different groups of flatworms. While some more basally branching groups - such as polyclad flatworms - retain the ancestral quartet spiral cleavage pattern, most flatworms have significantly diverged from this pattern and exhibit unique strategies to specify the common adult body plan. Most free-living flatworms (i.e. Platyhelminthes excluding the parasitic Neodermata) are directly developing, whereas in polyclads, also indirect developers with an intermediate free-living larval stage and subsequent metamorphosis are found. A comparative study of developmental diversity may help understanding major questions in evolutionary biology, such as the evolution of cleavage patterns, gastrulation and axial specification, the evolution of larval types, and the diversification and specialization of organ systems.
    [Show full text]