Arthropodization in the Hirudinea: evidence for a phylogenetic link with insects and other Uniramia?

The Hirudinea have a number of arthropod-like characters, including a true haemocoel and compound eyes. This 'arthropodization' may have bearing on the origin of the arthropod phylum Uniramia. Certain traits shared by Clitellata and Uniramia are interpreted as primitive and compatible with the view that a monophyletic link exists between these two groups: 'internal' fertilization, egg protected by cocoon/chorion, egg yolky with direct development not invoking a planktotrophic larval stage, formation and fate of presumptive areas in early embryology, uniramous lobopodial-like structures and the presence of uniramous mouthparts. Still other traits shared by Hirudinea and Hexapoda are interpreted as advanced and evidence for a paraphyletic relationship between leeches and pterygote insects: oogenesis with nurse cells, cephalization and segmental constancy. It is proposed that a major theme in uniramian evolution is the invasion of land from freshwater by a pre-lobopodial clitellate-like ancestor, the lobopodium being an adaptation to terrestrial locomotion. These views give new significance to the poorly studied fossil segmented worms from freshwater and terrestrial palaeohabitats.     

Evolution de la musculature desNéréidiens (Annélides polychètes) au cours de l'Epitoquie

Résumé Au cours de l'épitoquie des Nereidiens, les fibres musculaires longitudinales ne sont pas forméesde novo, à partir de cellules indifferenciées ou myoblastes, mais proviennent des fibres anciennes atoques. Celles-ci subissent une véritable dédifférenciation plus ou moins synchrone d'une redifférenciation. Les deux processus ne sont pas successifs mais simultanés, et une dédifférenciation complète est absente.Les premières cellules en évolution appartiennent à la couche musculaire externe; ensuite, les fibres des assises plus profondes se transforment à leur tour.Les transformations consistent en: 1) La dédifférenciation du bord interne ou coelomique de la fibre. Les structures contractiles disparaissent dans cette zone et de nombreuses particules de glycogène se différencient sans relation avec le reticulum endoplasmique ou les ribosomes. Aucun lysosome ou signe précurseur ne peuvent être observés avant la disparition des filaments contractiles et des éléments Z. 2) Le bord coelomique s'hypertrophie. Dans la région axiale de la fibre, de nombreuses mitochondries et particules et de glycogène remplacent le matériel contractile. Corrélativement, l'épaisseur des bandes A et I diminue. 3) La fibre hétéronéreidienne ou épitoque est constituée et présente deux parties: un cortex myoplasmique et une médulla sarcoplasmique, remplie de mitochondries et de glycogène. Le noyau renfermant un nucléole volumineux est situé dans une hernie sarcoplasmique latérale.

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Evolution of muscles inNereidae (Annelida polychaeta) during Epitoky. III. Dedifferentiation of the longitudinal fibres

Summary During epitoky inNereidae, the longitudinal muscle fibres are not formedde novo from undifferentiated cells or myoblasts, but arise from the old atokous fibres. These undergo a true dedifferentiation more or less synchronously with a redifferentiation. The two processes are not successive but simultaneous and there is no complete dedifferentiation.The first cells that develop are in the outside muscle layer; then the fibres of the inside layers are transformed in their turn.The transformations consist of: 1) Dedifferentiation of the edge of the inner or coelomic fibre. The contractile structures disappear in this part and numerous glycogen particles differentiate, unrelated to endoplasmic reticulum or ribosomes. No lysosomes or precursory markings are observed before the disappearance of contractile filaments and Z rods. 2) The coelomic edge becomes enlarged. In the axial region of the fibre, numerous mitochondria and and glycogen particles take the place of the contractile material. Consequently, the thickness of A and I bands decreases. 3) The heteronereid or epitokous fibre is formed and shows two parts: a myoplasmic cortex and a sarcoplasmic medulla, filled with mitochondria and glycogen. The nucleus with a voluminous nucleolus settles inside a lateral sarcoplasmic swelling.

     

Modeling mate-finding behavior of the swarming polychaete,Nereis succinea,with TangoInSilico,a scientific orkflow-based simulation system for sexual searching

Summary

Pheromones can be used as attractants for the opposite sex in many environments; however, little is known about the search strategies employed in responding to pheromones in the marine environment. The spawning behavior of males of the polychaete Nereis succinea is known to be triggered at close range by a high concentration (>～10^?7 M) of pheromone, cysteine glutathione disulfide (CSSG), released by females. Since CSSG also causes acceleration of swimming and increased turning, in addition to eliciting ejaculation, we proposed the hypothesis that these behaviors elicited by low concentrations of pheromone can be used by males to find females. The current study develops a computer simulation model of male and female N. succinea behavior for testing whether male responses to low concentrations of CSSG can facilitate finding females. Video recording of female swimming behavior in the field showed spontaneous loops, spirals, and circles that have been incorporated into the model. The scientific workflow paradigm within which the computer model has been developed also incorporates a data provenance system to enable systematic replay and testing of responses to individual parameters. Output of the model shows complex turning behavior leading to successful mating encounters at concentrations as low as 3×10^?9 M CSSG. Behavior resembling the output of the model was recorded in field observations. Application of the model in the future will be used to determine what pheromone concentrations produce significant increases in the probability of mating encounters.     

Earthworm leukocyte populations specifically harbor lysosomal enzymes that may respond to bacterial challenge

Earthworm leukocytes (coelomocytes) are responsible for innate cellular immune functions such as phagocytosis and encapsulation against parasites and pathogens. Microbial killing results from the combined action of the phagocytic process with humoral immune factors such as agglutinins (e.g., lectins), lysosomal enzymes (e.g., acid phosphatase, lysozyme), and various cytotoxic and antimicrobial molecules. There is also evidence of weak adaptive immune responses against foreign transplants. This study focused on aspects of the innate immune response. First, anti-human acid phosphatase (anti-AcP) polyclonal antibody characterized different acid hydrolase patterns in coelomocytes. Second, flow cytometry identified a strongly immunoreactive coelomocyte population. Third, ultrastructural and cytochemical analyses revealed acid phosphatase in discrete granules (lysosomes) of effector hyaline and granular coelomocytes but not in mature chloragocytes. Coelomocytes were exposed to bacteria to assess how phagocytosis influences: (a) the production of acid phosphatase using Western blot, and (b) release of acid phosphatase using ELISA from cell-free coelomic fluid. Fourth, after phagocytosis, acid phosphatase levels differed between controls and experimentals. Fifth, we found a 39-kDa molecule that reacted intensely with anti-AcP. Our results suggest that effector earthworm coelomocytes may not eliminate pathogens only by phagocytosis but also by extracellular lysis.E.L. Cooper has been supported by The Alexander von Humboldt Foundation, a GAAC grant from the Federal Republic of Germany and two grants from NATO, a Cooperative Research grant (971128) and an Advanced Research Workshop grant (976680)     

Lateral organs in sedentary polychaetes (Annelida) – Ultrastructure and phylogenetic significance of an insufficiently known sense organ

Lateral organs are sense organs visible as densely ciliated pits or papillae between the noto‐ and the neuropodia in certain taxa of sedentary polychaetes. Ultrastructural studies in about 10 species of the following taxa Maldanidae, Opheliidae, Orbiniidae, Paraonidae, Magelonidae, Spionidae, Poecilochaetidae and Terebellidae have been designed to evaluate whether these organs are homologous among polychaetes. In spite of great external diversity, the investigations revealed an overall ultrastructural similarity. Differences between species investigated mainly concern the size of the organs as well as the number and arrangement of cells. The organs comprise supportive cells and uniciliated penetrative sensory cells. Their dendrites are closely arranged and thus their cilia may resemble multiciliated cells. There are two types of sensory cells: one type possesses no or mainly thin microvilli of which usually only a few reach the cuticular surface, and in the other type the cilium is consistently surrounded by 10 strong microvilli, which form a pore‐like opening in the cuticle. Further differences occur in the structure of the rootlet system. Basally, a retractor muscle attaches to the organ. The systematic significance of these organs within Annelida is discussed with respect to the conflicting phylogenetic hypotheses explaining the relationships of annelid taxa.     

Fossilized spermatozoa preserved in a 50-Myr-old annelid cocoon from Antarctica

The origin and evolution of clitellate annelids—earthworms, leeches and their relatives—is poorly understood, partly because body fossils of these delicate organisms are exceedingly rare. The distinctive egg cases (cocoons) of Clitellata, however, are relatively common in the fossil record, although their potential for phylogenetic studies has remained largely unexplored. Here, we report the remarkable discovery of fossilized spermatozoa preserved within the secreted wall layers of a 50-Myr-old clitellate cocoon from Antarctica, representing the oldest fossil animal sperm yet known. Sperm characters are highly informative for the classification of extant Annelida. The Antarctic fossil spermatozoa have several features that point to affinities with the peculiar, leech-like ‘crayfish worms'' (Branchiobdellida). We anticipate that systematic surveys of cocoon fossils coupled with advances in non-destructive analytical methods may open a new window into the evolution of minute, soft-bodied life forms that are otherwise only rarely observed in the fossil record.     

Musculature in polychaetes: comparison of Myrianida prolifera (Syllidae) and Sphaerodoropsis sp. (Sphaerodoridae)

Abstract. The relationship of the polychaete taxa Syllidae and Sphaerodoridae within Phyllodocida is still unresolved: phylogenetic analyses either show them as sister groups or more widely separated. The present article aims to provide information about the structure of the muscular system that could be essential for understanding their relationship. A crucial point is whether the body wall contains circular muscles, which has recently been shown to be absent in more taxa than previously known. The F-actin filaments in members of Myrianida prolifera (Syllidae) and Sphaerodoropsis sp. (Sphaerodoridae) were labeled with phalloidin and their three-dimensional relationships reconstructed by means of confocal laser scanning microscopy. Among the noteworthy differences that emerged between the species are (1) members of M. prolifera possess four, those of Sphaerodoropsis sp. eight, longitudinal muscle strands; (2) the body wall in M. prolifera contains transverse fibers in a typical, supralongitudinal position, while in Sphaerodoropsis sp., corresponding fibers lie beneath the longitudinal strands; (3) pro- and peristomium in M. prolifera have no distinct F-actin fibers, while five longitudinal pairs and three single transverse muscular fibers shape the anterior end in Sphaerodoropsis sp.; (4) the proventricle of M. prolifera comprises primarily radial muscle fibers arranged in distinct rows, while in Sphaerodoropsis sp. the axial proboscis consists of longitudinal and circular fibers and radial fibers are lacking; (5) in M. prolifera, the proximal and distal sections of the two anteriormost pairs of dorsal cirri possess longitudinal myofilaments, which are separate from the body wall musculature; by contrast, all appendages in Sphaerodoropsis sp. do not; (6) both species have bracing muscles: in M. prolifera they are positioned above the longitudinal fibers, whereas in Sphaerodoropsis sp. they are uniquely positioned between longitudinal and sublongitudinal transverse fibers. These results do not support a sister-group relationship of Syllidae and Sphaerodoridae. In addition, Sphaerodoropsis sp. is yet another example in the list of polychaetes lacking typical circular muscles in the body wall.     

Observations of serotonin and FMRFamide-like immunoreactivity in palp sensory structures and the anterior nervous system of spionid polychaetes

Evidence suggests that ciliated sensory structures on the feeding palps of spionid polychaetes may function as chemoreceptors to modulate deposit-feeding activity. To investigate the probable sensory nature of these ciliated cells, we used immunohistochemistry, epi-fluorescence, and confocal laser scanning microscopy to label and image sensory cells, nerves, and their organization relative to the anterior central nervous system in several spionid polychaete species. Antibodies directed against acetylated alphatubulin were used to label the nervous system and detail the innervation of palp sensory cells in all species. In addition, the distribution of serotonin (5-HT) and FMRFamide-like immunoreactivity was compared in the spionid polychaetes Dipolydora quadrilobata and Pygospio elegans. The distribution of serotonin immunoreactivity was also examined in the palps of Polydora cornuta and Streblospio benedicti. Serotonin immunoreactivity was concentrated in cells underlying the food groove of the palps, in the palp nerves, and in the cerebral ganglion. FMRFamide-like immunoreactivity was associated with the cerebral ganglia, nuchal organs and palp nerves, and also with the perikarya of ciliated sensory cells on the palps.     

A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale

Wiwaxia corrugata and the indisputable polychaetes of the Middle Cambrian Burgess Shale, particularly Canadia spinosa, have figured prominently in recent hypotheses about the early evolution of polychaete annelids. Based on similarities between the sclerites of Wiwaxia and the notochaetae of Canadia with the broad notochaetae (paleae) of Recent chrysopetalid polychaetes, these two fossil taxa have been variously treated as closely related to the most highly derived stem forms of the polychaete (and annelid) crown group or as members of a specific, Recent subgroup within Polychaeta, the order Phyllodocida. Chrysopetalidae is a member of Phyllodocida, which is part of the major polychaete clade Aciculata; the latter two taxa are distinguished by four and six well defined autapomorphic characters, respectively. The best preserved or otherwise appropriate fossils of Wiwaxia corrugata, Canadia spinosa and the other polychaetes of the Burgess Shale have been studied in detail in order to determine whether they possess any characters that could support the homology of wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae. Most of these fossil taxa have significant autapomorphies but the specific characters of the Aciculata and Phyllodocida are entirely absent. It is demonstrated that constraining cladograms in such a manner that wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae are homologous leads to results that are markedly unparsimonious. Furthermore, Canadia and the other polychaetes of the Burgess Shale cannot be referred to any extant subgroup within the Polychaeta and cannot be used to polarize character evolution within the annelid crown group. Apart from its dubious sclerites, Wiwaxia has no characters that could indicate any close relationship with Polychaeta or Annelida.