BMC Evolutionary Biology BioMed Central Springer

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BMC Evolutionary Biology 2009 9 270 http www biomedcentral com 1471 2148 9 270. neuronal innervation patterns of organ systems may be. used as an independent test of homology prior to phyloge. netic analysis as illustrated by classical studies on the. structure of the polychaete nervous system 7 9 Along. these lines it seems reasonable to assume that investiga. tions of neuronal innervation patterns and neurogenesis. pathways might also aid in resolving long standing ques. tions regarding the internal phylogenetic relationships of. Annelida one major lophotrochozoan lineage In partic. ular the basal branches of the annelid tree remain unre. solved despite the additional use of large scale molecular. analyses 10 19 Current interest has therefore partly. shifted to lower taxonomic levels taking a top down. approach to reconstruct annelid phylogeny, One widely distributed and abundant group of polychaete. fan worms the Spirorbinae is characterized by a crown of. tentacles commonly referred to as radioles and a con. spicuous calcareous tube that can be closed with an oper. culum Figure 1A All spirorbins described to date brood. their embryos either in their coiled tube or in the opercu. lum unlike most species of the other two traditional sub. taxa of the serpulid tube worms the Serpulinae and. Filograninae 20 21 A recent cladistic analysis of the rela. tionships among spirorbid subfamilies 22 suggests, thereby that the opercular brooders occupy the most. derived position on the spirorbin tree Ever since M ller. 23 published his observations on the development of. Serpula Serpulinae the operculum has been viewed as a. modified tentacle radiole of the branchial crown and it. is often used as an important diagnostic character in ser. pulid systematics 21 24 Accordingly some authors, regard spirorbins as a separate taxonomic entity based on. their unique morphology and development e g 25, 29 However recent studies of serpulid phylogeny using. morphological and molecular data recognize Spirorbinae Figure 1. as one of altogether four monophyletic clades within Ser Serpulid morphology and phylogeny A Schematic. pulidae Figure 1B Moreover they appear to be the sister drawing of Spirorbis cf spirorbis with the collar co. group to a mixed clade of serpulin and filogranin taxa the branchial crown consisting of radioles ra and. pinnules pi and the operculum op protruding, 21 24 although the statistical analyses could not signif.
from the coiled shell The latter is dissected open in. icantly reject the hypothesis that Spirorbinae are instead the region of the anterior abdomen to unveil the. the basal most taxon of the Serpulidae 24 In addition membraneous tube surrounding the embryos B. the current knowledge and status of the taxonomy of the Current hypothesis of serpulid phylogeny based on. individual serpulid genera except the ones of the Spiror molecular analyses with radiole and opercular mor. binae has been reviewed only lately 30 phology illustrated for each clade Sketches redrawn. after Kupriyanova et al 21 The Serpulidae consist, In order to test the proposed homology and thus the of the Spirorbinae including Spirorbis and three new. potential usefulness for phylogenetic analyses of one of taxonomic entities referred to as the Protula Serp. the most crucial morphological characters in the serpulid ula and the Pomatoceros clade Thereby some spe. cies of the former Serpulinae cluster within the, bodyplan the operculum and to assess the hypothesis. Protula clade that comprises all species of the former. that all spirorbin opercula evolved from a cephalic radi Filograninae. ole we investigated the ontogeny of the innervation pat. tern of the prostomial appendages of the spirorbin. Spirorbis cf spirorbis In addition we provide a new set of. Page 2 of 13,page number not for citation purposes. BMC Evolutionary Biology 2009 9 270 http www biomedcentral com 1471 2148 9 270. characters for a broad comparison of neurogenesis in rotation is not complete in the posterior body region. polychaete species where the larval abdomen is only twisted by a mere 90. Therefore the former ventral and dorsal parts are now. Results located in a morphological lateral position In sharp. General development and FMRFamidergic contrast to the broad distribution of serotonin Figure 3. immunoreactivity Figure 4 Figure 5 in post settled juveniles the FMRFami. Each embryo of Spirorbis cf spirorbis is encased in an egg dergic expression is almost exclusively restricted to the. capsule and several dozens of embryos are surrounded by head and thorax Figure 2G Figure 5A All larval ciliary. a common membrane aligned along the trunk of the bands have been shed leaving the developing branchial. mother individual within the calcareous tube Figure 1A crown the only external ciliated body region The sinistral. Therein the embryos develop into three segmented leci direction of the coiled tube has already been determined. thotrophic larvae These larvae exhibit a short planktonic at this stage Figure 2H I. phase after hatching settle after a few hours metamor. phose into asymmetric juveniles and eventually secrete a Development of the serotonergic nervous system. coiled tube Figure 2C F I The anlage of the collar The neurotransmitter serotonin is present in Spirorbis cf. which surrounds the branchial crown of the adult is spirorbis from the early trochophore stage onwards Figure. already visible in the encapsulated larva Figure 2A At 3 Figure 4 The first serotonergic signals appear more or. the same time the first FMRFamidergic signals appear in less simultaneously with the FMRFamidergic immunore. structures of the precursors of the adult central nervous activity Initially serotonin is found in the early anlagen. system The cerebral commissures show an intense stain of the adult central nervous system such as the cerebral. ing and are in direct contact with the rudiments of the ganglion the circumesophageal connectives the ventral. adult circumesophageal connectives whose signal is faint nerve cords and the subesophageal commissure Figure. in the region of the prototroch nerve ring The ventral 3A In addition serotonin is distributed in a nerve ring. nerve cords consist of paired FMRFamidergic axons Fig underlying the larval prototroch This serotonergic inner. ure 2A Slightly later the cerebral commissures fuse with vation of the ciliated prototroch changes considerably. additional fibers forming the neuropil of the cerebral gan over time Figure 3 Figure 4 The primary prototroch. glion Figure 2B At the junction of the circumesophageal nerve ring expands on its ventral side shortly before hatch. connectives and the paired ventral nerve cords a ing and forms a network of cross linked serotonergic fib. subesophageal commissure interconnects both ventral ers Figure 3B C D Moreover two clusters of accessory. nerve cords Figure 2B The latter vary along the longitu prototroch neurons develop dorsolaterally in the free. dinal body axis with respect to the number of axons they swimming larva Figure 4A B and a broad gap appears. comprise Anteriorly the number of neurons with an in the serotonergic prototroch innervation Figure 3D. FMRFamidergic signal increases whereas only two axons Figure 4B After settlement the neurons of the prototroch. are present in the posterior region A second commissure are reduced to a semi circle on the dorsal side and degen. appears exactly in the region where this change of axon erate completely during metamorphosis Figure 3E F. number occurs Figure 2B Prior to hatching the, branchial rudiments and the first setae are already present The apical organ of free swimming Spirorbis larvae ini. Figure 2C Eventually after the release of the free swim tially comprises four serotonergic flask shaped perikarya. ming larva three body regions are recognized head tho Figure 3B Three of these cells lie in one plane with the. rax with collar and abdomen Figure 2D F Anteriorly fourth cell body being positioned medially above them. the circumesophageal connectives split into a dorsal and All apical perikarya possess projections that seem to be. a ventral root Both traverse the cerebral ganglion that connected to the rudiment of the adult cerebral ganglion. exhibits a very strong FMRFamidergic signal Figure 2D Interestingly the number and position of these apical ser. At this stage the larva possesses various ciliated regions otonergic somata varies over time as well as between indi. such as the prototroch the neurotroch the apical tuft and viduals with a maximum of seven being present in. several posterior sensory cilia Figure 2E One transi specimens competent to settle. tional FMRFamidergic perikaryon is located posterior to. the apical cilia Figure 2D Apart from that none of the The cerebral ganglion is one of the first adult features that. other ciliated structures shows FMRFamidergic innerva are established during neurogenesis It is divided into two. tion Figure 2E The same applies to the operculum that distinct serotonergic hemispheres left and right in the. develops dorsally on the left side in close proximity to the encapsulated and free swimming larval stages Posterior. rudiments of the branchial crown Figure 2F G In the to the two domains the cephalic commissures appear as. following the larva settles attaches its ventral side tempo one strand which is directly connected to the circumes. rarily to the substrate and finally turns by 180 resulting ophageal connectives Figure 4B After settlement the. in the dorsal side of the thorax facing the substrate This latter split anteriorly into the so called dorsal and ventral. 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BMC Evolutionary Biology 2009 9 270 http www biomedcentral com 1471 2148 9 270. FMRFamidergic neurogenesis in Spirorbiscf spirorbis Light micrographs and confocal micrographs showing. FMRFamide immunoreactivity red nuclei blue and cilia green anterior faces upwards Scale bars 50 m. A C 70 m D I A C Encapsulated larva dorsal view A Early anlage of the cerebral commissures cec. the circumesophageal connectives cc and the ventral nerve cords vnc together with two lateral cell clus. ters of the collar rudiments dashed circles B Still within the egg capsule ec the cerebral ganglion cg. the subesophageal commissure sc the neuronal innervation of the prototroch pn and a commissure c. are formed in the early trochophore larva C Anteriorly the paired branchial rudiments br are distinguish. able D F Free swimming larva ventral view D Three body regions can be recognized head h thorax. th with collar co and abdomen ab The ventral nerve cords vnc consist of multiple axons in the collar. region arrows but only of two axons in the posterior body Anteriorly one transitional perikaryon arrow. head appears and the circumesophageal connectives split into a dorsal drcc and a ventral root vrcc E. The larva possesses an apical tuft at a prototroch pt a mouth opening m a neurotroch nt several pos. terior sensory cilia psc and various ciliated fields asterisks F The operculum op starts to form and two. pigmented eyes e are present G I Settled juvenile Dorsal view in G ventral view of the same specimen in. H I G The FMRFamide expression is restricted to head and thorax H Ciliated bands are reduced to the. branchial crown bc I The direction of the coiled tube t has been determined. Page 4 of 13,page number not for citation purposes. BMC Evolutionary Biology 2009 9 270 http www biomedcentral com 1471 2148 9 270. roots Each root differentiates into a dorsal and a ventral crosses the dorsal body region Figure 3E Figure 4C One. commissure Accordingly four main commissures all lateral longitudinal nerve runs along each side of the tho. derived from the circumesophageal connectives traverse rax from the transversal thorax nerve to the main collar. the cerebral ganglion Figure 4C The cerebral commis nerve Figure 3E Figure 4C Here anterior segmental. sures come to lie in close proximity to the subesophageal neurons branch off They are associated with the first ven. commissure because the circumesophageal connectives tral commissure of the paired nerve cords Figure 4C Fig. are relatively reduced in length in juvenile specimens Fig ure 5B These anterior segmental neurons are connected. ure 3F Figure 4D Figure 5C to the subesophageal ganglion after metamorphosis due. to a forward migration of the first ventral commissure. Two thin axons represent the first rudiments of the devel Figure 4D Figure 5C Moreover the lateral longitudinal. oping serotonergic ventral nerve cords while the larva is nerves are shortened compared to the pre metamorphic. still within the egg capsule Figure 3A In later larval condition and the transversal thorax nerve is connected. stages the signal of the ventral nerve cords is discontinu to the ventral nerve cords at the position of the second. ous in the second segment Anteriorly two axons are ventral commissure Figure 4D. present on each side followed posteriorly by a discrete. perikaryon with one long axon pointing medially towards Innervation of the branchial crown and the operculum. the posterior end Figure 3B The ventral nerve cords con In adult Spirorbis specimens the branchial crown consists. sist of multiple fibers in the collar region during the free of two branches the left one carrying four and the right. swimming larval stage and are interconnected by an addi one bearing five tentacles radioli Figure 1A Terminal. BMC Evolutionary Biology By reconstructing the developmental neuroanatomy of the serpulid polychaete Spirorbis The cerebral commissures show an intense stain

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