Expression of Distal-less, dachshund, and optomotor blind in Neanthes arenaceodentata (Annelida, Nereididae) does not support homology of appendage-forming mechanisms across the Bilateria

The similarity in the genetic regulation of arthropod and vertebrate appendage formation has been interpreted as the product of a plesiomorphic gene network that was primitively involved in bilaterian appendage development and co-opted to build appendages (in modern phyla) that are not historically related as structures. Data from lophotrochozoans are needed to clarify the pervasiveness of plesiomorphic appendage forming mechanisms. We assayed the expression of three arthropod and vertebrate limb gene orthologs, Distal-less (Dll), dachshund (dac), and optomotor blind (omb), in direct-developing juveniles of the polychaete Neanthes arenaceodentata. Parapodial Dll expression marks premorphogenetic notopodia and neuropodia, becoming restricted to the bases of notopodial cirri and to ventral portions of neuropodia. In outgrowing cephalic appendages, Dll activity is primarily restricted to proximal domains. Dll expression is also prominent in the brain. dac expression occurs in the brain, nerve cord ganglia, a pair of pharyngeal ganglia, presumed interneurons linking a pair of segmental nerves, and in newly differentiating mesoderm. Domains of omb expression include the brain, nerve cord ganglia, one pair of anterior cirri, presumed precursors of dorsal musculature, and the same pharyngeal ganglia and presumed interneurons that express dac. Contrary to their roles in outgrowing arthropod and vertebrate appendages, Dll, dac, and omb lack comparable expression in Neanthes appendages, implying independent evolution of annelid appendage development. We infer that parapodia and arthropodia are not structurally or mechanistically homologous (but their primordia might be), that Dll’s ancestral bilaterian function was in sensory and central nervous system differentiation, and that locomotory appendages possibly evolved from sensory outgrowths

Induction d’une queue et de parapodes surnuméraires par déviation de l’intestin chez les Nereidae (Annélides Polychètes)

Supernumerary tail and parapodia induction by deviation of the intestine in nereids (Annelida: Polychaetd) Localized ablation of the intestine has been performed on normal and grafted specimens of the two polychaetes Nereis pelagica L. and Perinereis cultrifera G. with the following results: In the absence of the intestine, segments are not regenerated but parapodia grow on the plane of section.A segmented tail arises where the intestine is deviated; the regenerate we obtain in this case is of the ‘aneurogenic’ type and without anal cirri and parapodia.These results suggest that caudal regeneration results from the association of different tissues (intestine and the body wall) whereas the nerve cord exerts an influence upon the organization of the regenerate. Likewise the caudal regeneration of parapodia is the consequence of the juxtaposition of a dorsal and a ventral body wall in the presence of the central nervous system.</jats:p

Aspects ultrastructuraux et cytochimiques des hématies nucléées de deux Annélides Polychètes <i>Notomastus latericeus</i> Sars et <i>Glycera convoluta</i> Keferstein

The structure and cytochemistry (pseudoperoxidase activity and iron content) of the nucleated erythrocytes of two polychaetes, Notomastus latericeus Sars and Glycera convoluta Keferstein, have been investigated using light and electron microscopy. These cells contain few cytoplasmic organelles but do possess iron-rich structures in vacuoles and free in the cytoplasm. Both species show a pseudoperoxidase activity throughout the cytoplasm and, in the case of Notomastus, in the nucleus as well. The cytological and cytochemical aspects of these cells suggest a low metabolism and a limited lifetime, particularly in Notomastus. </jats:p