Nicolas Rabet

Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)

Nematogenesis, the production of stinging cells (nematocytes) in Cnidaria, can be considered as a model neurogenic process. Most molecular data concern the freshwater polyp Hydra, in which nematocyte production is scattered throughout the body column ectoderm, the mature cells then migrating to the tentacles. We have characterized tentacular nematogenesis in the Clytia hemisphaerica hydromedusa and found it to be confined to the ectoderm of the tentacle bulb, a specialized swelling at the tentacle base. Analysis by a variety of light and electron microscope techniques revealed that while cellular aspects of nematogenesis are similar to Hydra, the spatio-temporal characteristics are markedly more ordered. The tentacle bulb nematogenic ectoderm (TBE) was found to be polarized, with a clear progression of successive nematoblast stages from a proximal zone (comprising a majority of undifferentiated cells) to the distal end where the tentacle starts. Pulse-chase labelling experiments demonstrated a continuous displacement of differentiating nematoblasts towards the tentacle tip, and that nematogenesis proceeds more rapidly in Clytia than in Hydra. Compact expression domains of orthologues of known nematogenesis-associated genes (Piwi, dickkopf-3, minicollagens and NOWA) were correspondingly staggered along the TBE. These distinct characteristics make the Clytia TBE a promising experimental system for understanding the mechanisms regulating nematogenesis.

REVISION OF THE EXTANT GENERA OF LIMNADIIDAE (BRANCHIOPODA: SPINICAUDATA)

ABSTRACT The extant genera of the spinicaudatan clam shrimp family Limnadiidae are revised using morphological criteria built on previously published molecular analyses. The combined analyses demonstrate the presence of eight well defined genera, two of which are new to science and one (Paralimnadia) that is resurrected. We present the description of the new genus Afrolimnadia and the new genus and species Calalimnadia mahei n. sp. described from Mauritius Island. Both molecular and morphological data strongly support eight genera: Afrolimnadia n. gen., Calalimnadia n. gen., Eulimnadia, Imnadia, Limnadia, Limnadopsis, Metalimnadia and Paralimnadia.

Horizontal gene transfer and the evolution of cnidarian stinging cells

Summary Genes are regularly transmitted vertically, within one lineage, from one generation to the next, but they can also be exchanged between lineages by horizontal gene transfer (HGT). HGTs are frequent in prokaryotes and have been shown to play important roles in unicellular eukaryotes, whereas only a few instances are known in animals [1,2]. Here, we provide evidence that a subunit of bacterial poly-γ-glutamate (PGA) synthase was transferred to an animal ancestor by HGT. We suggest that this gene acquisition had important consequences on the evolution of the stinging cells (nematocytes) that cnidarians (sea anemones, jellyfish, corals etc.) essentially use to capture prey.

Molecular phylogeny of the Notostraca

We used a combined analysis of one nuclear (28S rDNA) and three mitochondrial markers (COI, 12S rDNA, 16S rDNA) to infer the molecular phylogeny of the Notostraca, represented by samples from the six continents that are inhabited by this group of branchiopod crustaceans. Our results confirm the monophyly of both extant notostracan genera Triops and Lepidurus with good support in model based and maximum parsimony analyses. We used branchiopod fossils as a calibration to infer divergence times among notostracan lineages and accounted for rate heterogeneity among lineages by applying relaxed-clock models. Our divergence date estimates indicate an initial diversification into the genera Triops and Lepidurus in the Mesozoic, most likely at a minimum age of 152.3-233.5 Ma, i.e., in the Triassic or Jurassic. Implications for the interpretation of fossils and the evolution of notostracan morphology are discussed. We further use the divergence date estimates to formulate a biogeographic hypothesis that explains distributions of extant lineages predominantly by overland dispersal routes. We identified an additional hitherto unrecognised highly diverged lineage within Lepidurus apus lubbocki and three additional previously unknown major lineages within Triops. Within T. granarius we found deep differentiation, with representatives distributed among three major phylogenetic lineages. One of these major lineages comprises T. cancriformis, the T. mauritanicus species group and two hitherto unrecognised T. granarius lineages. Samples that were morphologically identified as T. granarius diverged from the most basal nodes within this major lineage, and divergence dates suggested an approximate age of 23.7-49.6 Ma for T. cancriformis, indicating the need for a taxonomic revision of Triassic and Permian fossils that are currently attributed to the extant T. cancriformis. We thus elevate T. cancriformis minor to full species status as Triops minorTrusheim, 1938 and include in this species the additional Upper Triassic samples that were attributed to T. cancriformis. We further elevate T. cancriformis permiensis to full species status as Triops permiensisGand et al., 1997.

A 365-Million-Year-Old Freshwater Community Reveals Morphological and Ecological Stasis in Branchiopod Crustaceans

Branchiopod crustaceans are represented by fairy, tadpole, and clam shrimps (Anostraca, Notostraca, Laevicaudata, Spinicaudata), which typically inhabit temporary freshwater bodies, and water fleas (Cladoceromorpha), which live in all kinds of freshwater and occasionally marine environments [1, 2]. The earliest branchiopods occur in the Cambrian, where they are represented by complete body fossils from Sweden such as Rehbachiella kinnekullensis [3] and isolated mandibles preserved as small carbonaceous fossils [4-6] from Canada. The earliest known continental branchiopods are associated with hot spring environments [7] represented by the Early Devonian Rhynie Chert of Scotland (410 million years ago) and include possible stem-group or crown-group Anostraca, Notostraca, and clam shrimps or Cladoceromorpha [8-10], which differ morphologically from their modern counterparts [1, 2, 11]. Here we report the discovery of an ephemeral pool branchiopod community from the 365-million-year-old Strud locality of Belgium. It is characterized by new anostracans and spinicaudatans, closely resembling extant species, and the earliest notostracan, Strudops goldenbergi [12]. These branchiopods released resting eggs into the sediment in a manner similar to their modern representatives [1, 2]. We infer that this reproductive strategy was critical to overcoming environmental constraints such as seasonal desiccation imposed by living on land. The pioneer colonization of ephemeral freshwater pools by branchiopods in the Devonian was followed by remarkable ecological and morphological stasis that persists to the present day.

Mitochondrial gene trees support persistence of cold tolerant fairy shrimp throughout the Pleistocene glaciations in both southern and more northerly refugia

Fairy shrimp (Crustacea: Anostraca) are specialist inhabitants of temporary aquatic habitats. In many parts of the world and particularly in Western Europe, however, populations are declining while the development of adequate conservation strategies is impeded by a poor knowledge of the genetic structure and taxonomic status of remaining lineages. We reconstructed a phylogeography of the species Chirocephalus diaphanus Prévost, 1803 using partial sequences of the mitochondrial COI gene and discuss the importance of different Pleistocene refugia to explain current diversity patterns. In addition to 20 C. diaphanus populations, we also included populations of six presumably closely related chirocephalids to evaluate their taxonomic status. Based on molecular data, the Eastern European subspecies C. diaphanus romanicus deserves species status while the species status of two Italian chirocephalids, C. salinus and C. ruffoi is questionable. Results indicate European C. diaphanus lineages diverged well before the last glacial maximum and survived the Pleistocene glaciations in multiple (sub)refugia along the Iberian, Italian, and Balkan peninsula. Northern Europe was subsequently recolonized from Southern France, resulting in high levels of cryptic diversity around glacial refugia but also in more widespread haplotypes in mainland Europe.

First faunal inventory of large branchiopods (Crustacea: Branchiopoda) of Western Maharashtra, India with taxonomical and distributional comments

An updated inventory of large branchiopods of the Western Maharashtra state, India is presented for the first time. Employing current identification keys, we have detailed the egg morphology of Eulimnadia indocylindrova, which shares some similarities with E. geayi from South America and E. alluaudi from Madagascar, based on these egg characters. A thorough re-description of few morphological traits of the same species is also presented. We also have described the female of a Cyzicus species seen rarely from the studied region. Distributional comments on few species are also reported.

Acetylcholinesterase activity in Clytia hemisphaerica (Cnidaria)

Cholinesterase activity is known in representatives of all living organisms phyla but the origin of the cholinergic system as known in bilaterian animals is still undeciphered. In particular the implication of cholinesterases in the nervous system of non-bilaterian Metazoa is not well known. We thus chose to investigate this activity in the Clytia hemisphaerica (Cnidaria) medusa. In toto histochemical staining revealed an acetylcholinesterase activity in the tentacle bulbs but not in the nervous system. Sequences homologous to acetylcholinesterase were searched within Clytia ESTs and compared to other sequences found in public databases.

Gondwanalimnadia (Branchiopoda: Spinicaudata), replacement name for Afrolimnadia Rogers, Rabet and Weeks, 2012 (Limnadiidae), junior homonym of Afrolimnadia Tasch, 1987 (Lioestheriidae)

1 Kansas Biological Survey, Kansas University, Higuchi Hall, 2101 Constant Avenue, Lawrence, KS 66047-3759, USA 2 Unité Biologie des organismes et écosystèmes aquatiques (BOREA, UMR 7208), Sorbonne Universités, Muséum national d’Histoire naturelle, Université Pierre et Marie Curie, and Université de Caen Basse-Normandie, CNRS, IRD, 43 rue Cuvier, 75005 Paris, France 3 Program in Integrated Bioscience, Department of Biology, The University of Akron, Akron, OH 44325-3908, USA

Exploration of the egg shell structure of three Neotropical Eulimnadia species: a new insight into genus taxonomy (Crustacea: Branchiopoda: Spinicaudata)

In this work we describe the external morphology and shell cross section of eggs of three species of Brazilian Eulimnadia: E. geayi, E. colombiensis and E. magdalensis. In these species, cross sections show variation of vesicle size and strut thickness in the alveolar layer and also possible presence of hollows. With these new records, we increase the total number of known Brazilian Eulimnadia species to four, due to different egg structures. All four species were already reported from Venezuela, suggesting that Eulimnadia species have relatively wide distributions and a lower tendency for endemism than fresh water Anostracans in South America. Neste estudo, apresentamos a descrição da morfologia externa e interna da casca dos ovos de três espécies brasileiras de Eulimnadia: E. geayi, E. colombiensis e E. magdalensis. O corte transversal dos ovos revela uma relativa espessura das suas paredes, incluindo uma camada alveolar com vesículas de tamanho variável e eventual presença de cavidades. Além disso, com os novos registros aqui apresentados, elevamos para quatro o número total de espécies brasileiras de Eulimnadia identificadas através da morfologia dos ovos. O fato que todas estas já foram previamente registradas na Venezuela, sugere que as espécies de Eulimnadia têm uma relativamente ampla distribuição e uma menor tendência para o endemismo do que as espécies de Anostraca de água doce da América do Sul.