Contrasting biogeographical patterns in Margarella (Gastropoda: Calliostomatidae: Margarellinae) across the Antarctic Polar Front

Members of the trochoidean genus Margarella (Calliostomatidae) are broadly distributed across Antarctic and sub-Antarctic ecosystems. Here we used novel mitochondrial and nuclear gene sequences to clarify species boundaries and phylogenetic relationships among seven nominal species distributed on either side of the Antarctic Polar Front (APF). Molecular reconstructions and species-delimitation analyses recognized only four species: M. antarctica (the Antarctic Peninsula), M. achilles (endemic to South Georgia), M. steineni (South Georgia and Crozet Island) and the morphologically variable M. violacea (=M. expansa, M. porcellana and M. pruinosa), with populations in southern South America, the Falkland/Malvinas, Crozet and Kerguelen Islands. Margarella violacea and M. achilles are sister species, closely related to M. steineni, with M. antarctica sister to all these. This taxonomy reflects contrasting biogeographic patterns on either side of the APF in the Southern Ocean. Populations of Margarella north of the APF (M. violacea) showed significant genetic variation but with many shared haplotypes between geographically distant populations. By contrast, populations south of the APF (M. antarctica, M. steineni and M. achilles) exhibited fewer haplotypes and comprised three distinct species, each occurring across a separate geographical range. We hypothesize that the biogeographical differences may be the consequence of the presence north of the APF of buoyant kelps - potential long-distance dispersal vectors for these vetigastropods with benthic-protected development - and their near-absence to the south. Finally, we suggest that the low levels of genetic diversity within higher-latitude Margarella reflect the impact of Quaternary glacial cycles that exterminated local populations during their maxima.

Application of the ecosystem service concept at a small-scale: The cases of coralligenous habitats in the North-western Mediterranean Sea

The understanding of ecosystem services is essential to support sustainable use and preservation of ecosystems. Coralligenous habitats, main contributors of the Mediterranean marine biodiversity, are yet understudied in term of services provided. This study presents an original small-scale approach to investigate the services provided by coralligenous habitats of a French study area consisting of two marine sites (Marseille and Port-Cros sites) in order to cover two contrasted anthropogenic pressure despite the small-scale. Our results are based on the opinions of 43 experts who ranked 15 services in terms of existence and level of importance for human well-being: supporting ecological functions were considered the most important, then provisioning and cultural services. Regulating services were considered uncertain due to a lack of knowledge. The small-scale approach highlighted a need for a referential frame to determine the existence of services (e.g. geographical and temporal scales, benefits and beneficiaries levels).

Regional and local environmental conditions do not shape the response to warming of a marine habitat-forming species

The differential response of marine populations to climate change remains poorly understood. Here, we combine common garden thermotolerance experiments in aquaria and population genetics to disentangle the factors driving the population response to thermal stress in a temperate habitat-forming species: the octocoral Paramuricea clavata. Using eight populations separated from tens of meters to hundreds of kilometers, which were differentially impacted by recent mortality events, we identify 25 °C as a critical thermal threshold. After one week of exposure at this temperature, seven of the eight populations were affected by tissue necrosis and after 30 days of exposure at this temperature, the mean % of affected colonies increased gradually from 3 to 97%. We then demonstrate the weak relation between the observed differential phenotypic responses and the local temperature regimes experienced by each population. A significant correlation was observed between these responses and the extent of genetic drift impacting each population. Local adaptation may thus be hindered by genetic drift, which seems to be the main driver of the differential response. Accordingly, conservation measures should promote connectivity and control density erosion in order to limit the impact of genetic drift on marine populations facing climate change.

PCR survey of 50 introns in animals: cross-amplification of homologous EPIC loci in eight non-bilaterian, protostome and deuterostome phyla

Exon Primed Intron Crossing (EPIC) markers provide molecular tools that are susceptible to be variable within species while remaining amplifiable by PCR using potentially universal primers. In this study we tested the possibility of obtaining PCR products from 50 EPIC markers on 23 species belonging to seven different phyla (Porifera, Cnidaria, Arthropoda, Nematoda, Mollusca, Annelida, Echinodermata) using 70 new primer pairs. A previous study had identified and tested those loci in a dozen species, including another phylum, Urochordata (Chenuil et al., 2010). Results were contrasted among species. The best results were achieved with the oyster (Mollusca) where 28 loci provided amplicons susceptible to contain an intron according to their size. This was however not the case with the other mollusk Crepidula fornicata, which seems to have undergone a reduction in intron number or intron size. In the Porifera, 13 loci appeared susceptible to contain an intron, a surprisingly high number for this phylum considering its phylogenetic distance with genomic data used to design the primers. For two cnidarian species, numerous loci (24) were obtained. Ecdysozoan phyla (arthropods and nematodes) proved less successful than others as expected considering reports of their rapid rate of genome evolution and the worst results were obtained for several arthropods. Some general patterns among phyla arose, and we discuss how the results of this EPIC survey may give new insights into genome evolution of the study species. This work confirms that this set of EPIC loci provides an easy-to-use toolbox to identify genetic markers potentially useful for population genetics, phylogeography or phylogenetic studies for a large panel of metazoan species. We then argue that obtaining diploid sequence genotypes for these loci became simple and affordable owing to Next-Generation Sequencing development. Species surveyed in this study belong to several genera (Acanthaster, Alvinocaris, Aplysina, Aurelia, Crepidula, Eunicella, Hediste, Hemimysis, Litoditis, Lophelia, Mesopodopsis, Mya, Ophiocten, Ophioderma, Ostrea, Pelagia, Platynereis, Rhizostoma, Rimicaris), two of them, belonging to the family Vesicomydae and Eunicidae, could not be determined at the genus level.

From global to local genetic structuring in the red gorgonian Paramuricea clavata: the interplay between oceanographic conditions and limited larval dispersal

Defining the scale of connectivity among marine populations and identifying the barriers to gene flow are tasks of fundamental importance for understanding the genetic structure of populations and for the design of marine reserves. Here, we investigated the population genetic structure at three spatial scales of the red gorgonian Paramuricea clavata (Cnidaria, Octocorallia), a key species dwelling in the coralligenous assemblages of the Mediterranean Sea. Colonies of P. clavata were collected from 39 locations across the Mediterranean Sea from Morocco to Turkey and analysed using microsatellite loci. Within three regions (Medes, Marseille and North Corsica), sampling was obtained from multiple locations and at different depths. Three different approaches (measures of genetic differentiation, Bayesian clustering and spatially explicit maximum-difference algorithm) were used to determine the pattern of genetic structure. We identified genetic breaks in the spatial distribution of genetic diversity, which were concordant with oceanographic conditions in the Mediterranean Sea. We revealed a high level of genetic differentiation among populations and a pattern of isolation by distance across the studied area and within the three regions, underlining short effective larval dispersal in this species. We observed genetic differentiation among populations in the same locality dwelling at different depths, which may be explained by local oceanographic conditions and which may allow a process of local adaptation of the populations to their environment. We discuss the implications of our results for the conservation of the species, which is exposed to various threats.

Fine-scale genetic structure and inferences on population biology in the threatened Mediterranean red coral, Corallium rubrum

Identifying microevolutionary processes acting in populations of marine species with larval dispersal is a challenging but crucial task because of its conservation implications. In this context, recent improvements in the study of spatial genetic structure (SGS) are particularly promising because they allow accurate insights into the demographic and evolutionary processes at stake. Using an exhaustive sampling and a combination of image processing and population genetics, we highlighted significant SGS between colonies of Corallium rubrum over an area of half a square metre, which sheds light on a number of aspects of its population biology. Based on this SGS, we found the mean dispersal range within sites to be between 22.6 and 32.1 cm, suggesting that the surveyed area approximately corresponded to a breeding unit. We then conducted a kinship analysis, which revealed a complex half-sib family structure and allowed us to quantify the level of self-recruitment and to characterize aspects of the mating system of this species. Furthermore, significant temporal variations in allele frequencies were observed, suggesting low genetic drift. These results have important conservation implications for the red coral and further our understanding of the microevolutionary processes acting within populations of sessile marine species with a larval phase.

Genetic survey of shallow populations of the Mediterranean red coral [Corallium rubrum (Linnaeus, 1758)]: new insights into evolutionary processes shaping nuclear diversity and implications for conservation

Combined action from over-harvesting and recent mass mortality events potentially linked to ongoing climate changes has led to new concerns for the conservation of shallow populations (5-60 m) of Corallium rubrum, an octocorallian that is mainly found in the Mediterranean Sea. The present study was designed to analyse population structure and relationships at different spatial scales (from 10s of meters to 100s of kilometres) with a focus on dispersal pattern. We also performed the first analysis of the distribution of genetic diversity using a comparative approach between regional-clusters and samples. Forty populations dwelling in four distinct regions between 14 and 60 m in depth were genotyped using 10 microsatellites. Our main results indicate (i) a generalized pair-sample differentiation combined with a weak structure between regional-clusters; (ii) the occurrence of isolation by distance at the global scale, but also within two of the three analysed regional-clusters; (iii) a high level of genetic diversity over the surveyed area with a heterogeneous distribution from regional-cluster to sample levels. The evolutionary consequences of these results are discussed and their management implications are provided.

Defining reproductively isolated units in a cryptic and syntopic species complex using mitochondrial and nuclear markers: the brooding brittle star, Amphipholis squamata (Ophiuroidea)

At a time when biodiversity is threatened, we are still discovering new species, and particularly in the marine realm. Delimiting species boundaries is the first step to get a precise idea of diversity. For sympatric species which are morphologically undistinguishable, using a combination of independent molecular markers is a necessary step to define separate species. Amphipholis squamata, a cosmopolitan brittle star, includes several very divergent mitochondrial lineages. These lineages appear totally intermixed in the field and studies on morphology and colour polymorphism failed to find any diagnostic character. Therefore, these mitochondrial lineages may be totally interbreeding presently. To test this hypothesis, we characterized the genetic structure of the complex in the French Mediterranean coast using sequences of mitochondrial DNA (16S) and for the first time, several nuclear DNA markers (introns and microsatellites). The data revealed six phylogenetic lineages corresponding to at least four biological species. These sibling species seem to live in syntopy. However, they seem to display contrasted levels of genetic diversity, suggesting they have distinct demographic histories and/or life-history traits. Genetic differentiation and isolation-by-distance within the French Mediterranean coasts are revealed in three lineages, as expected for a species without a free larval phase. Finally, although recombinant nuclear genotypes are common within mitochondrial lineages, the data set displays a total lack of heterozygotes, suggesting a very high selfing rate, a feature likely to have favoured the formation of the species complex.

A presumptive developmental role for a sea urchin cyclin B splice variant

We show that a splice variant-derived cyclin B is produced in sea urchin oocytes and embryos. This splice variant protein lacks highly conserved sequences in the COOH terminus of the protein. It is found strikingly abundant in growing oocytes and cells committed to differentiation during embryogenesis. Cyclin B splice variant (CBsv) protein associates weakly in the cell with Xenopus cdc2 and with budding yeast CDC28p. In contrast to classical cyclin B, CBsv very poorly complements a triple CLN deletion in budding yeast, and its microinjection prevents an initial step in MPF activation, leading to an important delay in oocyte meiosis reinitiation. CBsv microinjection in fertilized eggs induces cell cycle delay and abnormal development. We assume that CBsv is produced in growing oocytes to keep them in prophase, and during embryogenesis to slow down cell cycle in cells that will be committed to differentiation.

The effect of somatic and gonadal size on the rate of oxygen consumption in the subantarctic echinoid Abatus cordatus (Echinodermata) from Kerguelen

1. The rate of oxygen consumption of this burrowing spatangoid was measured for individuals ranging in size from 3-month old 2.5-mm long juveniles to 39-mm long adults. 2. The decrease in the rate of oxygen consumption/dry weight with increasing body size is greater among mature adults than among juveniles because the increase in aerobic tissue (primarily the test) with body size is less than the increase in anaerobic tissues (mainly the gonads). 3. The rate of oxygen consumption/ash-free dry weight decreased more slowly with increasing body size because of the increase in the level of inorganic material. 4. Replacement of the common fresh weight or dry weight specific oxygen consumption by a more synthetic value calculated from ash-free dry weight specific oxygen consumption measurements, to annulate the body-size effect, is proposed for interspecific comparison over a wide range of body size, taking into account parameters such as temperature.