Genetic structure and demographic inference of the regular sea urchin Sterechinus neumayeri (Meissner, 1900) in the Southern Ocean: The role of the last glaciation

One of the most relevant characteristics of the extant Southern Ocean fauna is its resiliency to survive glacial processes of the Quaternary. These climatic events produced catastrophic habitat reductions and forced some marine benthic species to move, adapt or go extinct. The marine benthic species inhabiting the Antarctic upper continental shelf faced the Quaternary glaciations with different strategies that drastically modified population sizes and thus affected the amount and distribution of intraspecific genetic variation. Here we present new genetic information for the most conspicuous regular sea urchin of the Antarctic continental shelf, Sterechinus neumayeri. We studied the patterns of genetic diversity and structure in this broadcast-spawner across three Antarctic regions: Antarctic Peninsula, the Weddell Sea and Adélie Land in East Antarctica. Genetic analyses based on mitochondrial and nuclear markers suggested that S. neumayeri is a single genetic unit around the Antarctic continent. The species is characterized by low levels of genetic diversity and exhibits a typical star-like haplotype genealogy that supports the hypothesis of a single in situ refugium. Based on two mutation rates standardized for this genus, the Bayesian Skyline plot analyses detected a rapid demographic expansion after the Last Glacial Maximum. We propose a scenario of rapid postglacial expansion and recolonization of Antarctic shallow areas from a less ice-impacted refugium where the species survived the LGM. Considering the patterns of genetic diversity and structure recorded in the species, this refugium was probably located in East Antarctica.

An integrated method to evaluate and monitor the conservation state of coralligenous habitats: The INDEX-COR approach

A new method based on photographic sampling coupled with in situ observations was applied to 53 stations along the French Mediterranean coast, to assess the integrity of coralligenous reefs affected by different levels of anthropogenic pressure. The conservation state of the assemblages characterizing these habitats was then assessed by an index - the INDEX-COR - that integrates three metrics: (i) the sensitivity of the taxa to organic matter and sediment deposition, (ii) the observable taxonomic richness, and (iii) the structural complexity of the assemblages. The sensitivity of INDEX-COR was tested and showed good correlation with the Level of Pressure calculated for each station according to expert judgment and field observations.

CIGESMED for divers: Establishing a citizen science initiative for the mapping and monitoring of coralligenous assemblages in the Mediterranean Sea

Background

Over the last decade, inventorying and monitoring of marine biodiversity has significantly benefited from the active engagement of volunteers. Although several Citizen Science projects concern tropical reef ecosystems worldwide, none of the existing initiatives has yet specifically focused on their Mediterranean equivalents. Mediterranean coralline reefs, known as “coralligenous”, are bioherms primarily built by calcifying rhodophytes on hard substrates under dim-light conditions; they are considered hotspots of biodiversity and are extremely popular among divers due to their complex structure, conspicuous biological wealth and high aesthetic value. Nevertheless, data on their distribution, structure and conservation status is lacking for several Mediterranean areas while they are vulnerable to an increasing number of threats.

New information

In the framework of CIGESMED SeasEra (ERAnet) project a specialized Citizen Science project was launched, aiming to engage enthusiast divers in the study and monitoring of Mediterranean coralligenous assemblages through the gathering of basic information regarding their spatial occurrence, assemblage structure and associated pressures or threats. For its active implementation, a data collection protocol and a multilingual website were developed, comprising an educational module and a data submission platform. Georeferenced data reporting focuses on: (a) basic topographic and abiotic features for the preliminary description of each site, and the creation of data series for sites receiving multiple visits; (b) presence and relative abundance of typical conspicuous species, as well as (c) existence of pressures and imminent threats, for the characterization and assessment of coralligenous assemblages. A variety of tools is provided to facilitate end users, while divers have the choice to report additional information and are encouraged to upload their photographs. The long-term goal is the development of an active community of amateur observers providing widespread and ecologically significant data on coralligenous assemblages.

Does natural selection explain the fine scale genetic structure at the nuclear exon Glu-5' in blue mussels from Kerguelen?

The Kerguelen archipelago, isolated in the Southern Ocean, shelters a blue mussel Mytilus metapopulation far from any influence of continental populations or any known hybrid zone. The finely carved coast leads to a highly heterogeneous habitat. We investigated the impact of the environment on the genetic structure in those Kerguelen blue mussels by relating allele frequencies to habitat descriptors. A total sample comprising up to 2248 individuals from 35 locations was characterized using two nuclear markers, mac-1 and Glu-5', and a mitochondrial marker (COI). The frequency data from 9 allozyme loci in 9 of these locations were also reanalyzed. Two other nuclear markers (EFbis and EFprem's) were monomorphic. Compared to Northern Hemisphere populations, polymorphism in Kerguelen blue mussels was lower for all markers except for the exon Glu-5'. At Glu-5', genetic differences were observed between samples from distinct regions (F CT = 0.077), as well as within two regions, including between samples separated by <500 m. No significant differentiation was observed in the AMOVA analyses at the two other markers (mac-1 and COI). Like mac-1, all allozyme loci genotyped in a previous publication, displayed lower differentiation (Jost's D) and F ST values than Glu-5'. Power simulations and confidence intervals support that Glu-5' displays significantly higher differentiation than the other loci (except a single allozyme for which confidence intervals overlap). AMOVA analyses revealed significant effects of the giant kelp Macrocystis and wave exposure on this marker. We discuss the influence of hydrological conditions on the genetic differentiation among regions. In marine organisms with high fecundity and high dispersal potential, gene flow tends to erase differentiation, but this study showed significant differentiation at very small distance. This may be explained by the particular hydrology and the carved coastline of the Kerguelen archipelago, together with spatially variable selection at Glu-5'.

Potential for adaptive evolution at species range margins: contrasting interactions between red coral populations and their environment in a changing ocean

Studying population-by-environment interactions (PEIs) at species range margins offers the opportunity to characterize the responses of populations facing an extreme regime of selection, as expected due to global change. Nevertheless, the importance of these marginal populations as putative reservoirs of adaptive genetic variation has scarcely been considered in conservation biology. This is particularly true in marine ecosystems for which the deep refugia hypothesis proposes that disturbed shallow and marginal populations of a given species can be replenished by mesophotic ones. This hypothesis therefore assumes that identical PEIs exist between populations, neglecting the potential for adaptation at species range margins. Here, we combine reciprocal transplant and common garden experiments with population genetics analyses to decipher the PEIs in the red coral, Corallium rubrum. Our analyses reveal partially contrasting PEIs between shallow and mesophotic populations separated by approximately one hundred meters, suggesting that red coral populations may potentially be locally adapted to their environment. Based on the effective population size and connectivity analyses, we posit that genetic drift may be more important than gene flow in the adaptation of the red coral. We further investigate how adaptive divergence could impact population viability in the context of warming and demonstrate differential phenotypic buffering capacities against thermal stress. Our study questions the relevance of the deep refugia hypothesis and highlights the conservation value of marginal populations as a putative reservoir of adaptive genetic polymorphism.

Is the species flock concept operational? The Antarctic shelf case

There has been a significant body of literature on species flock definition but not so much about practical means to appraise them. We here apply the five criteria of Eastman and McCune for detecting species flocks in four taxonomic components of the benthic fauna of the Antarctic shelf: teleost fishes, crinoids (feather stars), echinoids (sea urchins) and crustacean arthropods. Practical limitations led us to prioritize the three historical criteria (endemicity, monophyly, species richness) over the two ecological ones (ecological diversity and habitat dominance). We propose a new protocol which includes an iterative fine-tuning of the monophyly and endemicity criteria in order to discover unsuspected flocks. As a result nine « full » species flocks (fulfilling the five criteria) are briefly described. Eight other flocks fit the three historical criteria but need to be further investigated from the ecological point of view (here called "core flocks"). The approach also shows that some candidate taxonomic components are no species flocks at all. The present study contradicts the paradigm that marine species flocks are rare. The hypothesis according to which the Antarctic shelf acts as a species flocks generator is supported, and the approach indicates paths for further ecological studies and may serve as a starting point to investigate the processes leading to flock-like patterning of biodiversity.

Role of evolutionary and ecological factors in the reproductive success and the spatial genetic structure of the temperate gorgonian Paramuricea clavata

Dispersal and mating features strongly influence the evolutionary dynamics and the spatial genetic structure (SGS) of marine populations. For the first time in a marine invertebrate, we examined individual reproductive success, by conducting larval paternity assignments after a natural spawning event, combined with a small-scale SGS analysis within a population of the gorgonian Paramuricea clavata. Thirty four percent of the larvae were sired by male colonies surrounding the brooding female colonies, revealing that the bulk of the mating was accomplished by males from outside the studied area. Male success increased with male height and decreased with increasing male to female distance. The parentage analyses, with a strong level of self-recruitment (25%), unveiled the occurrence of a complex family structure at a small spatial scale, consistent with the limited larval dispersal of this species. However, no evidence of small scale SGS was revealed despite this family structure. Furthermore, temporal genetic structure was not observed, which appears to be related to the rather large effective population size. The low level of inbreeding found suggests a pattern of random mating in this species, which disagrees with expectations that limited larval dispersal should lead to biparental inbreeding. Surface brooding and investment in sexual reproduction in P. clavata contribute to multiple paternity (on average 6.4 fathers were assigned per brood), which enhance genetic diversity of the brood. Several factors may have contributed to the lack of biparental inbreeding in our study such as (i) the lack of sperm limitation at a small scale, (ii) multiple paternity, and (iii) the large effective population size. Thus, our results indicate that limited larval dispersal and complex family structure do not necessarily lead to biparental inbreeding and SGS. In the framework of conservation purposes, our results suggested that colony size, proximity among colonies and the population size should be taken into consideration for restoration projects.

Differential reproductive timing in Echinocardium spp.: the first Mediterranean survey allows interoceanic and interspecific comparisons

Echinocardium cordatum had long been considered as cosmopolitan, but molecular data revealed it is a complex of cryptic species, with two non-hybridizing species (B1 & B2) in the Mediterranean Sea living in syntopy with Echinocardium mediterraneum. Histological analyses of the gonads from a 17-month sampling period revealed a statistically significant time lag between the Maturity Indices of E. cordatum and E. mediterraneum. The main environmental stimulus may be different for the two nominal species, possibly seawater temperature for E. cordatum and chlorophyll a concentration for E. mediterraneum. Within the E. cordatum complex, spawning timing and synchrony are different according to major geographic areas (Atlantic/Pacific/Mediterranean) and/or the corresponding genetic subdivision [A/P/(B1 & B2)]. In contrast, the effects of temperature on the reproductive cycle seem rather to mirror the genetic lineages than environmental similarities of the different localities. Between the sister species (B1 & B2) no differences could be detected, maybe due to small sample sizes.

Pleistocene separation of mitochondrial lineages of Mytilus spp. mussels from Northern and Southern Hemispheres and strong genetic differentiation among southern populations

Smooth-shelled mussels, Mytilus spp., have an antitropical distribution. In the Northern Hemisphere, the M. edulis complex of species is composed of three genetically well delineated taxa: M. edulis, M. galloprovincialis and M. trossulus. In the Southern Hemisphere, morphological characters, allozymes and intron length polymorphisms suggest that Mytilus spp. populations from South America and Kerguelen Islands are related to M. edulis and those from Australasia to M. galloprovincialis. On the other hand, a phylogeny of the 16S rDNA mitochondrial locus demonstrates a clear distinctiveness of southern mussels and suggests that they are related to Mediterranean M. galloprovincialis. Here, we analysed the faster-evolving cytochrome oxidase subunit I locus. The divergence between haplotypes of populations from the two hemispheres was confirmed and was found to predate the divergence between haplotypes of northern M. edulis and M. galloprovincialis. In addition, strong genetic structure was detected among the southern samples, revealing three genetic entities that correspond to (1) South America and Kerguelen Island, (2) Tasmania, (3) New Zealand. Using the trans-Arctic interchange as a molecular clock calibration, we estimated the time since divergence of populations from the two hemispheres to be between 0.5 million years (MY) and 1.3 MY (average 0.84 MY). The contrasting patterns observed for the nuclear and the organelle genomes suggested two alternative, complex scenarios: two trans-equatorial migrations and the existence of differential barriers to mitochondrial and nuclear gene flow, or a single trans-equatorial migration and a view of the composition of the nuclear genome biased by taxonomic preconception.

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.