Karin Gérard

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.

Divergence time estimations and contrasting patterns of genetic diversity between Antarctic and southern South America benthic invertebrates

La diversidad, abundancia y composicion de grupos taxonomicos en el oceano Austral difiere de otros lugares del planeta debido a que su biogeografia refleja la compleja interaccion de la tectonica, oceanografia, clima y elementos biologicos desde el Eoceno. Algunos grupos de organismos marinos bentonicos muestran altos niveles de divergencia genetica entre provincias de esta region, apoyando la existencia de procesos vicariantes por tectonica de placas mientras que otros grupos, con alta capacidad dispersiva, muestran procesos de divergencia mas recientes. Mas aun, el reciente descubrimiento de larvas de decapodos no Antarticos en peninsula Antartica sugiere que algunos grupos podrian cruzar la Corriente Circumpolar Antartica. Se analizaron los niveles de divergencia genetica en especies congenericas en invertebrados bentonicos del Oceano Austral con potencial dispersivo. Para esto, se incluyo en los analisis secuencias del gen COI de un equinoideo (Sterechinus), un gastropodo (Nacella), y un bivalvo (Yoldia). Considerando los niveles de diferenciacion genetica y asumiendo la Hipotesis del Reloj Molecular, estimamos la separacion entre grupos de ambos continentes. Tambien comparamos los niveles de variacion genetica entre especies antarticas y subantarticas de Nacella y Sterechinus para determinar el efecto de los episodios glaciales del Cuaternario en su demografia. Detectamos claras diferencias geneticas entre especies congenericas antarticas y subantarticas de Sterechinus, Nacella y Yoldia. La instalacion de una barrera efectiva entre antartica y subantartica ocurrio casi al unisono (3.7 a 5.0 Ma) para estos grupos de organismos, y muy posterior a la separacion fisica de ambos continentes. Comparaciones geneticas entre especies antarticas y subantarticas detectaron menores niveles de diversidad genetica en especies antarticas sugiriendo un efecto mas pronunciado de los episodios glaciales en Antartica que en Sudamerica. Estos resultados podrian reflejar el dramatico efecto de los ciclos glaciales del Cuaternario en los tamanos poblacionales antarticos, especialmente en grupos con rangos batimetricos estrechos. El presente estudio provee nueva evidencia de los procesos de diferenciacion entre organismos antarticos y sudamericanos. Ninguno de los generos analizados mostro evidencia de flujo genico recurrente a traves de la Corriente Circumpolar Antartica desde el Mio-Plioceno. Las comparaciones geneticas indican que especies antarticas y subantarticas fueron diferencialmente afectadas por los periodos glaciales.

Assessment of three mitochondrial loci variability for the crown-of-thorns starfish: A first insight into Acanthaster phylogeography

Acanthaster planci (L.) is one of the major threats to coral reefs, whose genetic diversity has been mainly studied with allozymes. Allozymes revealed the low genetic differentiation between A. planci populations in the Indo-Pacific area. We obtained sequences of A. planci from Kenya, Mayotte and Madagascar at the three loci cytochrome oxydase subunit I (COI), 16S rDNA (16S) and five tRNAs, analysed together with available sequences of Acanthaster from the Pacific Ocean. The level of genetic diversity varied among the three loci, tRNAs being on average three times less divergent than COI and 16S genes. The genus Acanthaster appeared monophyletic, the two species A. brevispinus (Fisher) and A. planci forming distinct clades in agreement with data from morphology and systematics. The A. planci clade split into a West Indian Ocean group and a Pacific group, in agreement with allozyme data on population differentiation.

Restricted geographic distribution and low genetic diversity of the brooding sea urchin Abatus agassizii (Spatangoidea: Schizasteridae) in the South Shetland Islands: A bridgehead population before the spread to the northern Antarctic Peninsula?

Los periodos glaciales del Pleistoceno han promovido los principales cambios climaticos del oceano Austral. Han generado el interes cientifico sobre las estrategias desarrolladas por la fauna bentonica para tolerar y superar la extension y contraccion de la capa de hielo sobre la plataforma continental. Un estudio reciente acerca de la zonacion batimetrica y distribucion de macro-invertebrados en una zona submareal somera de la Bahia Fildes (Isla Rey Jorge, islas Shetlands del Sur), resalta la presencia de una importante agregacion del erizo incubante Abatus agassizii, cuya distribucion geografica es conocida solo para localidades al sur de la convergencia antartica (Peninsula Antartica, islas Shetland del Sur e Isla Georgias del Sur en el Arco de Escocia). Sin embargo, su presencia es atipica dado a que, primero, sus poblaciones someras debieron haber sido extintas desde la vecindad de la Peninsula Antartica por los avances y retrocesos de la capa de hielo y segundo, la ausencia de estadios larvales asociado a la estrategia de incubacion como modo de desarrollo deberia limitar una recolonizacion desde areas subantarticas mas al norte. El objetivo de este estudio fue evaluar si A. agassizii puede haber sobrevivido a las glaciaciones en su estrecho rango batimetrico en las islas Shetland del Sur o ha recolonizado recientemente el area, a pesar de su baja capacidad de dispersion. A pesar de un intensivo esfuerzo de muestreo, la baja ocurrencia de A. agassizii indica que su distribucion es muy escasa a lo largo de las islas Shetland del Sur y la Peninsula Antartica y parece estar limitada a densas poblaciones someras en areas protegidas y libres del hielo en la Bahia Fildes en la Isla Rey Jorge. Ademas, estas poblaciones presentan una muy baja diversidad genetica asociada a una senal de una expansion demografica reciente. Finalmente, la reconstruccion de las relaciones filogeneticas entre las especies de Abatus, con secuencias mitocondriales del gen COI, establecieron la afinidad de la especie antartica A. agassizii con especies subantarticas. De acuerdo a los resultados obtenidos, consideramos que la presencia de esta especie en las islas Shetlands del Sur probablemente corresponde a una reciente recolonizacion desde las islas antarticas situadas mas al norte.

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.

Mating system and evidence of multiple paternity in the Antarctic brooding sea urchin Abatus agassizii.

Broadcasting is the predominant spawning behavior among benthic marine invertebrates, mainly associated with planktotrophic and planktonic lecitotrophic development. Broadcasting allows genetic mixing that should contribute to increase the genetic diversity of a female clutch. Conversely, in brooding species characterized by protected development, oocytes are retained and only sperm is released, which is supposed to limit the number of males that contribute to a female clutch. This spermcasting behavior together with egg retention, unusually frequent among Antarctic marine invertebrates, putatively give brooders low dispersal capacities which may reduce genetic mixing and generate genetic and kinship structure at a small spatial scale. Like many other Antarctic marine benthic invertebrates, the irregular sea urchin Abatus agassizii is a spermcaster that broods its young. In this study, we assessed the genetic diversity among 66 adults using 6 polymorphic microsatellite loci and performed progeny array analyses in order to evaluate the number of mates per female as well as genetic structure at a small spatial scale. A. agassizii exhibited a polyandric system with 2–5 mates per female regardless of population density. Bayesian analyses suggested the absence of genetic structure along our 20-m transect, while relatedness among individuals did not differ from that expected under panmixia. Finally, we conclude that a limited number of males contribute to a female clutch, probably as a consequence of limited sperm dispersal and that movement of adults may be sufficient to avoid kinship structure in the population.

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.

Fine-grained habitat-associated genetic connectivity in an admixed population of mussels in the small isolated Kerguelen Islands

Reticulated evolution -i.e. secondary introgression / admixture between sister taxa-is increasingly recognized as playing a key role in structuring infra-specific genetic variation and revealing cryptic connectivity patterns. When admixture zones coincide with ecological transitions, the connectivity patterns often follow environmental variations better than distance and introgression clines may easily be confounded with local adaptation. The Kerguelen mussels is an ideal system to investigate the potential role of admixture in enhancing micro-geographic structure, as they inhabit a small isolated island in the Southern Ocean characterized by a highly heterogeneous environment. Furthermore, genomic reticulation between Northern species (M. edulis, M. galloprovincialis and M. trossulus) and Southern species (M. platensis: South America and the Kerguelen Islands; and M. planulatus: Australasia) has been suspected. Here, we extended a previous analysis by using targeted-sequencing data (51,878 SNPs) across the three Northern species and the Kerguelen population. Spatial structure in the Kerguelen was then analyzed with a panel of 33 SNPs, including SNPs that were more differentiated than the genomic average between Northern species (i.e., ancestry-informative SNPs). We first showed that the Kerguelen lineage splitted very shortly after M. edulis and M. galloprovincialis initiated speciation, and it subsequently experienced admixture with the three Northern taxa. We then demonstrated that the Kerguelen mussels were significantly differentiated over small spatial distance, and that this local genetic structure was associated with environmental variations and mostly revealed by ancestry-informative markers. Simulations of admixture in the island highlight that genetic-environment associations can be better explained by introgression clines between heterogeneously differentiated genomes than by adaptation from admixture variation.

Trophic ecology of two co-existing Sub-Antarctic limpets of the genus Nacella: spatio-temporal variation in food availability and diet composition of Nacella magellanica and N. deaurata

Abstract Interactions between algae and herbivores can be affected by various factors, such as seasonality and habitat structure. Among herbivores inhabiting marine systems, species of the order Patellogastropoda are considered key organisms in many rocky coasts of the world. Nacella species are one of the most dominant macro-herbivores on the rocky shores of the sub-Antarctic ecoregion of Magellan. However, the importance of its key role must be associated with its trophic ecology. The objective of this work was to evaluate spatial and temporal variabilities in the dietary composition of two intertidal Nacella species, considering grazing on macro- (macroalgae) and microscopic (periphyton) food. The composition of periphyton and the availability of macroalgae in the winter and summer seasons were examined at two localities of the Magellanic province, alongside the gut contents of N. magellanica and N. deaurata. The dietary composition differed between the two Nacella species, as well as between seasons and locations. The differences observed in the diet of the two species of Nacella may be mainly due to their respective distributions in the intertidal zone. Both species presented a generalist strategy of grazing, which is relationed to the seasonality of micro- and macroalgae availability and to the variability of the assemblages between the localities. This research was the first to perform a detailed study of the diet of intertidal Nacella species.