Claudio A. González-Wevar

Molecular phylogeny and historical biogeography of Nacella (Patellogastropoda: Nacellidae) in the Southern Ocean

The evolution and the historical biogeography of the Southern Ocean marine benthic fauna are closely related to major tectonic and climatic changes that occurred in this region during the last 55 million years (Ma). Several families, genera and even species of marine organisms are shared between distant biogeographic provinces in this region. This pattern of distribution in marine benthic invertebrates has been commonly explained by vicariant speciation due to plate tectonics. However, recent molecular studies have provided new evidence for long-distance dispersion as a plausible explanation of biogeographical patterns in the Southern Ocean. True limpets of the genus Nacella are currently distributed in different biogeographic regions of the Southern Ocean such as Antarctica, Kerguelen Province, southern New Zealand Antipodean Province, North-Central Chile and South American Magellanic Province. Here, we present phylogenetic reconstructions using two mitochondrial DNA markers (Cytochrome Oxidase I and Cytochrome b) to look into the relationships among Nacella species and to determine the origin and diversification of the genus. Phylogenies were reconstructed using two methods, Maximum Parsimony and Bayesian Inference, while divergence time among Nacella species was estimated following a relaxed Bayesian approach. For this purpose, we collected inter- and subtidal species belonging to four biogeographic regions in the Southern Ocean: Antarctica, Kerguelen Province, Central Chile, and Magellanic Province. Our molecular results agree with previous morphological and molecular studies supporting the monophyly of Nacella and its sister relationship with Cellana. Two rounds of diversification are recognized in the evolution of Nacella. The first one occurred at the end of the Miocene and gave rise to the main lineages, currently distributed in Antarctica, South America or Kerguelen Province. Large genetic divergence was detected among Nacella species from these distant biogeographic provinces emphasizing the significance of trans-oceanic discontinuities and suggesting long-distance dispersal was relatively unimportant. The second diversification round consisted of a more recent Pleistocene radiation in the Magellanic region. In this province, different morphological species of Nacella exhibit extreme low levels of genetic divergence with absence of reciprocal monophyly among them. According to our time estimation, the origin and diversification of Nacella in the Southern Ocean is more recent (<15 MY) than the expected under the hypothesis of vicariant speciation due to plate tectonics. The evolution of this genus seems to be closely related to drastic climatic and oceanographic changes in the Southern Ocean during the middle-Miocene climatic transition. In spite of the high number of species described for the Magellanic Province, molecular results indicate that these species are the most derived ones in the evolution of the genus and therefore that the Magellanic region does not need to correspond to the origin center of Nacella. The absence of genetic divergence among these species supports a very recent radiation process accompanied by rapid morphological and ecological diversification.

Concerted genetic, morphological and ecological diversification in Nacella limpets in the Magellanic Province

Common inhabitants of Antarctic and Subantarctic rocky shores, the limpet genus Nacella, includes 15 nominal species distributed in different provinces of the Southern Ocean. The Magellanic Province represents the area with the highest diversity of the genus. Phylogenetic reconstructions showed an absence of reciprocal monophyly and high levels of genetic identity among nominal species in this Province and therefore imply a recent diversification in southern South America. Because most of these taxa coexist along their distribution range with clear differences in their habitat preferences, Nacella is a suitable model to explore diversification mechanisms in an area highly affected by recurrent Pleistocene continental ice cap advances and retreats. Here, we present genetic and morphological comparisons among sympatric Magellanic nominal species of Nacella. We amplified a fragment of the COI gene for 208 individuals belonging to seven sympatric nominal species and performed geometric morphometric analyses of their shells. We detected a complete congruence between genetic and morphological results, leading us to suggest four groups of Nacella among seven analysed nominal species. Congruently, each of these groups was related to different habitat preferences such as bathymetric range and substrate type. A plausible explanation for these results includes an ecologically based allopatric speciation process in Nacella. Major climatic changes during the Plio‐Pleistocene glacial cycles may have enhanced differentiation processes. Finally, our results indicate that the systematics of the group requires a deep revision to re‐evaluate the taxonomy of Nacella and to further understand the Pleistocene legacy of the glacial cycles in the southern tip of South America.

Towards a model of postglacial biogeography in shallow marine species along the Patagonian Province: lessons from the limpet Nacella magellanica (Gmelin, 1791)

BackgroundPatagonia extends for more than 84,000 km of irregular coasts is an area especially apt to evaluate how historic and contemporary processes influence the distribution and connectivity of shallow marine benthic organisms. The true limpet Nacella magellanica has a wide distribution in this province and represents a suitable model to infer the Quaternary glacial legacy on marine benthic organisms. This species inhabits ice-free rocky ecosystems, has a narrow bathymetric range and consequently should have been severely affected by recurrent glacial cycles during the Quaternary. We performed phylogeographic and demographic analyses of N. magellanica from 14 localities along its distribution in Pacific Patagonia, Atlantic Patagonia, and the Falkland/Malvinas Islands.ResultsMitochondrial (COI) DNA analyses of 357 individuals of N. magellanica revealed an absence of genetic differentiation in the species with a single genetic unit along Pacific Patagonia. However, we detected significant genetic differences among three main groups named Pacific Patagonia, Atlantic Patagonia and Falkland/Malvinas Islands. Migration rate estimations indicated asymmetrical gene flow, primarily from Pacific Patagonia to Atlantic Patagonia (Nem=2.21) and the Falkland/Malvinas Islands (Nem=16.6). Demographic reconstruction in Pacific Patagonia suggests a recent recolonization process (< 10 ka) supported by neutrality tests, mismatch distribution and the median-joining haplotype genealogy.ConclusionsAbsence of genetic structure, a single dominant haplotype, lack of correlation between geographic and genetic distance, high estimated migration rates and the signal of recent demographic growth represent a large body of evidence supporting the hypothesis of rapid postglacial expansion in this species in Pacific Patagonia. This expansion could have been sustained by larval dispersal following the main current system in this area. Lower levels of genetic diversity in inland sea areas suggest that fjords and channels represent the areas most recently colonized by the species. Hence recolonization seems to follow a west to east direction to areas that were progressively deglaciated. Significant genetic differences among Pacific, Atlantic and Falkland/Malvinas Islands populations may be also explained through disparities in their respective glaciological and geological histories. The Falkland/Malvinas Islands, more than representing a glacial refugium for the species, seems to constitute a sink area considering the strong asymmetric gene flow detected from Pacific to Atlantic sectors. These results suggest that historical and contemporary processes represent the main factors shaping the modern biogeography of most shallow marine benthic invertebrates inhabiting the Patagonian Province.

Extinction and recolonization of maritime Antarctica in the limpet Nacella concinna (Strebel, 1908) during the last glacial cycle: toward a model of Quaternary biogeography in shallow Antarctic invertebrates.

Quaternary glaciations in Antarctica drastically modified geographical ranges and population sizes of marine benthic invertebrates and thus affected the amount and distribution of intraspecific genetic variation. Here, we present new genetic information in the Antarctic limpet Nacella concinna, a dominant Antarctic benthic species along shallow ice‐free rocky ecosystems. We examined the patterns of genetic diversity and structure in this broadcast spawner along maritime Antarctica and from the peri‐Antarctic island of South Georgia. Genetic analyses showed that N. concinna represents a single panmictic unit in maritime Antarctic. Low levels of genetic diversity characterized this population; its median‐joining haplotype network revealed a typical star‐like topology with a short genealogy and a dominant haplotype broadly distributed. As previously reported with nuclear markers, we detected significant genetic differentiation between South Georgia Island and maritime Antarctica populations. Higher levels of genetic diversity, a more expanded genealogy and the presence of more private haplotypes support the hypothesis of glacial persistence in this peri‐Antarctic island. Bayesian Skyline plot and mismatch distribution analyses recognized an older demographic history in South Georgia. Approximate Bayesian computations did not support the persistence of N. concinna along maritime Antarctica during the last glacial period, but indicated the resilience of the species in peri‐Antarctic refugia (South Georgia Island). We proposed a model of Quaternary Biogeography for Antarctic marine benthic invertebrates with shallow and narrow bathymetric ranges including (i) extinction of maritime Antarctic populations during glacial periods; (ii) persistence of populations in peri‐Antarctic refugia; and (iii) recolonization of maritime Antarctica following the deglaciation process.

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.

DNA barcoding of marine polychaetes species of southern Patagonian fjords

Accurate species identification remains a basic first step in any study of biodiversity, particularly for global changes and their consequences. Thus, there is a pressing need for taxonomic expertise in a broad range of taxa. DNA barcoding has proved to be a powerful alternative method to traditional morphological approaches, allowing to complement identification techniques for living organisms. In this study, we assess intraspecific and interspecific genetic divergence among marine polychaetes from Patagonian fjords of southern Chile, using mitochondrial Cytochrome c Oxidase Subunit I (COI) gene. Our results showed that a total of 13 polychaetes species identified in this study exhibited high levels of interspecific variation among 31 analyzed sequences. Mean pairwise sequence distances comparisons based on K2P within species ranged from 0.2 to 0.4%. In contrast, interspecific comparisons were much higher and ranged between 18 to 47%, with the exception of the congeneric species Asychis chilensis and Asychis amphiglypta that showed high levels of genetic similarities and absence of reciprocal monophyly. This study presents the first information on DNA barcoding for polychaetes species in the southern Chile, and it establishes the effectiveness of DNA barcoding for identification of marine polychaetes species from Patagonian Fjords, thus making it available to a much broader range of scientists.

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.

Phylogeography in Galaxias maculatus (Jenyns, 1848) along Two Biogeographical Provinces in the Chilean Coast.

Major geologic and climatic changes during the Quaternary exerted a major role in shaping past and contemporary distribution of genetic diversity and structure of aquatic organisms in southern South America. In fact, the northern glacial limit along the Pacific coast, an area of major environmental changes in terms of topography, currents, and water salinity, represents a major biogeographic transition for marine and freshwater species. We used mitochondrial DNA sequences (D-loop) to investigate the consequences of Quaternary glacial cycles over the pattern of genetic diversity and structure of G. maculatus (Pisces: Galaxiidae) along two biogeographical provinces in the Chilean coast. Extreme levels of genetic diversity and strong phylogeographic structure characterize the species suggesting a low amount of influence of the last glacial cycle over its demography. However, we recognized contrasting patterns of genetic diversity and structure between main biogeographical areas here analyzed. Along the Intermediate Area (38°–41° S) each estuarine population constitutes a different unit. In contrast, Magellanic populations (43°–53° S) exhibited low levels of genetic differentiation. Contrasting patterns of genetic diversity and structure recorded in the species between the analyzed biogeographic areas are consistent with the marked differences in abiotic factors (i.e., different coastal configurations, Quaternary glacial histories, and oceanographic regimes) and to inherent characteristics of the species (i.e., salt-tolerance, physiology, and reproductive behavior).

Isolation Driven Divergence in Osmoregulation in Galaxias maculatus (Jenyns, 1848) (Actinopterygii: Osmeriformes).

Background Marine species have colonized extreme environments during evolution such as freshwater habitats. The amphidromous teleost fish, Galaxias maculatus is found mainly migrating between estuaries and rivers, but some landlocked populations have been described in lakes formed during the last deglaciation process in the Andes. In the present study we use mtDNA sequences to reconstruct the historical scenario of colonization of such a lake and evaluated the osmoregulatory shift associated to changes in habitat and life cycle between amphidromous and landlocked populations. Results Standard diversity indices including the average number of nucleotide differences (Π) and the haplotype diversity index (H) indicated that both populations were, as expected, genetically distinctive, being the landlocked population less diverse than the diadromous one. Similarly, pairwise GST and NST comparison detected statistically significant differences between both populations, while genealogy of haplotypes evidenced a recent founder effect from the diadromous stock, followed by an expansion process in the lake. To test for physiological differences, individuals of both populations were challenged with a range of salinities from 0 to 30 ppt for 8 days following a period of progressive acclimation. The results showed that the landlocked population had a surprisingly wider tolerance to salinity, as landlocked fish survival was 100% from 0 to 20 ppt, whereas diadromous fish survival was 100% only from 10 to 15 ppt. The activity of ATPase enzymes, including Na+/K+-ATPase (NKA), and H+-ATPase (HA) was measured in gills and intestine. Activity differences were detected between the populations at the lowest salinities, including differences in ATPases other than NKA and HA. Population differences in mortality are not reflected in enzyme activity differences, suggesting divergence in other processes. Conclusions These results clearly demonstrate the striking adaptive changes of G. maculatus osmoregulatory system, especially at hyposmotic environments, associated to a drastic shift in habitat and life cycle at a scale of a few thousand years.

Next-generation transcriptome characterization in three Nacella species (Patellogastropoda: Nacellidae) from South America and Antarctica

The southern tip of South America and Antarctica are particularly interesting due to many genera and also species currently sharing between both areas. The genus Nacella (Patellogastropoda: Nacellidae) is distributed in different regions of South America and Antarctica living preferentially on rocks and boulders and grazing on algae, diatoms and bacterial films. We described the transcriptomes of three Nacella species, Nacella concinna (Strebel, 1908), inhabiting the Antarctic Peninsula; Nacella magallanica (Gmelin, 1791), from Patagonia and Nacella clypeater (Lesson, 1831), from central Chile. In total, we obtained over 20,000 contigs with an average length of 583bp. Homologous protein coding genes (PCGs) for mitochondrial genome of the three species were characterized and a database of molecular markers was also generated. This study represents the first publicly available report on pyrosequencing data for patellogastropod species, and provides an important comparative resource for studies in ecophysiology and evolutionary adaptation in marine invertebrate species.