ALBERT

Description: The green mussel Perna viridis (Mytilidae) is a recent invader in the Atlantic basin including the Caribbean region and southeastern USA. In this study we examined the relative abundance of P. viridis across habitats in Tampa Bay, Florida, and density patterns at specific sample sites. Pilings, piers and floating objects were favourable habitats for P. viridis , while mangroves, flats and seawalls were not, although these were occasionally inhabited by P. viridis . Density and size-class distribution of P. viridis were estimated from approximately monthly samples collected between April 2002 and March 2003. Density of P. viridis varied significantly with sample date, reaching a maximum of 4,000 m –2 in September. Perna viridis exhibited the ability to dominate some habitats within the Tampa Bay ecosystem and the potential to compete for space with native species such as the eastern oyster, Crassostrea virginica , but the impacts were neither consistent nor widespread. Crassostrea virginica appears to have some refuges from P. viridis , while a small native mussel, Brachidontes cf. exustus , co-occurs with P. viridis in most sites; its density appears to be independent of P. viridis density.

Description: Homing endonucleases (HEs) can be used to induce targeted genome modification to reduce the fitness of pathogen vectors such as the malaria-transmitting Anopheles gambiae and to correct deleterious mutations in genetic diseases. We describe the creation of an extensive set of HE variants with novel DNA cleavage specificities using an integrated experimental and computational approach. Using computational modeling and an improved selection strategy, which optimizes specificity in addition to activity, we engineered an endonuclease to cleave in a gene associated with Anopheles sterility and another to cleave near a mutation that causes pyruvate kinase deficiency. In the course of this work we observed unanticipated context-dependence between bases which will need to be mechanistically understood for reprogramming of specificity to succeed more generally.

Description: We describe the identification and characterization of novel homing endonucleases using genome database mining to identify putative target sites, followed by high throughput activity screening in a bacterial selection system. We characterized the substrate specificity and kinetics of these endonucleases by monitoring DNA cleavage events with deep sequencing. The endonuclease specificities revealed by these experiments can be partially recapitulated using 3D structure-based computational models. Analysis of these models together with genome sequence data provide insights into how alternative endonuclease specificities were generated during natural evolution.

Description: The geochemistry of pyroclasts sampled from four volcanoes along the Kermadec arc in the SW Pacific is used to investigate the genesis of silicic magmas in a young (〈2 Myr), archetypical intra-oceanic arc setting. Raoul, Macauley and Raoul SW volcanoes in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all recently erupted dacitic to rhyolitic crystal-poor pumice. In addition to whole-rock analyses, we present a detailed study of mineral and glass chemistries to highlight the complex structure of the Kermadec magmatic systems. Major and trace element bulk-rock compositions mostly fall into relatively narrow compositional ranges, forming discrete groups by eruption for Raoul, and varying with relative crystal contents for Healy. In contrast, pumices from Macauley cover a wide range of compositions, between 66 and 72·5 wt % SiO 2 . At all four volcanoes the trace element patterns of pumice are subparallel to both those of previously erupted basalts and/or whole mafic blebs found both as discrete pyroclasts and as inclusions within pumices. Pb and Sr isotopic compositions have limited ranges within single volcanoes, but vary considerably along the arc, being more radiogenic in the southern volcanoes. Distinctive crystal populations and zonation patterns in pumices, mafic blebs and plutonic xenoliths indicate that many crystals did not grow in the evolved magmas, but are instead mixed from other sources including gabbros and hydrothermally altered tonalites. Such open-system mixing is ubiquitous at the four volcanoes. Oxygen isotope compositions of both phenocrysts (silicic origin) and xenocrysts or antecrysts (mafic origin) are typical for mantle-derived melts. Whole-rock, glass and mineral chemistries are consistent with evolved magmas being generated at each volcano through ~70–80% crystal fractionation of a basaltic parent. Our results are not consistent with silicic magma generation via crustal anatexis, as previously suggested for these Kermadec arc volcanoes. Although crystallization is the dominant process driving melt evolution in the Kermadec volcanoes, we show that the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected in small-scale chemical variations between eruption units.

Description: The population dynamics of the mediterranean snail, Cernuella virgata (Hygromiidae), which is a pest of grain crops and pastures in southern Australia, was studied in three adjacent fields used for continuous cropping on Yorke Peninsula, South Australia, for 20 years. The cropping sequence was commonly a rotation of faba beans, wheat and barley. Population densities were measured every month for the first 4 years and thereafter only in autumn and spring each year. Autumn and spring were the respective breeding season for adults and the resultant juvenile population that infested grain harvests. Populations varied greatly in abundance between years and fields. Spring abundance was correlated, albeit weakly, with prevailing spring and previous autumn rainfall. The latter can thus be used as an early warning of harvests suffering from snail contamination. Snails grew largest in faba bean crops and were more abundant in the wheat crops that followed faba beans than they were in the subsequent barley crop. No evidence was found of increased snail abundance near field edges, which would suggest biased seasonal movements of snails between adjacent fields, as has been reported for other studies of C. virgata in different agricultural systems (interfaces between crops and pastures, pastures and woody/weedy roadside vegetation). It seems probable, in this case, that the similarities in vegetative form of adjacent crops provided limited stimulation for differential dispersal between fields. The frequencies of banded (darkly coloured) shells also did not vary with distance from field edges, suggesting that the fence posts and tall weeds that occurred at the interfaces between fields were insufficient to provide (cooler, above-ground) shelter in summer and enhance local survival of banded C. virgata relative to locations further into fields.

Description: New geochemical and isotopic data are presented for volumetrically minor, depleted low-Ti basalts that occur in the Plateau Basalt succession of central East Greenland (CEG), formed during the initial stages of opening of the North Atlantic at 55 Ma. The basalts have mid-ocean ridge basalt (MORB)-like geochemistry (e.g. depleted light rare earth elements) and are distinct from the high-Ti lavas that dominate the sequence. Rare earth element geochemistry implies derivation from a source more depleted than the typical MORB source, and suggests polybaric melting and contributions from both spinel- and garnet-facies mantle. The low-Ti basalts have Sr–Nd–Pb–Hf isotopic characteristics that are similar to those of depleted magmas from Iceland (e.g. Theistareykir) and adjacent ridges (Kolbeinsey and Reykjanes) and distinct from global MORB (e.g. negative 207 Pb, and Hf and Nd isotope compositions that plot above the mantle reference line). Isotope and trace element data indicate the involvement of two depleted source components. One component has isotopic compositions similar to other depleted components identified in the North Atlantic and has high Rb/Zr and Ba/Nb. The second is isotopically less depleted with lower Rb/Zr and Ba/Nb. Small degrees of crustal contamination (〈 1%) by both amphibolitic and granulitic crust result in relatively large changes in isotopic composition ( c . 1% lower for 206 Pb/ 204 Pb and 0·1% higher for 87 Sr/ 86 Sr depending on the contaminant). Negative Nb suggests a MORB affinity for the low-Ti magmas; however, they are distinguished from global normal (N)-MORB on the basis of vertical deviations from the Northern Hemisphere Reference Line (negative 207 Pb and positive 208 Pb), and relative enrichments in Ba, Sr and Pb. The isotopic compositions of the low-Ti CEG basalts suggest correlation with modern depleted components beneath Iceland and adjacent ridges, considered to be derived from upper mantle sources polluted by the Iceland plume. However, small positive Pb peaks when normalized to MORB, and lower Nb distinguish the CEG low-Ti basalts from depleted Icelandic compositions. The lower Nb (〈 0) and 87 Sr/ 86 Sr, and suggestion of higher 206 Pb/ 204 Pb in crustally uncontaminated parental melts imply a closer affinity to compositions from the oceanic ridges surrounding Iceland (especially the Reykjanes Ridge), yet they are subtly distinct on the basis of available trace element data. We suggest that this depleted component was an integral part of the plume that melted primarily during the rapid lithospheric uplift and extension associated with continental break-up.