COVER Moonlit silhouette of the North American black cottonwood Populus trichocarpa. Because this tree has a small genome and has long been the subject of commercial and ecological studies, P. trichocarpa was selected as the first woody perennial plant to have its genome sequenced. See page 1596. Photo: David Hiser/Getty Images

EDITORS' CHOICE: HIGHLIGHTS OF THE RECENT LITERATURE

ECOLOGY: Single Symbionts for Corals

Caroline Ash

Tropical coral reefs are stressed by sea-level rise and higher water temperatures brought on by climate change. Stress prompts corals to shed their photosynthetic symbionts, or zooxanthellae, and large areas of reefs can "bleach," sometimes killing the coral. Controversy has centered on whether bleaching is adaptive to enable bleached corals to acquire different symbionts that could endow their hosts with different physiologies to cope with different conditions, in particular greater temperature tolerance. Symbiont shuffling could happen only if the host coral can naturally tolerate a variety of symbionts. Goulet has undertaken a meta-analysis and review of 43 papers containing genotype data for 442 coral-zooxanthellae associations. It seems that most mature hard coral individuals harbor only one strain of symbiont and will retain the same genotype for decades, even after transplantation from one site to another. It remains unclear how the remaining 23% of corals th at can host several symbionts respond to bleaching conditions.

CA

Mar. Ecol. Prog. Ser.321, 1 (2006).

CHEMISTRY: Flowing Precious Metals

Marc S. Lavine

et al. show that a particular ligand and counter-ion combination confers flowing properties to a range of precious metal nanoparticles ~2 nm in diameter. Crystalline particles of platinum and gold, and predominantly amorphous palladium and rhodium particles, were prepared with N,N-dioctyl-N-(3-mercaptopropyl)-N-methyl ammonium capping ligands (bound to the metal through sulfur) by reduction of metal salts in tetrahydrofuran solution. Exchange of bromide counter-ions with sulfonates bearing long hydrophobic tails yielded a substance that, after thorough drying under vacuum, exhibited highly viscous liquid-like flow at room temperature; a 50-mg droplet moved at a rate of just over 2 cm/hour down an inclined glass plane. The authors envi sion that these flowing nanoparticles may offer convenient routes to self-assembled materials, as well as applications in heat-transfer media. -- MSL

J. Am. Chem. Soc.128, 10.1021/ja064469r (2006).

GEOLOGY: Tales of Wander

H. Jesse Smith

In an investigation of possible true polar wander, Maloof et al. present paleomagnetic data from three Middle Neoproterozoic carbonate units in Svalbard, Norway, which show large shifts in paleomagnetic orientation coincident with abrupt changes in ¹³C and relative sea level. They conclude that the best explanation for the data is that this area experienced rapid shifts of paleogeography during a pair of true polar wander events. Their hypothesis can be further tested by analyzing sediments of the same age from other basins for predictable related changes. -- HJS

Geol. Soc. Am. Bull.118, 1099 2006).

MOLECULAR BIOLOGY: Circle of One

Guy Riddihough

Mycobacterium species. Oddly enough, as Pitcher et al. now show, Corndog and Omega both contain Ku homologs in their genomes. The viral Ku proteins can work together with the bacterial ligase LigD to repair ds breaks in a yeast system. This suggests that NHEJ is somehow involved in the viral life cycle, where previously there was no indication of such a requirement.

Corndog and Omega enter bacterial cells as linear viruses that must circularize to allow rolling circle replication-an essential part of the viral life cycle. Related viruses, such as Lambda, have long 9-nucleotide (nt) cohesive (cos) ends that provide a favorable equilibrium for self-association. Corndog and Omega have very short cos ends, of only 4 nt, which are too short to self-associate efficiently and promote genome circularization. Thus the viral Ku, working together with the host LigD, may help to bring the cos ends together, paralleling their function in dsDNA break repair. -- GR

Mol. Cell 23, 743 (2006).

ECOLOGY/EVOLUTION: In Perfect Symmetry

Andrew M. Sugden

et al. monitored the pollination rates of Erysimum mediohispanicum, a herbaceous plant of the southern Spanish mountains, which shows intraspecific variation in flower shape and is pollinated by beetles, bees, and hoverflies. The more bilaterally symmetric flowers were favored by the most abundant pollinating insect, the generalist beetle Meligethes maurus, and these flowers also produced the highest number of offspring. The significant fitness differences between flowers of differing shape suggest the adaptive route by which bilateral symmetry can evolve, even if the pollin ators are generalists like most beetles. -- AMS

Am. Nat.168, 10.1086/507048 (2006).

CLIMATE SCIENCE: Shedding Light on the Sun

Brooks Hanson

Bard and Frank provide a thorough critical review of both the problematic evidence for longer changes in solar irradiance and the possible climatic effects these changes could have induced. The authors point out that many proposed connections, for example between the records of cosmogenic nuclides such as ¹⁴C and ¹⁰Be and records of climate change, are based on correlations--some of which have large and perhaps unappreciated uncertainties--and on imperfect and indirect records. They conclude that there might still be a connection between solar changes and the Medieval Warm Period and Little Ice Age, but that overall solar changes, most of which remain unproven, probably represent a second-order influence on the behavior of Earth's recent climate. -- BH

Earth Planet. Sci. Lett.248, 1 (2006).

IMMUNOLOGY: Vascular Origins

Stephen J. Simpson

Sebza et al. extend previous work in which the hematopoietic immune signaling proteins Syk and SLP-76 were found to regulate the developmental separation of lymphatic and blood vessel systems [Science299, 247 (2003)]. Directed transgenic reexpression of SLP-76 in a subset of hematopoietic cells was sufficient to correct the defect in lymphatic-vascular connection apparent in mice that lack Syk and SLP-76. By generating chimeric animals bearing both wild-type and Syk/SLP-76-deficient cells, it was also possible to establish this phenomenon as an endothelial cell-autonomous effect. Thus, th e study demonstrates that under steady-state conditions, cells of hematopoietic origin can contribute directly to blood lymphatic-vascular separation as precursors of endothelial cells. It will now be interesting to pursue experiments that more precisely characterize the progenitor cells and their relationship with endothelium during the processes of blood and lymphatic vessel growth and repair. -- SJS

Dev. Cell11, 349 (2006).