Biomechanics: Jeepers creepers
Phys. Rev. Lett. 97, 184302 (2006)
How do climbing plants grasp the poles up which they wind? And why do some twine around thin poles but not thick ones? Researchers going back to Charles Darwin have puzzled over these questions. Now Alain Goriely at the University of Arizona in Tucson and Sébastien Neukirch at the Pierre and Marie Curie University in Paris, France, show that a simple model can explain the plants' behaviour.
They treated a twining plant stem as an elastic rod that has an intrinsic tendency to curl. They examined how friction between the rod and a supporting pole holds up the growing helix, and showed that the angle at which the curling tip meets the pole's surface determines the maximum radius of pole around which the stem can wind.
Cell biology: Spaced out
Nature Struct. Mol. Biol. doi:10.1038/nsmb1170 (2006)
A molecular machine that seems to act like a ruler in fact performs a dynamic balancing act, researchers report.
Biologists know that the enzyme ACF controls the spacing between DNA-packaging units called nucleosomes, in which DNA is wrapped around protein spools.
To find out more, Geeta Narlikar and her colleagues at the University of California, San Francisco, tagged nucleosomes with fluorescent dyes and tracked their movements in real time. It seems that ACF continuously samples the stretches of DNA bordering each nucleosome, and pushes longer pieces of DNA through the nucleosome faster than shorter stretches. This establishes an equilibrium state between neighbouring nucleosomes that favours even spacing.
Planetary science: Time heals
Earth Planet. Sci. Lett. 251, 79–89 (2006)
Planets such as Mars and Venus (pictured) with rigid, unmoving crusts can experience transient bursts of plate tectonics, according to new computer simulations.
Alexander Loddoch of the University of Münster in Germany and his colleagues built a model to simulate mantle convection beneath a 'stagnant lid'. Under some conditions the lid, or crust, breaks up and its parts begin to move. Previous studies have suggested that once such plate tectonics starts, it either continues or recurs in episodes so closely spaced that the crust remains unsettled. Loddoch's group found instead that bouts of plate tectonics can be far enough apart for the crust to completely reform. This would fit some theories of Mars' and Venus' histories, the team suggests.
Microbiology: Burrowing bacteria
Science 314, 985–989 (2006)
Researchers in Japan have figured out how Shigella bacteria, which cause dysentery, burrow through cells. Their observations may help efforts to combat the microbe, which kills 1.1 million people each year.
Chihiro Sasakawa of the University of Tokyo and his team used fluorescence- and electron microscopy to photograph cells infected with Shigella. The bacteria's movement is hindered by the cell's dense network of microtubulin, a cytoskeletal protein.
But Shigella produces a protein-chomping enzyme called VirA, which acts on microtubulin. Sasakawa's team saw that VirA sliced up the microtubules, creating tunnels through which the bacteria could spread. Knocking out VirA production in Shigella prevented the deaths of infected mice.
Cell Biology: Battle of the bulge
J. Cell Biol. 175, 477–490 (2006)
Researchers hoping to understand a part of the cell's skeleton known as the cortex have made headway by studying a process called 'blebbing'.
The cortex is a layer of cytoskeleton wrapped beneath the cell membrane, which helps to maintain cell shape and aid cell movement. Blebbing occurs when the membrane detaches from the cortex and inflates, forming a bulge on the cell surface.
Guillaume Charras and his colleagues at Harvard Medical School in Boston, Massachusetts, studied how the cortex reassembles beneath the bulging membrane. They show that it happens in steps, with actin and actin-bundling proteins following the anchor protein ezrin. Contractile proteins then pull the layer down, all within 30 seconds.
Nanotechnology: Shrink to fit
Nano Lett. doi:10.1021/nl061671j (2006)
Carbon nanotubes shrunk to order could be the next fashion in nanotube devices, thanks to a process developed by Alex Zettl at the University of California, Berkeley, and his colleagues.
They blasted a nanotube with an electron beam, knocking some atoms from its structure. At the same time, a current was passed through the damaged tube. This made it reform, with few defects, at a smaller diameter (pictured right). The appeal for device designers is that a tube's electrical properties are intimately linked to its diameter: as the tube shrinks, resistance increases.
Radiology: Tumour annihilation
Radiother. Oncol. doi:10.1016/j.radonc.2006.09.012 (2006)
Protons are the particles of choice to zap tumours in sensitive areas of the body because where they deposit their energy can be controlled precisely. But their antimatter cousins could pack even more of a knockout punch because antiprotons and matter annihilate one another, releasing a burst of energy.
Bradley Wouters of the University of Maastricht in the Netherlands and his colleagues used an antiproton facility at CERN, the European particle-physics laboratory near Geneva, Switzerland, to make the first measurements of the particles' effects on mammalian cells. They compared the number of cells killed in the target zone with the number that died in the surrounding tissue. For antiprotons, this ratio was some 3.75 times greater than for protons.
But practical therapeutic uses are far off: making antiprotons currently requires a circular accelerator with a diameter of at least 100 metres.
Neuroscience: Memory gene
Neuron 52, 437–444; 445–459; 461–474; 475–484 (2006)
Four papers published in Neuron help to demystify the mechanism of a gene implicated in the consolidation of memories.
The gene, known as Arc/Arg3.1, is expressed in the brain during learning. It has long been used as a marker of neuronal activity, even though its physiological role was not clear.
The new research shows that mice with the Arc/Arg3.1 gene knocked out fail to form long-lasting memories. Studies in vitro suggest that the gene controls the appearance and disappearance of receptors for the neurotransmitter AMPA on neuronal surfaces. Such receptor trafficking is known to modify the strength of connections between neurons, which is fundamental to learning and memory.
Astronomy: Recipe for X-rays
Mon. Not. R. Astron. Soc. 372, 1531–1539 (2006)
On the outskirts of the spiral galaxy M99 is an object that, in some X-ray frequencies, outshines all the rest of the galaxy put together. It has been suggested that such 'ultraluminous X-ray sources' are powered by black holes larger than can be formed in the death throes of any normal star.
Roberto Soria of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Diane Sonya Wong of the University of California, Berkeley, note that radio images of M99 show a cloud of fast-moving hydrogen impinging on the galactic disc near the X-ray source. They suggest that this influx of hydrogen could have compressed material at the edge of the disc into an anomalously large star, which later evolved into a black hole of a size suited to such X-ray brilliance.
Prickly all over
Science 314, 941–952; 960–962 (2006)
The genome of the purple sea urchin, Strongylocentrotus purpuratus, has been sequenced. This will hearten developmental and systems biologists for whom the sea urchin serves as a model organism.
Sea urchins, like humans, are members of the superphylum of deuterostomes and hence are more closely related to us than the other lab favourites fruitflies and nematodes. The transparent sea urchin embryos are also ideal for studying early development. The sequenced genome reveals that many genes thought to be unique to vertebrates crop up in the sea urchin too, including some important for human hearing and sight. The genome also boasts large families of genes involved in innate immunity. Around 12,000 of its more than 23,000 genes are active in the developing embryo.
Michael Sanderson University of Arizona, Tucson
A biologist turns his attention to evolution's neglected radiations.
Evolution's spectacular adaptive radiations get a lot of press: Darwin's finches and the Hawaiian silversword plants being textbook examples. These organisms, in adapting to environmental pressures, underwent both rapid speciation and radical morphological change.
Such episodes give rise to easily observable diversity and have stimulated extensive study. But how about those hyperdiverse clades in the tree of life in which many species have little morphological difference between them?
I first pondered this problem when musing about my thesis on the flowering-plant taxon Astragalus. I was cursed with perhaps 2,500 species, many remarkably similar. Their small differences were typically of uncertain adaptive significance.
Alas, I have counted barely ten papers since then that have addressed such radiations, which end up being labelled as 'non-adaptive'. I hope the most recent will shake things up a bit.
It analyses the speciation rate of North American Plethodon, a clade of salamanders most diverse in the woodlands of the Appalachian mountains (K. H. Kozak et al. Proc. R. Soc. B. 273, 539–546; 2006). This group has an evolutionary history that runs back 28 million years and has spun off about 46 species, many of which are only diagnosable by molecular markers.
Remarkably, the rate of speciation in the group's early days matched or exceeded rates seen in the textbook adaptive radiations. This suggests that we have a lot to learn about the evolutionary phenomena driving such radiations.
The authors make some interesting suggestions about the role of geography, ecology and adaptation in the salamanders' evolution. For example, the lineages may have evolved by tracking the ebb and flow of favourable habitats.