Genetics: Birds do it differently

J. Biol. 6, 2 (2007)

Gender parity may not be as important in the bird world as it is for other species, report Arthur Arnold of the University of California, Los Angeles, and his colleagues.

Animals such as humans and fruitflies have evolved various types of 'dosage compensation' to ensure that males and females express genes on the sex-related X chromosome(s) at the same level, even though females have two copies of the X to males' one.

Dosage compensation is thought to be crucial for the survival of a species. But Arnold's team found that zebra finches and chickens defy this expectation. On average, the males of these bird species express sex-linked genes at higher levels than females (in birds, it's the males that have two identical sex chromosomes).

The authors speculate that the unequal expression could be a novel way to generate sexual dimorphism.

Infectious Diseases: HIV modelled in mice

J. Exp. Med. doi:10.1084/jem.20062411 (2007)

Scientists in the United States report the development of a mouse model of HIV that, they say, could facilitate the search for new treatments and preventative measures against the disease.

It has been difficult to find appropriate small-animal models of HIV infection — especially those that model rectal transmission. J. Victor Garcia at the University of Texas Southwestern Medical Center in Dallas and his colleagues approached this problem by transplanting human tissue — including bone-marrow stem cells — into mice. The resulting mice developed many human cells, including the immune system's T cells. Garcia's team now reports that the mice respond to rectal HIV infection in a similar fashion to humans, including rapid depletion of gut T cells and the production of antibodies against the virus.

Materials Science: Alphabet soup

J. Phys. Chem. Lett. C 111, 4477–4480 (2007)

Credit: AM. CHEM. SOC.

Crisply rendered particles shaped like letters of the alphabet float about in the soup (pictured) prepared by Thomas G. Mason and Carlos Hernandez at the University of California, Los Angeles.

The soup shows off a lithography method developed by the team, which can mass-produce tiny particles with intricate shapes. The particles are formed in a layer of polymer on a silicon wafer, then float freely when an underlying layer is dissolved. Each of the letters measures about 4 by 7 micrometres — small enough that the particles dance the thermally induced random waltz of brownian motion when dispersed in a fluid.

The method could make any number of shapes, providing a tool to study how differently shaped particles interact, assemble, reconfigure and jam together.

Astronomy: Star bright

Astrophys. J. 659, L13–L16 and Astrophys. J. (submitted) preprint at (2007)

Astronomers are puzzling over what is probably the brightest supernova ever observed — a stellar explosion that, at its peak, pumped out almost 100 billion times more energy per second than is emitted by our Sun.

Telescopes on the ground and in space have tracked the fate of the supernova, SN 2006gy, since it was first detected in September 2006. Having analysed these data, two teams now present differing theories for what might have happened.

Eran Ofek of the California Institute of Technology in Pasadena and his colleagues suggest that the event's unusual brightness was due to the exploding star being enclosed in a dense gas cloud, which converted the mechanical energy of the burst into radiation. In a second paper submitted to the same journal, Nathan Smith of the University of California, Berkeley, and his colleagues argue that it was possibly a 'pair instability' supernova — the explosion of an extremely massive star, of a type that theorists might have expected to collapse quietly to form a black hole.

Biotechnology: Blood for all

Nature Biotech. doi:10.1038/nbt1298 (2007)

Bacterial enzymes that efficiently convert blood from groups A, B and AB into the 'universal' O group may lead to safer blood transfusions.

Red blood cells from groups A, B and AB contain antigens that cause life-threatening reactions if transfused into people with a different blood group. Henrik Clausen of the University of Copenhagen, Denmark, Gerlind Sulzenbacher of the Universities of Aix-Marseille, France, and their colleagues have identified two novel glycosidase enzymes that strip away these antigens. The enzyme-treated blood could then be used like group-O blood — group-O cells do not carry A or B antigens, so they can be safely given to anyone. The technology now needs to be tested in clinical trials.

Materials science: Waves of honey

Langmuir 23, 3732–3736 (2007)

Credit: AM. CHEM. SOC.

Even the simplest ingredients can give rise to complex patterns. Mark Buchanan of the University of Oslo, Norway, and his team have tackled one family of patterns — those formed when fluids containing suspended particles flow down a vertical surface.

Yoghurt running down the side of a pot, for example, is cut through by vertical, branched channels, whereas honey tends to break up into wavy horizontal bands. The researchers show that particle size determines which type of pattern is produced.

Smaller particles leave behind vertical channels, because the downward flow of the film becomes focused along certain paths by a feedback mechanism that amplifies flow rate. Wavy horizontal bands appear in suspensions of larger particles, as particles get trapped by random imperfections on the vertical surface and jam together to form stress-bearing arches. The pictures above show these effects; the fraction of large particles increases from left to right.

Molecular Electronics: A promising line-up

Nature Nanotechnol. doi:10.1038/nnano.2007.77 (2007)

Arrays of thousands of perfectly linear and parallel carbon nanotubes have been turned into transistors by John Rogers of the University of Illinois in Urbana-Champaign and his co-workers.

The researchers grew their nanotubes on quartz decorated with iron-oxide stripes that catalyse the tubes' growth. The tubes line up along the crystal axis of the substrate. Each tube could be divided into many transistors by careful placement of metal electrodes on the array's surface. Devices built in this way showed good performance — particularly in having high 'mobility', a measure of how readily current can move through them. The nanotube arrays could also be transferred from the quartz onto plastic substrates, which could be useful in flexible displays.

Cell Biology: Predictions on target

Nature Struct. Mol. Biol. doi:10.1038/nsmb1226 (2007)

A novel model might help biologists to predict the targets of microRNA molecules in animals. This is notoriously difficult, because in animals microRNAs can block the translation of messenger RNA (mRNA) into proteins without matching precisely the sequence to which they bind.

Devised by Ye Ding at the Wadsworth Center of the New York State Department of Health and Victor Ambros of Dartmouth Medical School in Hanover, New Hampshire, and their colleagues, the model takes into account the structure of the target mRNA, calculated with a program called Sfold. It assumes that microRNAs first find a short open stretch of mRNA to bind to, then unfurl the surrounding mRNA to complete binding.

This approach predicted strong interactions for some known microRNAs and their targets. It also correctly ruled out 11 of 12 targets predicted for a particular microRNA by sequence matching that were not borne out by experiments.

Eroding principles

Geology 35, 303–306 (2007)


A survey in southern Ecuador quantifies the dramatic effects of removing surface vegetation on soil erosion in tropical mountain regions. But the good news is that restoring dense plant cover, even if it is not native vegetation, cuts erosion back to near its natural rate.

Veerle Vanacker, now at the Catholic University of Louvain, Belgium, and her colleagues studied isotopes in river sediments to show that the natural erosion rate, averaged over the past 20,000 years, has been about 150 tonnes per square kilometre per year. The team showed that on slopes cleared by humans this rate increases up to 100-fold. They found reduced erosion rates for land covered by pine or eucalyptus plantations.

Journal club

Adina Paytan

Stanford University, California

A palaeoceanographer worries not about corals, but about coral reefs.

To understand what the consequences of human-induced CO2 increases might be, I study how atmospheric CO2 concentrations fluctuated in the past.

One outcome of high atmospheric CO2 that is inevitable is ocean acidification. Atmospheric CO2 dissolves in sea water, lowering the pH of the ocean's surface layer.

We expect this to create problems for marine creatures that precipitate their skeletons from calcium carbonate, because the mineral dissolves in acid. Some researchers have suggested that scleractinian corals might even be driven to extinction.

But what does the geological record tell us? Corals' reef-building fossils have appeared and disappeared over the past 200 million years and despite periods of elevated atmospheric CO2, the organisms did not go extinct.

A recent experiment (M. Fine & D. Tchernov Science 315, 1811; 2007) resolves this apparent paradox. The team grew scleractinian corals for a year in sea water with a lower-than-normal pH. They found that the corals reproduced and grew happily in this acidic environment — albeit without their hard skeletons. The corals adjusted their skeleton-forming physiology in response to the different growing conditions.

So corals seem to be quite adaptable. But I would like to know whether other calcifying organisms have such physiological versatility.

Moreover, we have to remember that although corals may survive in an ocean with a lower pH as sea-anemone-like organisms, they are currently major contributors to the intricate physical structure of coral reefs. What will be the future of these ecosystems if their calcium-carbonate scaffolding disappears? Will our grandchildren enjoy the spectacular beauty of these 'rainforests' of the ocean?