Entomology: Unlocking the sex secrets of cockroaches

Science 307, 1104–1106 (2005)

Humans have notched up a victory in the long-running battle with cockroaches. Satoshi Nojima et al. have purified and identified the sex pheromone of the female German cockroach (Blattella germanica), more than a decade after the gland that produces it was discovered. The finding could pave the way for more effective cockroach traps.

The pheromone had proved resistant to isolation by standard gas chromatography — in which components evaporate out of a mix one by one — because it fell apart in the heat. Using a new, lower-temperature version of the procedure, Nojima et al. isolated the contents of the gland and then used a cockroach antenna to single out the active pheromone. Finally they used nuclear magnetic resonance analysis to determine the structure of the compound.

The pheromone is gentsyl quinone isovalerate — or, the authors propose, ‘blattellaquinone’. When the team baited traps with it in the lab and at a roach-infested pig farm, male roaches came running. Given the compound's structure, Nojima et al. suggest that it may have been used for defence earlier in the insect's evolutionary history, although the quantities found in females are now too small to be useful for this purpose.

Emma Marris

Neurobiology: Beyond a supporting role

Cell 120, 421–433 (2005)

Biologists once viewed astrocytes as passive supports for neurons. But as actors in the bigger picture, these cells are not simply extras. That point has been made to striking effect in a study by Karen S. Christopherson and colleagues — their research provides evidence that astrocytes produce proteins called thrombospondins that encourage the growth of new neuronal synapses.

Christopherson et al. added a purified form of these proteins to rat brain cells and witnessed synapse growth strikingly similar to that induced by astrocyte secretions. Exactly how the proteins work remains unclear, but in the future the authors hope to pinpoint the neuronal receptor to which they bind.

The presence of thrombospondins in young, developing brains suggests that these molecules have an important role in the formation of synapses. Moreover, their levels are decreased in adults, perhaps helping to explain why adult brains are less ‘plastic’ than younger ones.

Roxanne Khamsi

Atmospheric science: Plant food from pollution

J. Geophys. Res. doi:10.1029/2004JD005082 (2005)

Iron is an essential nutrient for phytoplankton, the tiny aquatic plants that carry out almost half of all photosynthesis on Earth.

Dust storms in northern China and Mongolia carry iron from the soil of the Gobi desert to the northern Pacific Ocean. But the iron in desert dust is in a mineral form that has low solubility in seawater and so is not readily available to phytoplankton. Nicholas Meskhidze and colleagues have found that sulphur dioxide pollution from industrial plants in China can acidify the dust, which converts iron to a more soluble form.

The team tracked two dust storms from the Gobi desert that passed over Beijing and then the Pacific in 2001. Using satellite measurements, they saw an increase in phytoplankton growth after a storm in March, but, surprisingly, no increase after a larger storm in April. They attribute this difference to the fact that the dust in the April storm contained much more calcium carbonate, which neutralizes SO2 and therefore limits the process of making iron more soluble.

Natural sources of SO2, such as volcanoes, may also boost phytoplankton growth. This mechanism could be vital for fertilizing the oceans, the authors add, increasing the uptake of CO2 during photosynthesis.

Mark Peplow

Materials chemistry: Magnetic sensors

Chem. Mater. doi:10.1021/cm0486971 (2005)

Magneto-optic memories are now commercially available. But C. Michael Elliott et al. report that they have developed a material that changes its response to light in the presence of weak magnetic fields, and propose that magnetism and light can be coupled to give an optical read-out of data from a magnetic storage medium.

In their molecular ‘triad’ — comprising an electron-donating group, an electron acceptor and a light-absorbing chromophore (a dye) — charge is transferred between the donor and acceptor when the chromophore is illuminated. The length of time the acceptor remains in this charged state is altered by a weak magnetic field, providing a possible ‘detection’ signal. However, this usually only happens when triad species are in solution, and only solid materials can make magneto-optical storage practical.

Elliott et al. get around the problem by encapsulating droplets of an aqueous solution of their triad in a polymeric matrix. They first sequester the droplets inside reverse micelles of a surfactant dispersed in a polymerizable organic liquid. Crosslinking the solvent molecules then produces a hard, transparent polymer in which the charge-transfer dye retains its optical properties.

Laser light is used to excite the charged state, and a modest magnetic field induces a measurable shift in its lifetime.

Philip Ball

Pattern formation: Square rules

Phys. Rev. Lett. 94, 054503 (2005)

The archetype for the division of two-dimensional space into cells has long been the honeycomb. Its hexagonal symmetry is echoed in natural phenomena as diverse as bubble rafts and geological formations such as the Giant's Causeway in County Antrim, Northern Ireland. For biological cells, the paradigm tends to be threefold junctions of cell walls that meet at 120°.

But many naturally arising patterns have a different geometry, exemplified by the filigree of cracks in a ceramic glaze (pictured). Here the cells are predominantly four-sided, and cracks tend to meet at right angles.

Bohn et al. show that this pattern can be understood by considering the sequential, hierarchical manner in which the cracks form. Long, space-crossing rifts come first, and then the space in between is filled with ever-finer, stress-relieving fractures that do not alter the existing fissures.

The researchers use geometric principles such as Euler's theorem (relating the number of network vertices and edges to the number of cells they define) to show why, in this hierarchical formation, the cells will on average be four-sided but with six neighbours. Intriguingly, this kind of pattern is also seen in the street networks of old cities (before urban planning skewed the topology), where a few major routes radiating from the centre were gradually supplemented by ever-narrower roads, lanes and alleys in between.

Philip Ball