Evolution: Adapting to life on campus
Evolution 58, 166–174 (2004)
When colonizing a new habitat, a species can encounter new selection pressures. A population of dark-eyed juncoes (Junco hyemalis) that left its mountain habitat and set up home at the University of California, San Diego, has allowed researchers to see this effect in action. For the past two decades, the birds' sexual plumage displays have been evolving at a surprising rate.
Male juncoes signal their quality to mates through the amount of white plumage in their tail. But the San Diego birds have 22% less white in their tail than do their mountain counterparts, reports Pamela J. Yeh. As the birds arrived in 1983, this is impressively rapid evolution.
The move to a warmer climate, with a longer breeding season, may have forced males to concentrate on proving their worth as fathers, rather than as Adonises, Yeh suggests. With the opportunity for several clutches each summer, females may favour males who helped to raise previous broods that they had sired. Coupled with the campus population's low density, this reduces the need for striking plumage and may have driven its decline.
This could not have occurred without such selection, says Yeh, and the campus population is too big for such a large genetic change to have occurred randomly.
Chemistry: On with the flow
J. Am. Chem. Soc. 126, 1569–1576 (2004)
Water can form a cage-like lattice that traps other molecules, creating solid ‘clathrate hydrates’. For example, methane hydrates, trapped beneath the oceans, are believed to be the most abundant form of natural methane. But hydrates are bad news when they build up in pipelines and block gas flow. Although chemical additives can inhibit clathrate formation, they stop working if the pipes get cold enough for long enough.
Through a computational screening process, Mark T. Storr et al. have designed a new class of clathrate inhibitor. An exemplar, tributylammonium propylsulphonate (TBAPS), performed at least as well as one of the most commonly used inhibitors in a series of tests.
According to Storr and colleagues' analysis, one end of the TBAPS molecule attracts water molecules, which weakens the surrounding lattice; the opposite end repels water, strengthening the lattice in that region. This distortion effectively delays the formation of clathrate crystals. The authors believe that this new compound can be developed further to substantially increase its activity.
Transcription: It's about time
Proc. Natl Acad. Sci. USA 101, 1200–1205 (2004)
At a sufficiently high cell density, yeast cultures synchronize their efforts during the so-called respiratory cycle: in 40-minute bouts within the cell cycle, they alternate respiratory tasks, which generate energy, with reductive functions, during which they produce cellular constituents. While hunting for genes that support these oscillations, Robert R. Klevecz and colleagues uncovered genome-wide rhythms in gene transcription.
The researchers scrutinized gene activity in yeast by using gene chips, and found three distinct intervals in each respiratory cycle when most gene transcription occurred. The vast majority of genes were expressed during one of two periods in the reductive phase of the cycle, whereas the rest were active during a single period in respiration. Even those genes previously thought to be important in cellular housekeeping, and ‘constitutively’ expressed, showed small oscillations.
Klevecz et al. further showed that, every 40 minutes, a burst of DNA replication followed the respiratory phase in a constant fraction of the yeast culture. The authors believe that restricting DNA synthesis to the reductive phase of the cycle, when potentially harmful reactive-oxygen species are less abundant, may have evolved to reduce oxidative damage to DNA. Moreover, they propose that multiples of 40-minute oscillations underlie cell division.
Immunology: Toxic shock in the dock
Cell 116, 367–379 (2004)
Toxic shock syndrome, caused by the bacterium Streptococcus pyogenes, is one of the severest forms of septic insult. Normally, the bacterium brings on a mild throat or skin infection. But occasionally it goes on to cause blood-vessel leakage and organ failure, with rapidly fatal results. Working with both in vitro and in vivo models, Heiko Herwald et al. have characterized the chain of events by which S. pyogenes wreaks havoc in its unfortunate host.
The researchers find that a protein released from the surface of the bacterium, ‘M protein’, attaches itself to fibrinogen, a component of blood. The complex of M protein and fibrinogen activates neutrophils, a type of white blood cell, which then release heparin-binding protein — the molecule that produces the ultimately catastrophic failure of blood vessels.
Herwald et al. went further, and by pin-pointing a peptide that interferes with the fibrinogen–neutrophil interaction, they identified where the chain might be broken. As they say, 30–70% of patients die despite intensive efforts to save them, and more effective treatments for the syndrome are badly needed.
Materials science: Snow has no weakest link
Phys. Rev. E 69, 011306 (2004)
Skiers and mountain guides know that snow is treacherous stuff, but H. O. K. Kirchner and colleagues have demonstrated just how treacherous. One never knows, they say, when snow is going to crack — the process that typically triggers an avalanche.
The researchers quantify this unreliability of snow by measuring its so-called Weibull modulus, a measure of the variability of a material's strength. The smaller this parameter is, the broader the spread of different fracture strengths for ostensibly identical samples. High-performance steel has a modulus of 22, and for normal ice the value is 3–10. For snow, however, it is 1.5 ± 0.5. This means that even very low stresses have a chance of fracturing snow.
On the other hand, Weibull's original analysis postulated that the strength of a brittle material is limited by the strength of the weakest link in a random distribution of flaws. This implies that the strength declines as the sample gets larger (increasing the chance of it containing a weak link). But Kirchner et al. find that snow has no such size dependence — which means that lab-scale measurements of its mechanical properties can probably be extrapolated to the much larger scales relevant to avalanche prediction.