Physicists' understandable embarrassment that we don't know what most of the universe is made of prompts an eagerness, verging on desperation, to identify the missing ingredients. Dark matter, which is thought to comprise around 85 per cent of tangible material, is very much on the experimental agenda. This invisible substance is inferred on several grounds, especially that galaxies ought to fall apart without its gravitational influence. The favourite idea is that dark matter consists of unknown fundamental particles that barely interact with visible matter—hence its elusiveness.
One candidate is a particle predicted by theories that invoke extra dimensions of spacetime beyond the familiar four. So there was excitement at the recent suggestion that the signature of these particles has been detected in cosmic rays, which are electrically-charged particles (mostly protons and electrons) that whizz through space. Cosmic rays can be detected when they collide with atoms in the Earth's atmosphere. Some are probably produced in high-energy environments like supernovae and neutron stars, but their origins are poorly understood.
An international experiment called ATIC, which floats balloon-borne cosmic-ray detectors over Antarctica, has found an excess of cosmic-ray electrons with high energies, which might be the debris of collisions between the hypothetical dark-matter particles. That's the sexy interpretation anyway; they might instead come from more conventional sources.
The matter is further complicated by an independent finding, from a detector called Milagro near Los Alamos in New Mexico, that high-energy cosmic-ray protons seem to be concentrated in a couple of bright patches in the sky. It's not clear if the two results are related, but if the ATIC electrons come from the same source as the Milagro protons, that rules out dark matter, which is expected to produce no such patchiness. On the other hand, no other source is expected to do so either. It's all very perplexing, but nonetheless a demonstration that cosmic rays offer an unparalleled natural resource for particle physicists.
Genome sequenced while you wait
A Californian biotech company is promising, within five years, to sequence your entire genome while you wait. In under an hour, a doctor could take a sample and deduce all of your genetic predispositions to disease—at least, that's the theory.
Pacific Biosciences has developed a technique for replicating a piece of DNA in a form that contains fluorescent chemical markers attached to each "base," the fundamental building blocks of genes. Each of the four types of base gets a differently-coloured marker, and so the DNA sequence—the arrangement of bases along the strand—can be discerned as a string of fairy lights, using a sensor that can image individual molecules.
With a current read-out rate of about 4.7 bases per second, the method will need a speed increase to sequence all 3bn bases of a human genome in an hour. And it is plagued by mistakes about the "colour" of the markers which might wrongly identify as many as one in five of the bases. But these are early days; the basic technology works. The company hopes to start selling commercial products by 2010.
Faster genome sequencing will no doubt be valuable in medicine. To take one example, potential drugs that would be unusable because of genetically-based side-effects in a minority of cases could be rescued by screening that identifies those at risk. But many researchers admit that the notion of a genome-centred "personalised medicine" is easily over-hyped. Not all diseases have a genetic component, and those that do may involve complex, poorly understood interactions of many genes. Worse still, DIY sequencing kits could saddle people with genetic data that they don't know how to interpret or deal with, as well as running into a legal morass about privacy and disclosure. At this rate, the technology is far ahead of the ethics.
More evidence on epigenetics
Besides, it is becoming increasingly clear that genes can be over-ridden: to put it crudely, an organism can "disobey" its genes. There are now many examples of "epigenetic" inheritance, in which phenotypic characteristics (hair colour, say, or susceptibility to certain diseases) can be manifested or suppressed despite a genetic imperative to the contrary (see "What genes remember," Prospect May 2008). Commonly, epigenetic inheritance is induced by small strands of RNA, the intermediary between genes and the proteins they encode, which are acquired directly from a parent and can modify the effect of genes in the offspring.
An American team has now shown a new type of such behaviour. A rogue gene that can cause sterility in crossbreeds of wild and laboratory-bred fruit flies may be silenced by RNA molecules if the gene is maternally inherited, maintaining fertility in the offspring despite a "genetic" sterility. Most strikingly, this effect may depend on the conditions in which the mothers are reared: warmth boosts the fertility of progeny. It's a reminder, amid the impending Darwin celebrations, of how complicated the story of heredity has become.
