Moms and pups sniff out immune genes Given how similar newborns can look,

forgive parents if they have trouble pick- ing out their bundle of joy from the swad- dled masses in a maternity ward. Perhaps they should sniff each one. After all, among animals, many parents and off- spring recognize each other by scent.

The major histocompatibility complex (MHC), a cluster of genes important to the immune system, is at the heart of this olfactory ability, suggests a mouse study in the Sept. 12 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

“This is the first indication of a set of genes that is responsible for providing the individual signature that both the mother and offspring recognize,” says coauthor Gary K. Beauchamp of the Monell Chemi- cal Senses Center in Philadelphia.

The MHC genes encode cell-surface proteins that display bits of invading bac- teria, viruses, and other microbes, there- by helping the immune system recognize harmful germs. The MHC of a mouse or a person contains dozens of genes, some having more than 100 versions. MHC vari- ation creates a problem for people need- ing an organ transplant; their bodies may reject donor tissue if its MHC profile is too dissimilar.

In the 1970s. the diversity of the MHC prompted the late biologist Lewis Thomas t o speculate that this se t of genes confers identifying odors upon people. He even suggested that for a per- son needing a transplant, dogs could sniff out a donor having similar MHC genes. As Thomas predicted, many studies have now shown that the MHC influences odortype, the signature scent of a person or animal.

In recent years, studies on the role of the MHC in mating have backed the max- im that opposites attract. Animals, and even people, seem to prefer mates with dissimilar MHC genes. In one study, women favored the smell of sweaty T- shirts from men with MHC genes unlike their own. Such a preference may pro- mote MHC diversity, creating offspring with strong immune systems.

Beauchamp and his colleagues, who conducted some of the mating studies on mice, have now extended their MHC work to mother-pup recognition. They considered two strains of mice whose on- ly genetic differences are in the MHC.

First, the scientists observed female mice retrieving pups born to other females of both strains. In general, a foster mother first went after pups with her own MHC complement. This suggests that female mice use MHC-influenced odortypes to rec- ognize their young, says Beauchamp.

The scientists also tested pups by plac- ing them in a Y-shaped maze. One arm held soiled cedar shavings from cages of mice with the same MHC genes as the pups, while the other had shavings from

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the cages of the other strain. The pups preferred to spend time in

one arm over the other and based their choice on the foster mothers raising them. If reared by a female mouse with dissimilar MHC genes, says Beauchamp, pups chose the arm bearing the scent of the female’s strain over the one smelling of the pups’ own strain.

“This is the first study that has moved the MHC out of the mating realm,” notes Peter Hepper of the Queen’s University of Belfast in Northern Ireland.

While the study reveals that mice use MHC-influenced odors to identify each other, it suggests that experience governs whom the pups want to be around. “In

my mind, there’s no MHC preference. It’s a learned thing,” says Hepper.

Beauchamp suggests that human par- ents and their babies also establish a cru- cial bond via smell. A 1998 study, he notes, found that parents could pick out the amniotic fluid that had bathed their infant simply by odor. Some scientists are even exploring whether a mother’s smell soothes an infant.

Beauchamp and his colleagues contin- ue to investigate how the MHC generates an odortype. They contend that the pro- teins encoded by the genes bind to and transport molecules responsible for the body’s scent (SN: 3/13/99, p. 174).

“It’s quite clear that the MHC produces individually distinct odors. The way that it does that has yet to be fully under- stood,” says Hepper. -J. Travis

Device ups hydrogen energy from sunlight Here’s a recipe for a cleaner, healthier

planet: Take some water, add solar ener- gy, extract hydrogen, and use it to power fuel cells for running cars and other ma- chines. Then, collect their water emis- sions and start the procedure again.

One look at the list of ingredients in to- day’s fuel cells, however, shows that this ideal isn’t yet being followed. Because processes that use sunlight to extract hy- drogen remain costly and inefficient, fos- sil fuels still supply the hydrogen in most fuel cells.

Hoping to break the fossil fuel habit, a team of Israeli, German, and Japanese scientists has created a device that boosts the efficiency of solar-powered hydrogen extraction by 50 percent.

The group placed a photovoltaic cell on top of two flat, finger-long electrodes. The combination “is very efficient in con- verting solar energy [into an electric cur- rent] but also provides nearly the ideal voltage for splitting water” into hydrogen and oxygen, says team leader Stuart Licht of the Technion in Haifa, Israel. A water molecule splits, or undergoes elec- trolysis, at only 1.23 volts.

Licht and his colleagues describe their device in the Sept. 14 JOURNAL OF PHYSICAL CHEMISTRY B. The gadget converts sun- light t o an electrolysis current with 18.3 percent efficiency. In turn, the current creates hydrogen gas as it passes through acidic water.

The device is “showing the pathway to- wards higher efficiencies for direct solar- to-hydrogen production,” comments John A. Turner of the National Renewable Energy Laboratory (NREL) in Golden, Colo. The newly achieved efficiency may already be high enough for commercial hydrogen generators to be feasible. “That still needs to be figured out,” Turner says.

In 1998, he and Oscar Khaselev, then also of NREL, demonstrated a novel ap- paratus for solar-to-hydrogen conversion (SN: 4/18/98, p. 246). To achieve unprece-

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dented efficiency, the device used multi- ple layers of semiconductor materials. The researchers arranged the layers t o form two active regions, or junctions, that would absorb solar photons that dislodge electrons. Some of the less ener- getic photons weren’t captured in the first junction but passed to the second, where they generated more current.

The design gained an energy advantage by combining solar electricity and water splitting into one unit. Their cell’s 12.4 percent efficiency-nearly twice that of any previous solar-to-hydrogen de- vice-has held as the record until now.

Licht and his colleagues have im- proved upon that pioneering effort in several crucial ways. In one sense, the NREL device was all wet: It had to be completely immersed in water to oper- ate. That feature forced the researchers to select semiconductors that wouldn’t break down in solution.

By keeping their stack of semiconduc- tor layers high and dry, Licht and his group were free to optimize them for both converting sunlight to electricity and water splitting. Their design permits a low electrolysis current, which also re- duces energy waste.

Licht and his coworkers say that be- sides besting the solar-to-hydrogen con- version record, their work opens the way to efficiencies not considered possible be- fore. Using measured photoelectric effi- ciencies of seven semiconductor combi- nations not yet tested in hydrogen generation, they predict maximum solar- to-hydrogen conversion efficiencies of up to 31 percent.

Thermodynamics theory says the max- imum could range above 40 percent for a two-junction converter, but no one has previously predicted better than 24 per- cent performance for practical devices, Turner says. Experimentally achieving the new prediction “would be an accom- plishment indeed!” he adds. -P Weiss

SEPTEMBER 16,2000