American Scientist

In this roundup, associate editor Katie L. Burke summarizes notable recent developments in scientific research, selected from reports compiled in Sigma Xi's free electronic newsletter.

Elephant Shark Is a Living Fossil

Geneticists have sequenced the DNA of the elephant shark, the first member of the shark family to be decoded. Cartilaginous fishes—such as sharks, rays, and skates—diverged from bony vertebrates 450 million years ago. The new study revealed a family of genes that may help explain that transition. Zebrafish with these genes knocked out experienced reduced bone formation, a finding that could aid research on bone-related diseases. More surprising, the elephant shark lacked the CD4 genes related to immune function, including ones considered vital for defense against infections and for preventing autoimmune disorders. Nevertheless, the elephant shark has a robust immune system. Its genome also exhibits the slowest rate of evolution among those vertebrates whose genomes have been sequenced, trumping even the so-called living fossil, the coelacanth. That finding makes the elephant shark’s DNA a valuable resource for understanding the common ancestor to all jawed vertebrates.

Venkatesh, B., et al. Elephant shark genome provides unique insights into gnathostome evolution. Nature doi:10.1038/nature12826 (Published online January 8)

Earthquake Lights Explained?

In 2007 in Lima, Peru, a surveillance camera recorded mysterious flashes of light just as a magnitude 8.0 tremor struck, providing the first definitive documentation of a long-rumored phenomenon called earthquake lights. Now a study of 65 locations where such lights have been reported shows that they occur almost exclusively on steep rifts, where the Earth’s crust is pulled apart. The authors say their finding supports the p-hole theory, proposed by physicist Friedemann Freund of the SETI Institute, for the origin of earthquake lights. According to this theory, oxygen atoms bound up in minerals are sometimes short an electron. As rocks grind together during an earthquake, these bonds are broken, releasing positive charges, or p-holes. These electric charges may be channeled up the near-vertical faults of rift zones, creating a glow when they reach the surface and interact with Earth’s atmosphere. But an online Nature news report describing this new study sparked a heated debate among seismologists about the validity of the p-hole theory.

Thériault, R., F. St-Laurent, F. T. Freund, and J. S. Derr. Prevalence of earthquake lights associated with rift environments. Seismological Research Letters 85:159 (January 2)

Deformed Frog Hotspots

A 10-year study of frogs and toads in U.S. National Wildlife Refuges shows that, overall, fewer than 2 percent of the animals have deformities such as missing or shortened limbs—but in some hotspots, including locations in the Mississippi River Valley and eastern Alaska, up to 40 percent exhibit deformities. It is unknown whether the deformities are caused by the presence of a pollutant or by a natural process. The researchers examined more than 68,000 amphibians in 152 nature refuges between 2000 and 2009. The highly variable incidence of abnormalities in different places suggests a local-scale cause. However, the frequency of malformation varied episodically from year to year, a pattern often associated with natural dynamics such as parasite epidemics. The authors have made their data freely available online so that other scientists can search for more answers.

Reeves, M. K., et al. Localized hotspots drive continental geography of abnormal amphibians on U.S. Wildlife Refuges. PLoS One doi:0.1371/journal.pone.0077467 (November 18)

Solar Giant Cells Substantiated

As heat moves around the Sun through convection, it flows in huge, consistent patterns known as granules, which measure about 1,000 kilometers across, and supergranules, which measure about 30,000 kilometers across. For 45 years, solar physicists suspected that the Sun also experienced another type of convection called giant cells, similar to but even larger and slower than the supergranules, but such cells had never been substantiated. Since 2010, NASA’s orbiting Solar Dynamics Observatory has been imaging the Sun continuously, providing the data needed to track the motion of supergranules and detect the giant convective cells, which span some 200,000 kilometers and move at about 29 kilometers per hour. The finding could help predict the movements of solar magnetic fields, which would aid the forecasting of solar storms.

Hathaway, D. H., L. Upton, and O. Colegrove. Giant convection cells found on the Sun. Science 342:1217 (December 6)

New Hominid Genetics

Two recent genetic discoveries are complicating the story of human evolution. Mitochondrial DNA analysis of a 130,000-year-old toe bone from Denisova Cave in China revealed that it was from a Neandertal, not a Denisovan as expected. By comparing the new Neandertal genome to those of Denisovan and modern humans, the researchers substantiated that Neandertals and Denisovans diverged from one another after their common ancestor diverged from modern humans. The comparison also showed that the three human lineages interbred multiple times after they diverged, although these events were rare. Meanwhile, a femur found in Spain was thought to belong to a Neandertal, but upon analysis of the 400,000-year-old DNA, it turned out to match more closely Denisovan DNA. The Spanish fossil could represent a common ancestor of Neandertals and Denisovans; it could be from a different hominid lineage; or it could force anthropologists to rethink where Denisovans lived, since they were previously thought to have inhabited Asia, not Europe.

Prüfer, K., et al. The complete genome sequence of a Neanderthal from the Altai Mountains. Nature doi:10.1038/nature12886 (Published online December 18)

Meyer, M., et al. A mitochondrial genome sequence of a hominin from Sima de los Huesos. Nature doi:10.1038/nature12788 (Published online December 4)