American Scientist

In this roundup, digital features editor Katie L. Burke summarizes notable recent developments in scientific research, selected from reports compiled in the free electronic newsletter Sigma Xi SmartBrief.

Black Holes at Galaxy’s Center

The detection of x-rays from binary black holes at the center of our galaxy confirms a fundamental theory that thousands of smaller black holes surround the Milky Way’s central supermassive black hole, Sagittarius A*. Astronomers have searched before for bursts of x-rays from Sagittarius A*, but they are strong enough to detect only once every 100 to 1,000 years. Instead, researchers looked for fainter, steadier x-rays emitted by stellar-mass binary black holes. They found 12 binary black holes within three light years of Sagittarius A*. Extrapolating from what is known about the fraction of black holes that will pair up with low-mass stars, the researchers calculated how many isolated black holes there are in the Milky Way’s center. They estimate that there are 300 to 500 low-mass binary black holes (those in orbit around ordinary stars) and about 10,000 isolated black holes in this region of the galaxy. This study offers insights into the dynamic evolution of the center of the Milky Way. It also opens up new potential for studying gravitational waves, ripples in space-time, from collisions between the black holes in the center of the galaxy.

Hailey, C. J., et al. A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy. Nature 556:70–73 (April 5)

Frog Disease Evolution

Studies of a fungal disease in frogs caused by the chytrid fungus Batrachochytrium dendrobatidis provide insights into the global amphibian decline and also offer hope from animals that are developing resistance. In one study, frogs from a wild population decimated by the disease and descendents of that population before the fungus was introduced, which had been bred in captivity, were both infected with strains of the pathogen—one strain from 2004 when it was first found in the region and another recent strain. The frogs from the wild population fought off the infection better than the captive-bred frogs did, but did so equally well for both strains, showing that the fungus was not weakening its attack so much as the frogs were fortifying their defenses. This rebound is only happening in a handful of frog species in a few places; most susceptible frogs are still at risk. Further study of this development of resistance could inform captive breeding methods and management strategies to encourage contexts under which resistance is likely to arise. Another study unraveled the fungus’s evolution and tracked its spread alongside the global amphibian trade, pinpointing its origin to the Korean peninsula and more broadly to a region of East Asia with high genetic diversity in the pathogen. This genomics study lays the groundwork for research that could identify and prevent other harmful disease introductions and advance models of worldwide pandemics of conservation importance.

O’Hanlon, S. J., et al. Recent Asian origin of chytrid fungi causing global amphibian declines. Science 360(6389):621–627 (May 11)
Voyles, J., et al. Shifts in disease dynamics in a tropical amphibian assemblage are not due to pathogen attenuation. Science 359(6383):1517–1519 (March 30)

Earliest Middle Stone Age Tools

Archaeological finds from Kenya’s Rift Valley are the earliest evidence of certain tool innovations and social behaviors among hominins. For hundreds of thousands of years, hominins there were using stone axes that changed very little. Then, 500,000 years ago, a period of tectonic activity and variable climate caused a disruption in the archaeological record at the site. When the record resumed, 320,000 years ago, the lifestyles of the hominins there had changed markedly. Resources were different and more scarce. New, more variable tools showed up: small blades made of obsidian. Such tools are associated with the beginning of the Middle Stone Age and with our species, Homo sapiens. The stone axes from earlier in the record had originated from rocks within a 5-kilometer radius of the site. By contrast, the nearest sites with the dark glass used to make the new tools were up to 95 kilometers away, so people must have either traveled long distances or traded with other groups to acquire this material. Along with the novel blades, the researchers found chiseled red and black rocks that suggest pigment use—a find associated with early symbolic behavior. These finds predate the earliest known fossils of Homo sapiens, which are about 300,000 years old and were found in Morocco. No fossils indicate which hominin made the tools found at the Kenya site, but these behaviors are associated with early Homo sapiens.

Science 360(6384):86–98 (April 6) [Full list of papers]

Gene That Determines Turtle Sex

In turtles, as in many reptiles, sex is determined by the temperature at which eggs are incubated, and a study identifies the gene responsible for this switch. This gene, KDM6B, was already thought to be involved in the process, so the next step was to turn off the gene in incubating eggs to see what happened. Eggs kept at a temperature of 26 degrees Celsius normally develop into males, but with the gene knocked out, 80 percent developed into females, which normally develop at a higher temperature, 32 degrees Celsius. As the climate warms, conservation biologists have been concerned that turtle populations are beginning to skew toward females, so understanding how sex determination works will be important to managing reptile populations in a warming climate. The next step to understanding this process is to figure out how the temperature cues the gene to turn on or off.

Ge, C., et al. The histone demethylase KDM6B regulates temperature-dependent sex determination in a turtle species. Science 360(6389):645–648 (May 11)