American Scientist

In this roundup, Katie Burke summarizes notable recent items about scientific research, selected from news reports compiled in the free electronic newsletter Sigma Xi SmartBrief.

Dogs Origins Decoded

A genetic study of 1,872 Old World dogs and 347 New World dogs showed that American purebreds, such as Chihuahuas, are more closely related to Asian dogs than to European dogs. The finding indicates that ancient humans brought dogs with them when they crossed the land bridge from Asia to North America during the Pleistocene; it is corroborated by archaeological and historic records of dogs predating European settlement. The new study used mitochondrial DNA, which is passed down maternally and is strongly conserved between generations. A related genetic examination of free-ranging and street dogs in the Americas showed that European breeds largely comprise the genetics of American dogs, as these breeds became more common. A few exceptions exist, including an indigenous American dog with dingo-like characteristics, called the Carolina dog, which lives wild in remote areas of the southeastern United States.

Van Asch, B., et al. Pre-Columbian origins of Native American dog breeds, with only limited replacement by European dogs, confirmed by mtDNA analysis. Proceedings of the Royal Society B (Published online July 10)

Poached Animal Forensics

Concentrations of radioactive carbon-14 in the atmosphere almost doubled between 1952 and 1962 because of nuclear weapons testing and then dropped when testing was banned. A half-century later, this pattern, known as the bomb-curve, turns out to be useful for precisely measuring the ages of biological samples. Carbon-14 in the atmosphere is incorporated into plants through photosynthesis, and then into animals that eat either plants or other organisms that have eaten plants. To explore how accurately tissues from organisms could be dated using the bomb-curve, carbon-14 concentrations in 29 animal and plant tissues of known age were compared to their concentrations of other carbon isotopes. Samples harvested after 1955 were dated to an accuracy of 0.3 to 1.3 years, depending on tissue type. This method has applications in ecology and environmental policy. Most notably, it can identify the time of harvest for ivory and other potentially poached animal parts. Often, age of these items establishes whether they can be legally traded.

Uno, K. T., et al. Bomb-curve radiocarbon measurement of recent biologic tissues and applications to wildlife forensics and stable isotope (paleo)ecology. Proceedings of the National Academy of Sciences of the U.S.A. (Published online July 1)

Faux Planets Around Infant Stars

Astronomers thought they were indirectly watching planets being born in debris disks around certain young stars: Some contain asymmetrical rings that have generally been attributed to the gravitational effects of one or more exoplanets. But new computer models show that interactions between gas and dust in the disks could give rise to similar patterns, even if no planets are present. The finding calls into question some planets identified within the disks surrounding nearby stars. It also underscores the need for a better model of how planet formation really occurs within these dusty disks.

Lyra, W., and M. Kuchner. Formation of sharp eccentric rings in debris disks with gas but without planets. Nature 499:184 (July 10)

Crohn’s Potential Viral Cause

The cause of Crohn’s disease is unknown, but a recent study offers intriguing evidence. This disease leads to diarrhea and stomach pain due to chronic inflammation in the intestinal tract; a severe version called advanced ileocecal Crohn’s disease also produces lesions in the part of the nervous system governing gastrointestinal function. Previous work showed that the disease is associated with variations in two genes called NOD2 and ATG16L1. Because these genes regulate immune response to viruses with single-stranded RNA (ssRNA), researchers hypothesized that Crohn’s could be associated with such a virus. The new study bolsters the case for a particular virus, human enterovirus species B (HEV-B), associated with mild gastrointestinal inflammation. Of the 24 patients with Crohn’s disease, all had the NOD2 and ATG16L1 genetic variations, and all showed more elevated levels of ssRNA viruses known to cause gastrointestinal symptoms than a control group. Patients who had advanced ileocecal Crohn’s disease all exhibited HEV-B infection in particular. Although based on a small sample, the findings strengthen the case for a viral cause.

Nyström, N., et al. Human enterovirus species B in ileocecal Crohn’s disease. Clinical and Translational Gastroenterology (Published online June 27)

Photosynthesis Precursor

Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The critical component of the photosynthetic system is the water-oxidizing complex, made up of manganese atoms and a calcium atom. This system splits water molecules and delivers some of their electrons to other molecules that help build up carbohydrates. Given the central role of manganese, researchers hypothesized that the anaerobic missing link in photosynthesis evolution might be a process called manganese-oxidizing photosynthesis. Earth scientists recently studied 2.415 billion-year-old rocks from South Africa rich in manganese oxide. The rock’s physical and chemical properties indicate that the ancestor of cyanobacteria used manganese-oxidizing photosynthesis. Next up the researchers hope to mutate cyanobacteria to re-create their long-lost predecessor for direct study.

Johnson, J. E., et al. Manganese-oxidizing photosynthesis before the rise of cyanobacteria. Proceedings of the National Academy of Sciences of the U.S.A. 110:11238 (July 9)