EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES
Arctic greening revealed in ancient DNA
Researchers working from inflatable boats on Lake CF8 in the Canadian High Arctic to collect sediment cores for DNA analysis. Image credit: Zach Montes (photographer).
Arctic greening darkens typically snow-covered surfaces at high northern latitudes and can enhance warming. The Last Interglacial Period, approximately 125,000 years ago, is the most recent period in which the Arctic was warmer than at present. Through analysis of plant DNA from Arctic lake sediment, Sarah Crump et al. uncovered ecosystem changes during this ancient warm period, offering potential insights into the effects of the current summer warming. The authors analyzed sediment that was up to 130,000 years old and removed from a small lake on Baffin Island to gauge vegetation response to previous Arctic warming. Next, the authors sequenced plant DNA found in the sediment to reconstruct plant communities at the site. Fossil pollen was used to reconstruct July temperatures through the same period. During the times of peak warmth during the Last Interglacial Period, dwarf birch, a woody shrub, expanded its range 400 km northward relative to current times. Similar patterns of vegetation changes, albeit with different plant communities, are noted in both the Last Interglacial Period and the current Holocene, with woody shrubs becoming established toward the middle of each period. According to the authors, amplified warming from this increased shrub cover may produce similar effects in the future. — T.H.D.
PNAS e2019069118 (2021)
ENGINEERING
Jet fuel from food waste
The production of jet fuel from used grease and fats is limited by supply owing to competition with food grown for consumption. Further, jet fuel production from food waste requires an additional step that lowers yield compared with renewable diesel production. Nabila Huq et al. report a method for converting volatile fatty acids (VFAs) from widely prevalent food waste into sustainable aviation fuel that produces relatively less soot and greenhouse gases. The authors explored the feasibility of using wet waste such as manure, sludge, and food scraps. Due to their water content, these wet wastes have conventionally been converted to methane. The authors recovered molecules from food waste undergoing fermentation. Using a heterogeneous catalyst, the authors converted the VFAs into paraffin, a combustible hydrocarbon used in aviation fuel. By altering the paraffin structure and blending to address viscosity and flashpoint constraints, the authors achieved a 70% blend, which produced 34% less soot than fossil jet fuel. According to the authors, lifecycle analysis revealed that diverting food waste from landfills into VFA production for sustainable aviation fuel could reduce greenhouse gas emissions up to 165%, compared with conventional fossil jet fuels. — T.H.D.
PNAS e2023008118 (2021)
EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES
Greenland Ice Sheet melting over the past million years
Greenland is home to the second largest ice sheet on Earth, potentially posing a major risk under future warming scenarios. Relatively little is known about the history of the Greenland Ice Sheet or the island’s ice-free ecosystems before the Last Interglacial. Andrew Christ et al. analyzed samples of subglacial sediment at the bottom of the Camp Century ice core, frozen under nearly 1.4 km of ice in northwestern Greenland. The authors found that the sediment preserves a unique, multimillion-year-old record of glaciation and vegetation. Enriched stable isotopes in the pore ice suggest that precipitation must have occurred at lower elevations than the present ice-surface elevation, implying the absence of ice sheets. In addition, the isotope ratios in the upper-most sediment were similar to those in subglacial bedrock from central Greenland, indicating that the Greenland Ice Sheet persisted through interglacial periods for much of the Pleistocene. However, there were at least two episodes of warm, ice-free, vegetated conditions—one in the Early Pleistocene and the other during the past 1.1 million years. According to the authors, these insights into the history of the Greenland Ice Sheet could help predict future sea-level rise in response to climate change. — J.W.
PNAS e2021442118 (2021)
ENVIRONMENTAL SCIENCES
US flood risk, hydrology, and race
Flooding events can initiate changes in risk-averse communities such as migration, construction of flood control projects, or uptake of insurance. In contrast, risk-enduring communities tend to live with risk, despite direct experience with flooding events.
Global costs of river flooding are projected to exceed $500 billion by midcentury. Perceptions of and strategies for mitigating flood risk vary within and across communities and reflect complex interactions between hydrologic and socioeconomic factors. James Knighton et al. applied a sociohydrological model to 50 US metropolitan areas to identify patterns in community responses to flood risk. The authors calibrated the model using local data on flood insurance claims and policies, population density, and peak streamflow. Two main behavior types emerged: “risk-enduring” cities, which exhibit low flooding defenses and long memories of past floods, and “risk-averse” cities, which feature high flooding defenses but short memories of past floods. Risk-enduring cities tended to have more predictable local streamflow and smaller dams than risk-averse cities. Risk-enduring cities also tended to have a lower proportion of White residents than risk-averse cities. The results suggest that community responses to flood risk may reflect awareness of local hydrologic conditions. However, the data may also reflect systemic racial inequity in flood exposure and resilience in the United States, according to the authors. — B.D.
PNAS e2016839118 (2021)
NEUROSCIENCE
Cysteinyl leukotriene signaling mediates chronic itching in mice
In this illustration, RNAscope expression of Cysltr2 (white), Nppb (blue), and Hrh1 (green) in DRG along the spinal cord is overlaid on a mouse scratching an itch.
Chronic itch is a debilitating symptom that accompanies skin conditions such as atopic dermatitis (AD) and allergic contact dermatitis. Previous studies have established that inflammatory mediators in the skin stimulate the sensory neurons that produce an itch, but the molecular pathways behind chronic itching remain largely unknown. Tiphaine Voisin et al. examined cysteinyl leukotrienes (CysLTs)—lipid signaling molecules thought to mediate allergic skin reactions—and found that a signaling molecule called LTC4 has a functional role in chronic itching. The authors report that the CysLT receptor 2 (CysLT2R) is highly expressed in dorsal root ganglia (DRG) associated with itching in mice and is expressed more broadly in human DRG neurons. Furthermore, using an AD mouse model, the authors found that the CysLT signaling pathway, specifically the coupling of LTC4 with CysLT2R, produces scratching in mice in a dose-dependent manner, but without corresponding pain behaviors. According to the authors, LTC4 may provide a link between immune cells and sensory neurons in mice and may represent a therapeutic target for chronic itch associated with inflammatory skin pathologies in humans. — T.J.
PNAS e2022087118 (2021)
PSYCHOLOGICAL AND COGNITIVE SCIENCES
False memories can be reversed
Remembering events that never happened has important legal implications, potentially resulting in wrongful accusations or confessions. Relatively little is known about pragmatic strategies for successfully reversing false memories of autobiographical events. Aileen Oeberst et al. demonstrated the effectiveness of two such approaches: source sensitization, which involves alerting participants that their memories could come from external sources, and false memory sensitization, which involves informing individuals that repeatedly being asked to recollect events can produce false memories. Over the course of three repeated interviews over 2 weeks, 52 participants were asked to recollect childhood events, which were verified through questionnaires sent to their parents. Two of the events occurred, and the other two were plausible but never happened. Using suggestion techniques, the interviewers implanted false memories in up to 56% of the participants. Next, the authors attempted to reverse the false memories through source and false memory sensitization. Together, the two approaches returned the incidence of false memories to baseline levels, namely 15 to 23% of participants. False memory rates decreased even further, to 5%, when 38 of the participants were tested 1 year later. According to the authors, the two strategies could be widely implemented in real-world settings and do not require interviewers to know any ground truths. — J.W.
PNAS e2026447118 (2021)
MICROBIOLOGY
Characterizing an equid hepatitis B virus that causes chronic infections
Zebras were found to harbor a new hepatitis B virus species, Equid HBV.
Chronic hepatitis B (CHB) is a major human health problem, but development of drugs against this disease has been hampered by the lack of suitable preclinical animal models. Andrea Rasche, Felix Lehmann, et al. screened 2,917 specimens from equids from 13 countries on 5 continents for hepatitis B virus (HBV), which causes CHB. They characterized a new HBV species (Equid HBV or EqHBV) globally in 3.2% of donkeys and zebras by PCR and found antibodies against EqHBV in 5.4% of donkeys and zebras. Antibodies against an equid hepatitis C virus (HCV) were codetected in 26.5% of donkeys sero-positive for EqHBV. The researchers found several similarities between EqHBV and HBV, including liver tropism, moderate liver damage, and development of chronic infection with characteristics resembling human CHB, but the two viruses differed in receptor usage. In vitro infection studies in primary horse hepatocytes suggested that horses might also be susceptible to EqHBV. Evolutionary analyses suggested that EqHBV originated in Africa several thousand years ago around the same time as donkey domestication. According to the authors, EqHBV naturally infects a variety of equids globally, with similar infection patterns as HBV, and the findings offer opportunities to develop animal models to study CHB and HBV/HCV coinfections. — S.R.
PNAS e2013982118 (2021)
ANTHROPOLOGY
History of migration to the Philippines
The Philippine Islands are situated at a crossroads of human migration, yet the history of the population and colonization of the islands is not fully understood. Maximilian Larena et al. collected around 2.3 million genotypes from 1,028 people representing 115 Philippine indigenous populations, as well as ancient genome sequences from two individuals, around 8,000 years old, from the island of Liangdao in the Taiwan Strait. The results suggest that the Philippines were populated in at least five waves, beginning with Negritos, distantly related to Australian and Papuan populations, and followed by Manobo, Sama, Papuan, and Cordilleran groups. The Cordilleran group diverged from indigenous Taiwanese groups at least 8,000 years ago, predating the arrival of rice paddy agriculture around 2,500 years ago and contradicting a previous theory that language, culture, and agriculture arrived together in the Philippines from Taiwan. The authors also found that less than 1% of individuals exhibited genetic signs from West Eurasia, suggesting a limited effect of Spanish colonization on the genetic character of the Philippines. According to the authors, the results show a multilayered genetic history of the Philippines, which served as a gateway for migration that changed the genetic landscape of the Asia-Pacific region. — P.G.
PNAS e2026132118 (2021)
EVOLUTION
Massive study suggests chimp populations mixed more recently than previously reported
Findings cast doubt on whether four distinct chimp populations in Africa are true subspecies. Image credit: ScienceSource/Jean-Michel Labat.
Posted on March 19, 2021
David Adam
A recent study in Communications Biology reports results from the largest such survey of chimpanzee genetics so far. It suggests that animals from the different groups have mixed more recently than many researchers in the field previously believed. The findings cast doubt on whether the four distinct chimp populations are true subspecies; their DNA suggests that despite the physical barriers, there’s been gene flow among the populations in the last few thousand years. Continue Reading⇒