Highlights in PNAS

New for December 23, 2020

Anthropology

Food trade with South Asia revealed by Near East food remains

Long-distance trade has had a significant impact on cuisines worldwide, and although long-distance food trade is well-documented since the Roman era, the preceding history of food trade and subsequent cuisine evolution in Europe and Asia is unclear. Ashley Scott et al. analyzed remains of food preserved in the dental calculus of 16 people who lived in the Southern Levant during the 2nd millennium BCE to ascertain their diet. The authors found expected staple foods such as cereals, sesame, and dates, and also evidence of exotic foods including soybean, banana, and turmeric. The results suggest that sesame had become established as a crop in the region by the 2nd millennium BCE and that bananas, for which little archaeological evidence is preserved, had made an appearance in the region. In the case of turmeric, the results push the earliest known appearance in the Mediterranean back by centuries; in the case of soybeans, the earliest known appearance is pushed back by millennia. According to the authors, the results suggest that a robust trade network had developed between the Near East and South Asia by the 2nd millennium BCE and offer physical evidence to support textual and other lines of evidence used to reconstruct past diets and agricultural expansions in prehistory. — P.G.

"Exotic foods reveal contact between South Asia and the Near East during the second millennium BCE," by Ashley Scott, et al.
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Applied Physical Sciences

Elucidating how ice transitions to a ferroelectric state

Protons in ordinary bulk ice are typically disordered down to the lowest temperatures, but pure crystalline ice is thermodynamically expected to enter a ferroelectric (FE) state in which its protons occupy bond sites in a unique order. This proton-ordered FE phase is only achieved through metal hydroxide dopants such as KOH, and the mechanisms of this transition are not completely clear. In order to initially understand the FE ordering mechanisms of pure ice, J. Lasave et al. developed a bare-bones lattice model of ice that includes dipole–dipole interactions and only configurations which exactly satisfy the ice rule. They applied Monte Carlo strategies that allowed simulating large samples at low temperatures in this lattice model, and indeed they found, in full equilibrium, a first-order phase transition between proton-disordered, defect-free ice and low-temperature FE proton-ordered ice. Once inserted in the same model, dopants mimicking KOH were found to nucleate the transition by generating FE hydrogen-bond strings growing inside the proton-disordered bulk structures. The resulting kinetics qualitatively reproduce aspects of real KOH-doped ice, such as the dependence of the FE fraction on dopant concentration, quenching temperature, and the time evolution of the order parameter. The researchers conclude that their lattice model is able to shed light on the transition of ice into and out of its ferroelectric phase. — S.R.

"Proton strings and rings in atypical nucleation of ferroelectricity in ice," by J. Lasave, S. Koval, A. Laio, and E. Tosatti
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Environmental Sciences

Reducing lead bioavailability in soil

Childhood exposure to lead can lead to long-term adverse health effects. A significant source of lead exposure in children is contaminated soil and dust. Because of the expense and complexity of removing and replacing contaminated soil, Ranju Karna et al. examined the feasibility of reducing the bioavailability of soil-borne lead by altering its physiochemical properties in situ. The authors converted soil lead into plumbojarosite, an insoluble lead–iron hydroxysulfate mineral, by addition of iron sulfate and sulfuric acid. X-ray absorption spectra of contaminated soils before and after treatment indicated that more than 90% of soil lead was converted to plumbojarosite after 8 hours of treatment. Lead remained in plumbojarosite after post-treatment neutralization of soil pH. To measure the relative bioavailability of soil lead, the authors fed mice diets containing untreated or treated lead-contaminated soils. Mice exposed to treated soils had at least 90% lower lead concentrations in blood and bone than mice exposed to untreated soils. According to the authors, in situ plumbojarosite formation could be a cost-effective method for remediating soil lead contamination and preventing exposure in children. — B.D.

"Bioavailable soil Pb minimized by in situ transformation to plumbojarosite," by Ranju R. Karna, et al.
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Environmental Sciences

Forced labor in global fisheries

Forced labor in fisheries around the world has been documented, but the extent to which it occurs remains unclear. Gavin McDonald et al. combined machine learning with satellite-monitoring data from 2012 to 2018 for more than 16,000 industrial fishing vessels, including long-liners, trawlers, and squid-jiggers. The authors also reviewed investigative reports published between 2015 and 2019 by Greenpeace, the Environmental Justice Foundation, Oceana, and The New York Times about instances of forced labor case accounts detailing observable vessel behavior from vessel-monitoring data. Additionally, the authors consulted experts from six nonprofit organizations about observable vessel behavior to identify suspected forced labor. Compared with vessels not using forced labor, long-liners and trawlers at a high risk of forced labor traveled further from ports and shores, fished more hours per day, and had fewer voyages but longer voyage durations. Between 57,000 and 100,000 crewmembers working on 2,300–4,200 high-risk vessels potentially represented forced labor. Fisheries with the largest number of high-risk vessels were Chinese squid-jiggers and Chinese, Japanese, South Korean, and Taiwanese long-liners. High-risk vessels predominantly visited Africa, Asia, and South America. The findings suggest that forced labor is widespread in global fisheries, according to the authors. — M.S.

"Satellites can reveal global extent of forced labor in the world’s fishing fleet," by Gavin G. McDonald, et al.
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Genetics

Sex determination in pigs

Although the sex-determining region of the Y chromosome, or SRY, is thought to regulate development of male mammals, the mechanism of the region is unknown, as is whether other regions of the chromosome also contribute to sex characteristics. Stefanie Kurtz et al. utilized CRISPR-Cas gene editing to knock out a central group within SRY in pig genomes, causing downstream mutation effects. The knockout resulted in pigs that were genetically male, with a Y chromosome, but physiologically female; although, by 9 months of age the sex organs were significantly smaller in the gene-edited pigs than in control females of a similar age, leading to sterility in gene-edited pigs. Incomplete female genitalia development in the XY pigs, the researchers write, demonstrates the key role of a second X chromosome in normal development. The results provide a possible alternative to surgical castration in commercial pig production to prevent boar taint and, owing to the genetic, physiological, and anatomical similarities between pigs and humans, provide a large animal model for investigating mammalian sex determination and sex development disorders. According to the authors, the results highlight the role of the central group of the SRY in male sex determination in pigs. — P.G.

"Knockout of the HMG domain of the porcine SRY gene causes sex reversal in gene-edited pigs," by Stefanie Kurtz, et al.
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Neuroscience

Signaling protein regenerates taste cells, maintains taste tissue homeostasis in mice

Like all cells, the 50 to 100 taste cells in the human taste bud age and lose their integrity over time. First shown more than a century ago, signals from distant, gustatory ganglion neurons tell adult taste stem cells when to differentiate and replenish taste buds; however, researchers have yet to determine exactly which neuron-supplied factors mediate the production of differentiated taste cells. Taking advantage of a recently published RNAseq data set of mouse gustatory neurons, Xiaoli Lin et al. use data mining and in situ hybridization to reveal that R-spondin-2—a member of the R-spondin family of proteins involved in Wnt signaling and stem cell differentiation—is predominantly expressed in gustatory neurons. To support their findings, the authors use an in vivo mouse model to demonstrate that exogenous R-spondin promotes taste stem cell differentiation and taste tissue homeostasis in the absence of the neuronal signaling and that R-spondin is required to produce taste cells in ex vivo organoid cultures. The study argues that R-spondin-2 may be the unknown neuron-supplied factor that maintains taste cells throughout the human lifespan, as well as a potential treatment for patients who lose their sense of taste. — T.J.

"R-spondin substitutes for neuronal input for taste cell regeneration in adult mice," by Xiaoli Lin
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Some of the highlights have previously appeared on the PNAS media tipsheet. The articles in PNAS report original research by independent authors and do not necessarily represent the view of the National Academy of Sciences.

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