Highlights in PNAS

New for February 13, 2019

Anthropology, Statistics

Origin of European megaliths

Around 35,000 megaliths—structures or monuments constructed from large stones—exist throughout Europe, most of which date from the Neolithic and Copper ages and are concentrated in coastal areas. Whether the use of megaliths spread through Europe from a single origin or developed independently in different locations remains unclear. B. Schulz Paulsson analyzed more than 2,400 radiocarbon dates from megalithic, premegalithic, and contemporaneous nonmegalithic sites throughout Europe. The earliest megalithic graves emerged during a period of 200–300 years in the second half of the 5th millennium BC, in northwest France, the Atlantic coast of the Iberian Peninsula, and the Mediterranean. Premegalithic monumental constructions were found only in northwest France. Thousands of megalithic passage graves were built in the Iberian Peninsula, the British Isles, and France during the first half of the 4th millennium BC and in Scandinavia during the second half of the millennium. The distribution of the graves suggests diffusion of the megalithic tradition via sea routes. According to the author, the results support the view that megalith-building originated in northwest France and diffused via sea routes in three main phases over the subsequent millennium, and suggest that the maritime skills and technology of megalithic societies may have been more advanced than previously believed. — B.D.

"Radiocarbon dates and Bayesian modeling support maritime diffusion model for megaliths in Europe," by B. Schulz Paulsson
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Earth, Atmospheric, and Planetary Sciences

Organism mobility in the Paleoproterozoic Era

The oldest trace fossils are approximately 635 million years old, and records show that organisms from that period could move on the surface of and within sediment. Using chemical analyses, 3D tomographic reconstructions, and scanning electron microscopy, Abderrazak El Albani et al. examined string-shaped, 6- to 170-mm-long pyritized structures preserved in shale from Gabon’s Franceville basin. Chemical conditions, such as high levels of sulphate, within the basin allowed the rapid pyritization of the structures, which were composed of organic matter and were likely mucus strands left behind by organisms moving through mud. The size and complexity of the fossilized strands suggest that they were created by the movements of aggregated eukaryotes that formed multicellular organisms. Mobility in these organisms may have stemmed from the need to find food sources. The fossils dated back to 2.1 billion years ago, when Earth was oxygen-rich and harbored ideal conditions for the emergence of complex life forms. Although the Francevillian biota may represent an anomaly, the findings suggest that complex biological processes, such as mobility and multicellularity, had evolved 300 million years earlier than the oldest documented eukaryotes and more than 1.5 billion years before the emergence of mobile animals, according to the authors. — M.S.

"Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago," by Abderrazak El Albani, et al.
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Genetics

Gene drive strategy to alter populations

Gene drives enable desired beneficial traits to outpace competing alleles and propagate in a self-sustaining manner throughout wild populations. One proposed strategy to engineer gene drives would adapt naturally occurring genetic elements found in eukaryotes and bacteria called toxin–antitoxin (TA) systems, in which a tightly linked pair of genes encode a trans-acting toxin and cis-acting antidote. Georg Oberhofer, Tobin Ivy, et al. describe a synthetic genetic element that, in an analogous manner to TA systems, works to rapidly alter the genetic composition of a population. Dubbed CleaveR, for Cleave and Rescue, the technique uses two components: a Cleaver consisting of Cas9 and guide RNAs designed to disrupt the function of an essential gene, as well as a recoded copy of the gene resistant to subsequent cleavage by CleaveR that restores essential function. Because progeny that lack CleaveR perish, the frequency of genes linked to Cleaver, including desired beneficial traits, increases rapidly in the population, according to the authors. — T.J.

"Cleave and Rescue, a novel selfish genetic element and general strategy for gene drive," by Georg Oberhofer, Tobin Ivy, and Bruce A. Hay
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New for February 6, 2019

Agricultural Sciences

Protein structure reveals mode of action against western corn rootworm

The Gram-positive soil bacterium Bacillus thuringiensis produces more than 100 insecticidal proteins that target particular pests without harming vertebrates, beneficial insects, or the environment. Genetically modified crops that carry genes for such proteins have reduced pest damage and boosted crop yields, but some insects evolve resistance to B. thuringiensis insecticidal products. Jelena Zaitseva et al. applied a method called single isomorphous replacement with anomalous scattering to the recently discovered Gram-negative insecticidal protein dubbed GNIP1Aa and solved the protein’s crystal structure at 2.5-Å resolution. The authors found that the protein comprises two structurally distinct domains with unique properties that appear to act differently from all other known insecticidal proteins. Isolated from a strain of Chromobacterium piscinae, GNIP1Aa exhibits specific toxicity toward western corn rootworm larvae, a major crop pest that takes a heavy economic toll. According to the authors, GNIP1Aa might represent a candidate for commercial development that can potentially reduce corn crop damage and help slow the emergence of resistance in the western corn rootworm. — T.J.

"Structure–function characterization of an insecticidal protein GNIP1Aa, a member of an MACPF and β-tripod families," by Jelena Zaitseva, et al.
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Ecology

Genetically modified crops and monarch butterfly decline

Recent decreases in the abundance of monarch butterflies and their food plants, milkweeds, have been linked to genetically modified crops and the associated use of herbicides. However, the role of genetically modified crops in milkweed and monarch declines has not been clear, partly because previous monarch population datasets only extend back to 1993. J. Boyle et al. gathered online digitized museum and herbaria records for 1,191 monarch butterfly specimens and 39,510 milkweed specimens collected from 1900 to 2016. Analysis of long-term trends revealed that both monarchs and milkweeds suffered a twofold decline between approximately 1950 and 2016. Moreover, the abundance of the common milkweed, the primary host plant of monarch butterflies, was negatively associated with the number of farms, which declined from 1950 to 2006 as smaller farms consolidated. Taken together, the findings suggest that a decrease in the number of farms may predict common milkweed trends more strongly than herbicide-resistant crops, which were not introduced until 1996. According to the authors, while the findings do not exclude a role for changes in agricultural practices, such as agrochemical use, factors other than genetically modified crops may influence the abundance of monarchs and milkweeds. — J.W.

"Monarch butterfly and milkweed declines substantially predate the use of genetically modified crops," by J. H. Boyle, H. J. Dalgleish, and J. R. Puzey
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Genetics

Zika-resistant mosquitoes

Preventing the spread of Zika virus (ZIKV) will require novel vector control strategies. Gene drive systems can be used to rapidly spread disease resistance genes throughout vector populations, but no genes conferring resistance to ZIKV are known. Anna Buchman, Stephanie Gamez, et al. engineered a synthetic gene to confer ZIKV resistance on mosquitoes. The engineered gene encodes a set of synthetic RNAs designed to inhibit synthesis of various ZIKV proteins via RNA interference. The authors introduced the gene into mosquitoes, which expressed the small RNAs. Mosquitoes homozygous for the resistance gene were immune to infection by multiple ZIKV strains and unable to transmit the virus, whereas heterozygous mosquitoes exhibited significantly lower rates of viral infection, replication, and transmission than wild-type mosquitoes. When ZIKV-infected wild-type mosquitoes fed on ZIKV-susceptible mice, all of the mice died within 8 days. However, nearly all mice fed on by ZIKV-infected heterozygous mosquitoes survived and did not exhibit symptoms of ZIKV infection. The results suggest that the engineered gene could be combined with a gene drive system to reduce the ability of mosquito populations to transmit ZIKV. According to the authors, a similar strategy might help prevent the spread of other vector-borne diseases. — B.D.

"Engineered resistance to Zika virus in transgenic Aedes aegypti expressing a polycistronic cluster of synthetic small RNAs," by Anna Buchman, Stephanie Gamez, et al.
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New for January 30, 2019

Evolution

Protein compositions of dinosaur and bird feathers

Analyzing fossilized dinosaur feathers is key to uncovering how feathers evolved into appendages required for avian flight. Flight feathers are typically constructed of β-keratin proteins with a peptide deletion that provides biomechanical hardness and flexibility. Yanhong Pan et al. used electron microscopy and chemical analyses to compare flight feathers of a chicken to preserved forelimb feathers of the bird-like dinosaur Anchiornis. Whereas Anchiornis feathers lack the biomechanical properties needed for flight, they exhibit some of the necessary molecular structures. Anchiornis feathers are comprised of both α-keratins and β-keratins, which are also found in reptilian tissue and embryonic bird feathers. Although α-keratins, predominantly found in mammals, dominate the dinosaur feathers, β-keratins are essential proteins in the mature flight feathers of modern birds. Compared with modern bird feathers and fossilized feathers of four other dinosaurs from latter geological ages, Anchiornis feathers also contain lower concentrations of sulfur. The findings suggest that although Anchiornis feathers are not suitable for flight, the presence of β-keratins in the feathers’ molecular structure may signify an intermediate stage in the evolution of avian flight feathers, according to the authors. — M.S.

"The molecular evolution of feathers with direct evidence from fossils," by Yanhong Pan, et al.
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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.

[02/19]