MARCH 10, 2020

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www.pnas.org/cgi/doi/10.1073/iti1020117 PNAS | March 10, 2020 | vol. 117 | no. 10 | 5081–5083

Animal lacking a mitochondrial genome

Aerobic respiration is a hallmark of eukaryotes but has been lost in several single-celled eukaryotic lineages adapted to low-oxygen environments. Whether certain animals also rely exclusively on anaerobic metabolism and have mitochondrion- related organelles (MROs) has been debated. Through deep sequencing and microscopy, Dorothée Huchon et al. (pp. 5358–5363) found that the parasitic cnidarian Henneguya salminicola, which can live in low-oxygen environments in hosts, has lost its mitochondrial genome, the capacity for aero-bic respiration, and nearly all nuclear genes involved in transcription and replication of the mitochondrial genome. In addition, the anaerobic MROs of this species are unusual due to the presence of cristae, or folds in the mitochondrial inner membrane. The findings show that aerobic respiration, a crucial met-abolic pathway, is absent in at least one multicellular animal, not just in certain single-celled eukaryotes. The findings could shed light on the evolutionary transition from aerobic metabolism to exclusively anaerobic metabolism. Moreover, the results might lead to strategies to combat the substantial negative economic impact of parasitic cnidarians on fisheries and aquaculture, according to the authors. — J.W.

Timing and location of HIV-1 core uncoating Before HIV-1 viral DNA can integrate into host chro-mosomes, its retroviral core needs to be disassem-bled, a process known as uncoating. Where and when uncoating occurs remain unclear, although it is thought to occur in the cytoplasm in coordination with reverse transcription. Ryan Burdick et al. (pp. 5486–5493) developed methods to investigate HIV-1 uncoating by directly labeling the capsid protein with GFP and using live-cell microscopy to track infectious viral cores in living infected cells. The authors found that infectious viral cores in the nuclei of infected cells are intact or nearly intact, and complete reverse transcription after nuclear import. Viral complexes were found to uncoat within ∼1.5 µm of the sites of genomic integration and less than 1.5 hours before integration. The mechanism of entry of viral cores was found to involve interactions at the nuclear envelope

involving recruitment of host protein cleavage and polyadenylation specificity factor 6, a nuclear host factor that interacts with the viral core. The authors conclude that HIV-1 uncoating occurs in the nucleus near the sites of chromosomal integration. Addition-ally, the results carry implications for understanding HIV-1 infection, including how the virus is imported into the nucleus, integrates into the host genome, and evades host innate immunity, according to the authors. — S.R.

Analyzing trade-offs of bacterial CRISPR-based immunity Bacteria and archaea can use the CRISPR-Cas sys-tem to recall past viral infections and mount targeted immune defenses against specific viral threats. Such bacterial immune memory depends on the number of spacers, or snippets of viral DNA stored in CRISPR

Light microscope image of spores of H. salminicola from Chinook salmon.

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cassettes and used by CRISPR-Cas to identify and clear recurring viral threats. Serena Bradde et al. (pp. 5144–5151) developed a statistical method to quan-tify trade-offs involved in the CRISPR-based adaptive immune system. Prokaryotes have been found to have a few dozen to a few hundred spacers, but given the potentially large size of bacterial genomes, it is unclear why relatively little genomic space is devoted to antiviral adaptive immune defense. The authors found that the optimal size of the bacterial immune repertoire depends on the interplay between phage diversity and the number of Cas complexes present. With a very diverse viral landscape, as expected in nature, the size of the immune repertoire stored in CRISPR cassettes is constrained by the number of Cas complexes that the bacteria can mobilize to mount a functional immune response. The model predicted that the optimal range of expressed spacers would lie between 10 and 100, consistent with observations. Thus, the study identifies a trade-off between bacte-rial immune memory and the effectiveness of antiviral response, according to the authors. — S.R.

Pattern analysis in forensic identificationPhotographic pattern analysis, in which features on a perpetrator’s face, hands, or clothing, obtained from crime scene photographs, are matched to those of a suspect, has been widely used in criminal cases over the past 50 years. However, the reliability and reproducibility of such analyses have not been rig-orously tested. Sophie Nightingale and Hany Farid (pp. 5176–5183) analyzed a technique for matching purportedly distinct patterns of wear and tear on denim jeans. The authors quantified the differences

in such patterns between different pairs of jeans, and between different images of the same pair of jeans taken under varying conditions. From the distribu-tions of these differences, the authors calculated the probability of correctly matching the same seams—true positive rate—and of incorrectly matching differ-ent seams—false positive rate. Differences between multiple images of the same pair were comparable to differences between different pairs. As a result, even at a relatively high false positive rate of 1 in 1,000, true positive rates were less than 40%. The results suggest that identification based on denim jean pat-terns should be used with extreme caution and that other forensic photographic pattern analyses should be validated using similarly rigorous studies, accord-ing to the authors. — B.D.

Insecticides and visual motion detectionNeonicotinoids are a widely used class of insecticides implicated in declines of nontarget insect species, such as bees. The recently developed sulfoximine

insecticides act on the same target as the neonico-tinoids, but can affect insects resistant to the latter. The sublethal effects of neonicotinoids and sulfoxi-mines remain unclear, however, preventing an under-standing of the full risks of these compounds. Rachel Parkinson et al. (pp. 5510–5515) investigated the effects of a neonicotinoid, imidacloprid, and sulfoxa-flor, a sulfoximine, on collision avoidance behavior in locusts. Locusts exposed to sublethal doses of either insecticide were presented with a visual loom-ing stimulus. Locusts exposed to sublethal doses of sulfoxaflor exhibited a normal jumping escape response to the stimulus, whereas those exposed to imidacloprid did not exhibit this response, despite both insecticides having similar lethality. The authors identified several distinct populations of neurons responsive to the looming stimulus and found significantly reduced spontaneous firing rates and neural habituation relative to controls in imidacloprid-treated but not sulfoxaflor-treated locusts. The results suggest that neonicotinoids, but not sulfoximines, may inhibit tuning of visual sensory circuits at sublethal doses, and that the two insecti-cide classes might exert their effects through differ-ent mechanisms, according to the authors. — B.D.

5082 | www.pnas.org/cgi/doi/10.1073/iti1020117 In this issue

Migratory locust (Locusta migratoria).

Representative examples of wear and tear on the seam of denim jeans.

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www.pnas.org/cgi/doi/10.1073/iti1020117 PNAS | March 10, 2020 | vol. 117 | no. 10 | 5083

Light-driven locomotion of liquid crystal gels

The locomotion of soft-bodied aquatic organisms, such as sea slugs and snails, has inspired efforts to develop soft-bodied robots that can operate under water. Difficulties in precisely controlling shape changes and an inability to overcome under-water drag forces hinder the use of existing mate-rials for such robots. Hamed Shahsavan et al. (pp. 5125–5133) developed liquid crystalline gels capa-ble of underwater photothermal actuation. The gels were designed so that light-driven heating would cause a change in the liquid crystal molecular align-ment, leading to macroscopic shape changes that could drive locomotion. The material exhibited a

sharp ordered-to-disordered phase transition at temperatures in the desired range—above ambi-ent temperature but below the boiling point of water—thus causing the material to bend with increasing temperature between approximately 30–60 °C. Laser illumination of the gels in water induced localized bending of the gel at the point of illumination, and pulsed illumination led to oscil-lation between bent and relaxed conformations. By controlling the position and timing of illumi-nation, the authors were able to induce various types of underwater locomotion, including crawl-ing, walking, jumping, and swimming. These liquid crystalline gels could be used to design light-fueled aquatic soft robots, according to the authors. — B.D.

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