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Highlights in PNAS
New for April 22, 2020
Ecology
Contextual habitat intactness for biodiversity. Dark shades
indicate high intactness, and colors indicate levels of human
influence (Blue = wilderness, Green = low disturbance;
Orange = modified; Pink = highly modified).
Conserving biodiverse habitats
Natural habitat loss contributes to biodiversity loss, and by some estimates nearly half a million species are in danger of extinction over the coming decades. However, it is unclear whether protecting large and intact natural systems, such as wilderness, is as important as protecting small areas of highly threatened habitats within degraded environments. To identify high-value biodiversity habitats, Karel Mokany et al. combined global data on habitat conditions with models of species assemblage turnover that included data for more than 400,000 species. The analysis revealed that large, intact habitats are important to protect, but fragmented and degraded habitats can prove essential to conserving biodiversity because they may support species that have lost much of their habitat elsewhere through anthropogenic modification. The authors also found that 18.6% of high-value biodiversity habitats are located within protected areas. Furthermore, only 6% of high-value biodiversity habitats in regions subjected to high levels of human modification are protected. The largest areas of unprotected high-value biodiversity habitat are within wilderness and locations that experience low disturbance from humans. Both intact and threatened habitats may be important to protect, given the importance of regional and environmental factors to biodiversity conservation, according to the authors. — M.S.
"Reconciling global priorities for conserving biodiversity habitat," by Karel Mokany, et al.
Article
Evolution; Earth, Atmospheric, and Planetary Sciences
Artist's conception of a metriorhynchid, an extinct crocodile relative.
Shapes represent evolution of the inner ear (bony labyrinth) from
terrestrial (Left) through semiaquatic (Middle) to pelagic (Right).
Image credit: Bryan Christie Design.
How reptiles adapted to marine life
During major evolutionary transitions, the development of new body plans allows animals to adapt to novel habitats and lifestyles. One of the most well-known examples are cetaceans, which evolved from terrestrial mammals into open-ocean swimmers, but relatively little is known about the earlier, land-to-sea transition of reptiles during the Mesozoic Era. Julia Schwab et al. explored evolutionary trends in a key vertebrate sensory system, the inner ear sensory system, which is involved in balance and equilibrium. Using computed tomography, the authors examined the anatomy of the inner ear vestibular system of 18 extinct reptile species and 14 modern relatives. The authors focused on the bony labyrinth, a cavity in the inner ear that includes structures called the vestibule and semicircular canals. Compared with terrestrial species, open-ocean thalattosuchians, which are extinct crocodile relatives, had shorter, more compact labyrinths, wider semicircular canals, and an enlarged vestibule, similar to cetaceans. However, unlike adaptations of the inner ear vestibular structures of cetaceans, those of reptiles developed after a prolonged semiaquatic stage and appeared after the first skeletal modifications for swimming. Taken together, the results demonstrate that distinct evolutionary routes resulted in similar vestibular system changes for different types of terrestrial animals that transitioned to open-ocean swimming, according to the authors. — J.W.
"Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water," by Julia A. Schwab, et al.
Article
Neuroscience
Cannabis plant, White Widow strain. Image credit: Flickr/Théo.
Cannabinoid exposure and cocaine response
The endocannabinoid system modulates reward and cognitive processes in the brain and plays a central role in neurodevelopment. Cannabis use among adolescents has been associated with increased risk of subsequent cocaine use and thought to alter the brain’s response to cocaine and other addictive substances. Maria Scherma et al. examined epigenetic responses, gene expression, and other biochemical changes in the brains of rats that resulted from cocaine exposure, with and without prior exposure to a synthetic cannabinoid. Prior exposure to cannabinoids resulted in an enhanced response to cocaine’s stimulatory effects in adolescent rats, but not adult rats. The enhanced behavioral response was associated with increased histone acetylation and reduced levels of the enzyme histone deacetylase 6 in the brain’s prefrontal cortex. Cannabinoid exposure also led to alterations in cocaine-induced gene expression patterns, alternative splicing events in genes related to neurotransmitter receptor membrane localization, and enhanced effects of cocaine on protein phosphorylation. The results suggest that cannabinoid exposure during adolescence alters the initial behavioral, molecular, and epigenetic responses to cocaine, potentially enhancing susceptibility to addiction in vulnerable individuals, according to the authors. — B.D.
"Cannabinoid exposure in rat adolescence reprograms the initial behavioral, molecular, and epigenetic response to cocaine," by Maria Scherma, et al.
Article
Social Sciences
Population age structure and COVID-19 mortality
The progression and impact of coronavirus disease 2019 (COVID-19) may depend on the age structure of the population, given that mortality risk is concentrated among the elderly. Jennifer Beam Dowd et al. calculated the expected number of deaths for various countries based on the age distribution within the population and assuming an infection prevalence of 10% and the age-specific mortality of Italy as of March 30, 2020. Around 23% of Italy’s population is over age 65, making it one of the oldest populations in the world, and Italy has experienced a correspondingly high overall case fatality rate of 10.6%. In this scenario, the authors projected more than 300,000 expected fatalities for Italy. In South Korea, where only 4.5% of cases have occurred in people aged 80 years or older, expected fatalities were fewer than 180,000. The authors also considered Brazil and Nigeria, which have similar populations but different age distributions. Brazil, with 2% of the population aged 80 years or older, was expected to have more than three times the number of fatalities as Nigeria, where only 0.2% of the population is aged 80 years or older. According to the authors, the results suggest that disease mitigation policies, such as social distancing, should consider both the age distribution within the population as well as intergenerational social contacts. — B.D.
"Demographic science aids in understanding the spread and fatality rates of COVID-19," by Jennifer Beam Dowd, et al.
Article
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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