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Migratory routes of prehistoric baleen whales
Baleen whales tend to feed in cool, high-latitude waters during summer before returning to warm waters to breed in winter, and researchers have hypothesized that migration is integral to the evolution of baleen whales. Although fossil records of baleen whales’ migration history are limited, whale barnacles can provide a record of the whales’ migration cycles by preserving the oxygen isotopic (δ18O) signature of the whales’ movements. Larry Taylor et al. (pp. 7377–7381) compared the oxygen isotope composition of modern whale barnacle shells from Alaska and California to fossilized Pleistocene-age barnacle shells from California and Panama. Both the prehistoric and modern specimens exhibited δ18O profiles with a wide range of values, indicating time spent in varied waters, as well as a continuous progression between the 18O-enriched and 18O-depleted values, suggesting that barnacles grow throughout the whales’ migration. The δ18O profiles of the fossil barnacles exhibited signatures of a summer feeding season and winter breeding season, mirroring the migration patterns of the modern specimens. The findings suggest that baleen whale migration patterns have been similar for hundreds of millennia, and that the Pacific coast of Panama has served as a meeting point for several whale subpopulations for at least 270,000 years, according to the authors. — M.S.
Canopies, impervious surfaces, and urban heat
The urban heat island effect causes increased temperatures in urban environments compared with surrounding areas, with potential human health consequences. As the climate warms, cities explore strategies to mitigate heat. Carly Ziter et al. (pp. 7575–7580) examined the relationship between canopy cover, which reduces heat, and impervious surface cover, which increases heat, on urban air temperature in Madison, Wisconsin. In the summer of 2016, the authors used a bicycle-mounted sensor to repeatedly sample the air temperature approximately every 5 m along 10 urban transects during the day, as well as four transects at night, spanning gradients of both impervious and canopy cover. Daytime temperature decreased nonlinearly with increasing canopy cover, with the greatest cooling occurring when canopy cover was more than 40% and at the scale of a typical city block (60–90 m). However, canopy cover exhibited limited cooling at night, whereas air temperatures increased with the amount of impervious surfaces both during the day and at night. The results suggest that effective urban heat mitigation requires efforts to both increase urban canopy cover and decrease impervious surface cover, according to the authors. — T.G.
Biologists have engineered synthetic gene circuits into mammalian cells to perform simple computational operations, but creating complex circuits that perform sophisticated computation has been a challenge. Hyojin Kim, Daniel Bojar, et al. (pp. 7214–7219) developed a CRISPR-based central processing unit (CRISPR-CPU) that uses a Cas9-based processor combined with input guide RNAs (igRNAs) to build programmable gene circuits in mammalian cells. The authors used Cas9 fused to a KRAB domain to serve as a transcriptional master repressor, and combined this component with various igRNAs and reporter constructs to create systems that perform complex computational functions. The authors created an OFF system and an ON system, and used these systems as building blocks for other gene circuits, including a NOR gate, a NIMPLY gate, an AND gate, and an XOR gate. Next, the authors combined these logic gates to perform binary arithmetic, in addition to combining two CRISPR processors in a single cell by using two different Cas9 proteins, from Streptococcus pyogenes and Staphylococcus aureus. According to the authors, the CRISPR-CPU could be used to create even more complex circuits that combine a greater number of processors in a single cell for potential therapeutic and biotechnological applications. — S.R.
Biodiversity changes in the Early Paleozoic Era
Marine biodiversity underwent large and abrupt changes during the Early Paleozoic Era. However, the temporal resolution of current Paleozoic biodiversity estimates is insufficient to attribute these changes to potential environmental causes. Christian Rasmussen et al. (pp. 7207–7213) constructed a high-resolution reconstruction of genus-level biodiversity during the first 120 million years of the Paleozoic Era based on fossil occurrences from a paleobiology database. The reconstruction exhibited two distinct bursts of biodiversity accumulation corresponding to the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE), separated by 50 million years of slow accumulation. The rapid onset of the latter coincided with equatorial sea-surface temperatures falling to present-day levels, suggesting that 470 million years ago, marine biodiversity required a narrow temperature window. Fifteen million years after the start the GOBE, diversity declined sharply in three successive phases corresponding to the end Ordovician mass extinctions, after which biodiversity did not rebound for tens of millions of years. The mass extinctions were associated with the formation of large igneous provinces and abrupt increases in 13C enrichment, consistent with increased volcanic activity. The results suggest that the end Ordovician extinctions may have begun several million years earlier than previously thought, were triggered by increased volcanism, and had a longer lasting impact on global diversity than recognized, according to the authors. — B.D.
Phasing out air pollution emissions
Significant excess mortality rates related to cardiovascular and respiratory disease are attributed to air pollution. J. Lelieveld et al. (pp. 7192–7197) used an atmospheric chemistry and general circulation model to assess how air pollution from anthropogenic and fossil fuel emissions affects climate, precipitation, and public health. According to the model, fossil fuel use accounted for approximately 65% of excess mortality rates from air pollution worldwide and 70% of climate cooling from aerosols, which also affects precipitation patterns. A reduction in aerosols could increase rainfall in parts of Africa, China, India, and Central America, which would support food and water security in those regions, especially in areas of rapid population growth. The authors found that removing fossil fuel-related and all anthropogenic emissions could prevent approximately 3.6 million and 5.5 million avoidable deaths each year, respectively. Moreover, the authors suggest, rapid emissions reduction is needed to prevent rising temperatures. Although removing pollution emissions is unlikely to limit global warming to 1.5 °C above preindustrial levels, reaching the 2 °C target of the Paris Agreement by midcentury is feasible if emissions are rapidly phased out, according to the authors. — M.S.
Whitefly manipulation of plant odor signals
Whiteflies are major invasive crop pests. Upon infestation by whiteflies, plants mobilize salicylic acid-dependent defenses, which target pathogens, while suppressing jasmonic acid-dependent defenses, rendering the plants susceptible to insects. Peng-Jun Zhang et al. (pp. 7387–7396) demonstrate that such host–plant manipulation extends to neighboring plants through airborne signals. Using interconnected glass chambers, the authors exposed healthy tomato plants to volatiles released by other plants that were either uninfested or infested by whiteflies. Compared with plants that had been exposed to volatiles from uninfested plants, plants that had been exposed to volatiles from infested plants were more susceptible to whiteflies. Exposure to airborne signals from infested plants helped accelerate the development of whitefly nymphs and suppressed the production of jasmonic acid, while increasing salicylic acid levels in response to whitefly infestation. Together, the findings suggest that whitefly-infested plants release volatiles that prime a defense against pathogens in neighboring plants, at the cost of defenses against insect herbivores, making the neighboring plants more suitable for whitefly development. According to the authors, the ability of whiteflies to manipulate plant defense responses through induced volatile emissions might explain the rapid spread of whiteflies, and the findings could help develop effective control strategies. — J.W.