Cesarean birth and brain development
Dying neurons (black) in the hippocampus of a newborn mouse.
Previous studies have reported behavioral and cognitive differences between children delivered vaginally versus by cesarean section (C-section), suggesting that birth mode affects brain development. However, medical complications, altered birth timing, and maternal factors associated with C-sections complicate studies of birth mode effects in humans. Alexandra Castillo-Ruiz et al. (pp. 11826–11831) examined neuronal cell death immediately before and after vaginal or C-section birth in mice. Female mice were impregnated within the same 12-hour period, and for each mouse that gave birth vaginally, a C-section was simultaneously performed on another randomly selected mouse, thereby controlling for effects of gestation length and birth timing. Vaginally delivered pups exhibited significantly reduced cell death in several brain regions 3 hours after birth, compared with that observed in pups just before birth. The abrupt decline in cell death across birth was not observed in pups delivered by C-section. Cesarean-born mice emitted quieter ultrasonic vocalizations than vaginally born mice, and at weaning had fewer neurons expressing the hormone vasopressin in the hypothalamus and greater body weight than vaginally born mice. The results suggest that birth mode can influence neurodevelopment, potentially leading to lasting impacts on brain function and behavior, according to the authors. — B.D.
Characterizing nuclease activity in Streptococcus pyogenes
Ribonucleases (RNases) are important for the regulation of bacterial gene expression, and 3’-to-5’ exoRNases play a key role in this process by degrading regulatory and protein-coding transcripts. Anne-Laure Lécrivain, Anaïs Le Rhun, et al. (pp. 11814–11819) conducted a genome-wide study of 3’-to-5’ exoRNase global processing sites—or targetomes—in the human pathogen Streptococcus pyogenes. The authors characterized the transcripts targeted by three S. pyogenes 3’-to-5’ exoRNases: RNase R, YhaM, and polynucleotide phosphorylase (PNPase). The authors report that PNPase was the major 3’-to-5’ exoRNase involved in RNA turnover and fully degraded short RNA fragments produced by endoRNases, as well as regulatory elements from 5’ untranslated regions. YhaM is relatively nonspecific and trims a few nucleotides from the majority of transcript ends generated either by transcription termination or endoRNases. RNase R activity was limited under standard culture conditions. Single and double deletions of the exoRNases did not have a major impact in cells grown at 37 °C. However, deleting either YhaM or RNase R affected bacterial growth at cold temperatures, and the combined deletion had a synergistic effect, suggesting that the exoRNases have distinct roles in S. pyogenes cold adaptation. According to the authors, the study elucidates the distinct activities of exoRNases in S. pyogenes. — S.R.
Engineering endosymbiosis
Confocal microscopy images of chimeric yeast (blue)–E. coli (purple) cells.
The endosymbiotic theory holds that mitochondria, an integral part of animal cells, evolved from an independent bacterium that entered an archaeal cell and developed a symbiotic relationship sustained over succeeding generations. Angad Mehta, Lubica Supekova, et al. (pp. 11796–11801) sought to establish such a symbiotic relationship between Escherichia coli bacteria and yeast cells by engineering deficiencies in each cell that could be complemented by the other. The authors created a yeast mutant with deficient mitochondria unable to produce ATP and inserted an E. coli that expressed the enzyme ADP/ATP translocase, enabling the yeast to grow on a nonfermentable medium. Additionally, the authors engineered the E. coli with a deficiency in thiamin that was offset by the yeast, thus establishing a symbiotic E. coli–yeast chimera. The E. coli additionally required expression of proteins to protect from lysosomal degradation inside the cell. Because the E. coli also expressed green fluorescent protein, the authors tracked the chimera through successive generations and found that the E. coli persisted in the yeast for more than 40 generations. According to the authors, the symbiotic chimera provides a mechanism for exploring the origins of mitochondrial endosymbiosis. — P.G.
Income mobility and life-expectancy inequality
Previous studies have found that people at the top of the income distribution have longer life expectancies than those at the bottom. However, people might not stay in the same income class over their entire lives, and the life expectancies of those who move to a different income class may be significantly different from those who stay in the same class. Claus Thustrup Kreiner, Torben Heien Nielsen, and Benjamin Ly Serena (pp. 11754–11759) devised a method of accounting for income mobility when calculating the relationship between income and life expectancy by incorporating a classic model of social mobility from the literature. The authors demonstrated their approach by calculating life expectancy at age 40 in Denmark based on official income and mortality records spanning the period 1983–2013. Without accounting for income mobility, the life-expectancy difference between the 80th and 20th income percentiles was 4.6 years for males and 4.1 years for females. The differences were only half as large after accounting for income mobility. Inequality in life expectancy steadily increased over the 30 years represented in the data. However, the increase was only half as great when accounting for income mobility. The results suggest that measures of inequality in life expectancy are exaggerated when mobility is disregarded, according to the authors. — B.D.
Calcium carbonate dissolution at the seafloor
Anthropogenic fraction of total calcite dissolution at the seafloor in 2002.
Dissolution of calcium carbonate (CaCO3) from sediments on the seafloor converts excess carbon dioxide (CO2) in the ocean into bicarbonate, preventing runaway ocean acidification and regulating atmospheric CO2 levels over timescales of centuries to millennia. Numerous examples exist in the geological record of increased CaCO3 dissolution in response to ocean acidification. Olivier Sulpis et al. (pp. 11700–11705) compared preindustrial and modern seafloor dissolution rates to estimate the impact of anthropogenic CO2 on dissolution rates. The authors estimated the spatial distributions of the rates based on databases of dissolved inorganic carbon in the deep ocean, the CaCO3 content of sediments, and sea-bottom current speeds, and a model of carbonate ion transport across the sediment–water interface. The difference between present-day and preindustrial dissolution rates corresponds to the anthropogenic contribution to dissolution. Although this difference was minimal throughout most of the oceans, the authors identified localized hot spots of pronounced anthropogenic dissolution, most notably in the northwestern Atlantic Ocean, where the anthropogenic component contributed 40–100% of the total dissolution rate. Additional hot spots were found in the southern Atlantic, Indian, and Pacific Oceans, in locations reflecting high anthropogenic CO2 intrusions and fast bottom currents. According to the authors, the results provide evidence of a significant anthropogenic contribution to CaCO3 dissolution at the seafloor, suggesting that human activity is altering the deep-sea geological record. — B.D.
How Darwin’s finches diversify
The medium ground finch Geospiza fortis on Daphne Major Island.
Darwin’s finches on the Galápagos Islands have exemplified the mechanisms of species divergence since the genesis of evolutionary theory. Various species of finches have diverged from a common ancestor and display adaptations in size and beak shape, although the processes underlying the genetic isolation of new species are unclear. Peter Grant and B. Rosemary Grant (pp. E10879–E10887) tracked finches on the island of Daphne Major between 1976 and 2012, recording the body and beak dimensions of individuals and identifying parent–offspring relationships. The authors found that two species of finches selected mates with body and beak sizes, as well as songs, similar to those of their parents, particularly those of the father. Such imprinting ensures that successive generations breed with birds of a similar lineage, thereby avoiding interbreeding with other species. The authors note that hybridization with other species occasionally happens, when species express similar adaptive traits, and such hybridization can itself drive speciation. According to the authors, conservation of diverse environmental conditions, such as those that lead to species radiation, including geographic heterogeneity, is essential to ensure future species diversification. — P.G.