Modeling the spread of a citrus epidemic
Measures to control the spread of emerging epidemics can hamper the assessment of the epidemiological parameters needed to predict the further spread of the epidemic and the effectiveness of control strategies. Matthew Parry et al. (pp. 6258–6262) used the spread of Huanglongbing (HLB), an extremely destructive citrus disease, through citrus groves as a case study for modeling an emerging epidemic in the presence of control measures. HLB has no known commercially viable cure, and over the last decade has inflicted severe economic losses in many of the world’s major citrus-growing areas. Using data from more than 250,000 trees in the Southern Gardens plantation in south Florida, the authors modeled the tree-to-tree spread of HLB through trees of different ages and susceptibility, while accounting for measures used to control the epidemic’s spread. The authors’ model was able to characterize the disease transmission process and the temporal and spatial patterns of HLB invasion through the citrus groves, despite the fact that many epidemiological parameters had to be estimated from limited and imprecise data. The study provides insights into the epidemiology of HLB, and the authors suggest that their model could help estimate the costs and benefits of potential strategies used to control the citrus disease. — S.R.
Wasp’s partner choice in ancient symbiosis
Male beewolf (Philanthus loefflingi) feeding on wild fennel.
The beewolf, a solitary wasp, cultivates a symbiotic relationship with a Streptomyces bacterium (CaSP), which lives in specialized gland reservoirs in the antennae of beewolf females. From these reservoirs, CaSP is secreted into the brood cell, taken up by the larva, and incorporated into the cocoon, where CaSP produces antimicrobial compounds that protect against pathogens. Martin Kaltenpoth et al. (pp. 6359–6364) investigated the wasp’s role in maintaining specificity in the association. To reconstruct the evolutionary history of the symbiosis, the authors performed cophylogenetic analyses of 50 beewolf species and their antennal symbionts, dating the beewolf–CaSP association to a single origin in the late Cretaceous between 68.3 and 110 million years ago. While vertical transmission dominated over evolutionary timescales, the authors found evidence of horizontal transmission of CaSP strains across host beewolf species. To investigate the role of the host in partner selection, the authors infected symbiont-free female beewolves with either a native (CaSP) or nonnative symbiont. Although both strains successfully colonized the antennal reservoir, only females carrying CaSP deposited antennal gland secretion in the brood cell and produced symbiont-positive cocoons. According to the authors, partner choice during transmission likely promoted stability of this symbiotic relationship since its inception. — C.B.
Genome sequencing reveals evolution of flesh-eating bacteria
Despite decades of study, researchers have yet to pinpoint critical molecular events that allow certain microbial pathogens to emerge and trigger epidemics. Focusing on a devastating infection, Waleed Nasser et al. (pp. E1768–E1776) coupled animal virulence studies with a genomic analysis of 3,615 strains of serotype M1 group A Streptococcus, often called “flesh-eating” bacteria. The authors describe how the contemporary epidemic clone emerged in a series of steps from a precursor cell, which contained a phage expressing an extracellular virulence factor. The authors report that the precursor subsequently acquired a second phage, which itself evolved into a third phage prior to the horizontal transfer of a large chromosomal region in the early 1980s. The acquisition of this 36 kb chromosomal region, according to the authors, likely represents the final major molecular event preceding the pathogen’s emergence and intercontinental spread. The study demonstrates that such analyses may help predict the emergence of new strains, guide effective public health interventions, and improve the development of vaccines, according to the authors. — T.J.
Airway epithelial cells generated from human stem cells
Multiciliated airway epithelial cells differentiated from iPSCs. Centrioles at the base of cilia (red), cilia projections (cyan), and nuclei (blue).
Mouse models fail to accurately recapitulate human lung disease, a leading cause of mortality in the United States. Amy Firth et al. (pp. E1723–E1730) developed a protocol to generate a renewable source of functional human airway epithelial cells, including secretory cells called Clara cells, mucus-secreting goblet cells, and multiciliated cells, which use hundreds of beating cilia to propel mucus over airway surfaces and protect the lungs from infection. Using induced pluripotent stem cells (iPSCs) derived from human skin fibroblasts and an air–liquid interface that mimics the lung environment, the authors generated a pseudostratified, polarized layer of epithelial cells, including Clara cells, goblet cells, and multiciliated cells with an active gene switch that controls cilia development. The authors report that the polarized epithelial layer contained demonstrably functional ion channels, namely the cystic fibrosis transmembrane conductance regulator, the activity of which is crucial to lung function and goes awry in cystic fibrosis. The authors’ protocol provides a means to study lung diseases caused by defective cilia, such as primary ciliary dyskinesia, or by deregulated secretion, such as asthma and chronic obstructive pulmonary disease. According to the authors, self-renewing lung stem cells generated in the pool of polarized human epithelial cells might pave the way toward patient-specific iPSCs for therapeutic gene editing and engraftment into the lungs. — P.N.
Homeostasis in intraneuronal functional maps
Neurons’ astounding computational capabilities depend on the topography of voltage-gated ion channels within the cells. Researchers have long been puzzled by the ability of neurons to maintain stereotypic, topographically continuous maps of physiological function despite the high plasticity and constant turnover of ion channels. Rahul Rathour and Rishikesh Narayanan (pp. E1787–E1796) explored whether the composition of individual ion channels within the neuronal membrane is constrained by the need to sustain several functional maps along the same topograph. The authors used a technique known as global sensitivity analysis to determine the validity of 20,420 random models of hippocampal neurons that coexpressed six well-characterized functional maps along the neuronal trunks. The analysis revealed that overall homeostasis can be maintained among several coexisting functional maps, even if the properties of individual ion channels are not maintained at constant levels. The authors report that so-called collective channelostasis—a phenomenon in which several ion channels regulate their properties and expression profiles in an uncorrelated manner—can allow for homeostasis in the functional maps of neurons. According to the authors, collective channelostasis may explain the robust emergence of analogous functional maps across a population of neurons, and could have implications for the localization and targeting of the ion channels and enzymes that regulate neural coding and homeostasis. — A.G.