Golgi maturation visualized in living yeast

The Golgi apparatus is composed of biochemically distinct early (cis, medial) and late (trans, TGN) cisternae. There is debate about the nature of these cisternae. The stable compartments model predicts that each cisterna is a long-lived structure that retains a characteristic set of Golgi-resident proteins. In this view, secretory cargo proteins are transported by vesicles from one cisterna to the next. The cisternal maturation model predicts that each cisterna is a transient structure that matures from early to late by acquiring and then losing specific Golgi-resident proteins. In this view, secretory cargo proteins traverse the Golgi by remaining within the maturing cisternae. Various observations have been interpreted as supporting one or the other mechanism. Here we provide a direct test of the two models using three-dimensional time-lapse fluorescence microscopy of the yeast Saccharomyces cerevisiae. This approach reveals that individual cisternae mature, and do so at a consistent rate. In parallel, we used pulse-chase analysis to measure the transport of two secretory cargo proteins. The rate of cisternal maturation matches the rate of protein transport through the secretory pathway, suggesting that cisternal maturation can account for the kinetics of secretory traffic.     

The adult Drosophila posterior midgut is maintained by pluripotent stem cells

Vertebrate and invertebrate digestive systems show extensive similarities in their development, cellular makeup and genetic control. The Drosophila midgut is typical: enterocytes make up the majority of the intestinal epithelial monolayer, but are interspersed with hormone-producing enteroendocrine cells. Human (and mouse) intestinal cells are continuously replenished by stem cells, the misregulation of which may underlie some common digestive diseases and cancer. In contrast, stem cells have not been described in the intestines of flies, and Drosophila intestinal cells have been thought to be relatively stable. Here we use lineage labelling to show that adult Drosophila posterior midgut cells are continuously replenished by a distinctive population of intestinal stem cells (ISCs). As in vertebrates, ISCs are multipotent, and Notch signalling is required to produce an appropriate fraction of enteroendocrine cells. Notch is also required for the differentiation of ISC daughter cells, a role that has not been addressed in vertebrates. Unlike previously characterized stem cells, which reside in niches containing a specific partner stromal cell, ISCs adjoin only the basement membrane, differentiated enterocytes and their most recent daughters. The identification of Drosophila intestinal stem cells with striking similarities to their vertebrate counterparts will facilitate the genetic analysis of normal and abnormal intestinal function.     

Local dispersal promotes biodiversity in a real-life game of rock-paper-scissors

One of the central aims of ecology is to identify mechanisms that maintain biodiversity. Numerous theoretical models have shown that competing species can coexist if ecological processes such as dispersal, movement, and interaction occur over small spatial scales. In particular, this may be the case for non-transitive communities, that is, those without strict competitive hierarchies. The classic non-transitive system involves a community of three competing species satisfying a relationship similar to the children's game rock-paper-scissors, where rock crushes scissors, scissors cuts paper, and paper covers rock. Such relationships have been demonstrated in several natural systems. Some models predict that local interaction and dispersal are sufficient to ensure coexistence of all three species in such a community, whereas diversity is lost when ecological processes occur over larger scales. Here, we test these predictions empirically using a non-transitive model community containing three populations of Escherichia coli. We find that diversity is rapidly lost in our experimental community when dispersal and interaction occur over relatively large spatial scales, whereas all populations coexist when ecological processes are localized.     

Increased forest ecosystem carbon and nitrogen storage from nitrogen rich bedrock

Nitrogen (N) limits the productivity of many ecosystems worldwide, thereby restricting the ability of terrestrial ecosystems to offset the effects of rising atmospheric CO(2) emissions naturally. Understanding input pathways of bioavailable N is therefore paramount for predicting carbon (C) storage on land, particularly in temperate and boreal forests. Paradigms of nutrient cycling and limitation posit that new N enters terrestrial ecosystems solely from the atmosphere. Here we show that bedrock comprises a hitherto overlooked source of ecologically available N to forests. We report that the N content of soils and forest foliage on N-rich metasedimentary rocks (350-950?mg?N?kg(-1)) is elevated by more than 50% compared with similar temperate forest sites underlain by N-poor igneous parent material (30-70?mg?N?kg(-1)). Natural abundance N isotopes attribute this difference to rock-derived N: (15)N/(14)N values for rock, soils and plants are indistinguishable in sites underlain by N-rich lithology, in marked contrast to sites on N-poor substrates. Furthermore, forests associated with N-rich parent material contain on average 42% more carbon in above-ground tree biomass and 60% more carbon in the upper 30?cm of the soil than similar sites underlain by N-poor rocks. Our results raise the possibility that bedrock N input may represent an important and overlooked component of ecosystem N and C cycling elsewhere.     

The let-7-Imp axis regulates ageing of the Drosophila testis stem-cell niche

Adult stem cells support tissue homeostasis and repair throughout the life of an individual. During ageing, numerous intrinsic and extrinsic changes occur that result in altered stem-cell behaviour and reduced tissue maintenance and regeneration. In the Drosophila testis, ageing results in a marked decrease in the self-renewal factor Unpaired (Upd), leading to a concomitant loss of germline stem cells. Here we demonstrate that IGF-II messenger RNA binding protein (Imp) counteracts endogenous small interfering RNAs to stabilize upd (also known as os) RNA. However, similar to upd, Imp expression decreases in the hub cells of older males, which is due to the targeting of Imp by the heterochronic microRNA let-7. In the absence of Imp, upd mRNA therefore becomes unprotected and susceptible to degradation. Understanding the mechanistic basis for ageing-related changes in stem-cell behaviour will lead to the development of strategies to treat age-onset diseases and facilitate stem-cell-based therapies in older individuals.     

Body plan innovation in treehoppers through the evolution of an extra wing-like appendage

Body plans, which characterize the anatomical organization of animal groups of high taxonomic rank, often evolve by the reduction or loss of appendages (limbs in vertebrates and legs and wings in insects, for example). In contrast, the addition of new features is extremely rare and is thought to be heavily constrained, although the nature of the constraints remains elusive. Here we show that the treehopper (Membracidae) 'helmet' is actually an appendage, a wing serial homologue on the first thoracic segment. This innovation in the insect body plan is an unprecedented situation in 250 Myr of insect evolution. We provide evidence suggesting that the helmet arose by escaping the ancestral repression of wing formation imparted by a member of the Hox gene family, which sculpts the number and pattern of appendages along the body axis. Moreover, we propose that the exceptional morphological diversification of the helmet was possible because, in contrast to the wings, it escaped the stringent functional requirements imposed by flight. This example illustrates how complex morphological structures can arise by the expression of ancestral developmental potentials and fuel the morphological diversification of an evolutionary lineage.     

Controlled exchange interaction between pairs of neutral atoms in an optical lattice

Ultracold atoms trapped by light offer robust quantum coherence and controllability, providing an attractive system for quantum information processing and for the simulation of complex problems in condensed matter physics. Many quantum information processing schemes require the manipulation and deterministic entanglement of individual qubits; this would typically be accomplished using controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress towards this goal by demonstrating entanglement among an ensemble of atoms confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between atoms in isolated pairs. Here we use an optical lattice of double-well potentials to isolate and manipulate arrays of paired (87)Rb atoms, inducing controlled entangling interactions within each pair. Our experiment realizes proposals to use controlled exchange coupling in a system of neutral atoms. Although 87Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, the wavefunction exchange symmetry of these identical bosons leads to state-dependent dynamics. We observe repeated interchange of spin between atoms occupying different vibrational levels, with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits). Its 'half-implementation', the root SWAP gate, is entangling, and together with single-qubit rotations it forms a set of universal gates for quantum computation.