Thursday, December 23, 2010

Hot off the presses! Dec 21 curr biol

The Dec 21 issue of the curr biol is now up on Pubget (About curr biol): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

  • Current Biology at 20
    - curr biol 20(24):R1049-R1051 (2010)
  • Researchers ponder a 4°C temperature rise
    - curr biol 20(24):R1052-R1053 (2010)
    As the 2°C target in the Copenhagen Accord begins to appear increasingly elusive, researchers have drawn up an overview of the challenges the world will face if global temperatures rise by 4°C or more. Michael Gross reports.
  • Maldives take a climate lead
    - curr biol 20(24):R1053-R1054 (2010)
    The Maldives hopes to be the first state to be carbon neutral but threats from sea-level rise still dominate. Nigel Williams reports.
  • Warming puzzle for Tibetan plants
    - curr biol 20(24):R1054-R1055 (2010)
    Milder winters may delay the onset of spring in some species. Nigel Williams reports.
  • UK body backs food from cloned animals
    - curr biol 20(24):R1055-R1056 (2010)
    Mediawatch: Bernard Dixon looks at the mixed response to the Food Standard Agency's report.
  • New support for polar bears and tigers
    - curr biol 20(24):R1056-R1057 (2010)
    Two events have bolstered conservation efforts. Nigel Williams reports.
  • Ice stars in new climate exhibition
    - curr biol 20(24):R1057-R1058 (2010)
    Following criticism of its earlier efforts to present an exhibition on climate change for the public, the Science Museum in London launched this month a new gallery aiming to focus on the science. The previous exhibition last October was seen by many as an effort to persuade people of the dangers of global warming. 'Prove it! All the evidence you need to believe in climate change' grated with many at this time of widespread questioning of some of the activities of researchers.
  • Giacomo Rizzolatti
    - curr biol 20(24):R1058-R1060 (2010)
  • Bats
    - curr biol 20(24):R1060-R1062 (2010)
  • Adhesion signalling complexes
    - curr biol 20(24):R1063-R1067 (2010)
    Intercellular communication in metazoa not only requires autocrine, paracrine and exocrine signalling systems, but it also relies on the structural and positional information encoded in extracellular matrices (ECMs). Most cells in tissues are structurally and functionally integrated with their surrounding ECM in a highly organised manner involving thousands of dynamic connections. On the intracellular face of these linkages, adhesion receptors — principally integrins and syndecans — link the cytoskeleton to the plasma membrane and compartmentalise cytoplasmic signalling events, whereas at the extracellular face the same receptors direct and organise the deposition of the ECM itself. Adhesion receptors transduce mechanical force bidirectionally across the plasma membrane by tethering variably deformable ECMs to the contractile cytoskeleton (Figure 1), and they translate the topography and composition of the ECM into chemical signals that determine behaviour. The mem! brane-proximal functions of adhesion receptors in turn trigger distal processes within cells, such as alterations in the direction of cell movement and the regulation of gene transcription, and long-range effects outside cells, such as the construction of ECM networks and consequent shaping of higher-order tissue structure. Given the diverse and fundamental roles attributed to adhesion, it is understandable that adhesion receptor engagement has been reported to alter the flux through virtually all major signalling pathways.
  • Sex differences in chimpanzees' use of sticks as play objects resemble those of children
    - curr biol 20(24):R1067-R1068 (2010)
    Sex differences in children's toy play are robust and similar across cultures [1] and [2]. They include girls tending to play more with dolls and boys more with wheeled toys and pretend weaponry. This pattern is explained by socialization by elders and peers, male rejection of opposite-sex behavior and innate sex differences in activity preferences that are facilitated by specific toys [1]. Evidence for biological factors is controversial but mounting. For instance, girls who have been exposed to high fetal androgen levels are known to make relatively masculine toy choices [3]. Also, when presented with sex-stereotyped human toys, captive female monkeys play more with typically feminine toys, whereas male monkeys play more with masculine toys [1]. In human and nonhuman primates, juvenile females demonstrate a greater interest in infants, and males in rough-and-tumble play. This sex difference in activity preferences parallels adult behavior and may contribute to differ! ences in toy play [1]. Here, we present the first evidence of sex differences in use of play objects in a wild primate, in chimpanzees (Pan troglodytes). We find that juveniles tend to carry sticks in a manner suggestive of rudimentary doll play and, as in children and captive monkeys, this behavior is more common in females than in males.
  • Centrosome Biogenesis: Centrosomin Sizes Things Up!
    - curr biol 20(24):R1069-R1071 (2010)
    Centrosomes are composed of a centriole pair surrounded by a proteinaceous matrix of pericentriolar material that contains protein complexes required for microtubule nucleation and anchoring. Recent work reveals an intriguing link between centrioles and pericentriolar material that is involved in modulating centrosome size.
  • Social Cognition: Feeling Voices to Recognize Emotions
    - curr biol 20(24):R1071-R1072 (2010)
    Our understanding of how we simulate other people's actions and feelings to recognize their emotional states is extended by a new study which finds that premotor and somatosensory cortices are required to process the emotional meaning of sounds.
  • Speciation Genetics: Search for the Missing Snowball
    - curr biol 20(24):R1073-R1074 (2010)
    Theory predicts that, as species diverge from one another, the number of genetic incompatibilities causing sterility or inviability in interspecies hybrids grows faster than linearly, or snowballs. Two new genetic analyses now provide the first empirical support for this snowball effect.
  • Primatology: Monkey Bromance
    - curr biol 20(24):R1074-R1076 (2010)
    Male macaques form strong social bonds that enhance competitive ability and mating success, belying theoretical predictions that mate competition should prevent males from cooperating with one another.
  • Aging: miRacles of Longevity?
    - curr biol 20(24):R1076-R1078 (2010)
    The inventory of processes that miRNAs regulate has continued to expand since their relatively recent discovery. A new study reveals not only that the expression of miRNAs changes with age, but also that these miRNAs can act in both pro- and anti-longevity regulatory pathways.
  • Eukaryotic Evolution: The Importance of Being Archaebacterial
    - curr biol 20(24):R1078-R1079 (2010)
    Approximately half of all eukaryotic genes show signs of prokaryotic origin. Genes derived from eubacteria are more abundant than those from archaebacteria, but the latter are functionally more important. This supports archaebacteria as founding ancestors of the eukaryotic nucleus.
  • Sex Allocation: Size Matters for Red Spider Mites
    - curr biol 20(24):R1080-R1081 (2010)
    Many animals can adjust the sex ratio of their offspring according to their parental ability to invest. In spider mites, larger eggs are likely to be fertilized and produce diploid females, whereas smaller eggs produce haploid males.
  • Fruit Development: New Directions for an Old Pathway
    - curr biol 20(24):R1081-R1083 (2010)
    A recent study investigating the molecular mechanisms of seed pod shattering has shown that the basic helix-loop-helix (bHLH) proteins INDEHISCENT and ALCATRAZ appear to regulate fruit patterning through gibberellic acid (GA)–DELLA signalling, revealing a central role for bHLH family members in GA response specificity.
  • Kinetochores: NDC80 Toes the Line
    - curr biol 20(24):R1083-R1085 (2010)
    Kinetochore-associated NDC80 complexes serve as the primary binding site for the plus-ends of spindle microtubules in mitosis. A recent study proposes a novel mechanism for regulating kinetochore-microtubule binding involving NDC80 complex oligomerization, which could be mediated by Aurora B kinase.
  • Ambient Thermometers in Plants: From Physiological Outputs towards Mechanisms of Thermal Sensing
    - curr biol 20(24):R1086-R1092 (2010)
    Plants respond to ambient temperature changes over a series of timescales. Genetic and physiological studies over the last decades have revealed myriad thermally sensitive pathways in plants. A recent study provides a genetic and biochemical mechanistic description of how thermal changes can be transduced to influence gene expression. What remains to be revealed in this, and other thermally controlled responses, is a description of the primary temperature-sensing event. Cooling and warming alter membrane fluidity and elicit intracellular free-calcium elevations, a process that has been considered the primary event controlling plant responses to temperature. Such direct thermal sensors appear to process temperature information. Future efforts will be required to identify the effector proteins linking perception to response. This review considers the evidence for plant thermometers to date, provides a description of several notable physiological and developmental process! es under ambient temperature control, and outlines major questions that remain to be addressed in the understanding of thermometers in plants.
  • MicroRNAs Both Promote and Antagonize Longevity in C. elegans
    - curr biol 20(24):2159-2168 (2010)
    Background Aging is under genetic control in C. elegans, but the mechanisms of life-span regulation are not completely known. MicroRNAs (miRNAs) regulate various aspects of development and metabolism, and one miRNA has been previously implicated in life span. Results Here we show that multiple miRNAs change expression in C. elegans aging, including novel miRNAs, and that mutations in several of the most upregulated miRNAs lead to life-span defects. Some act to promote normal life span and stress resistance, whereas others inhibit these phenomena. We find that these miRNAs genetically interact with genes in the DNA damage checkpoint response pathway and in the insulin signaling pathway. Conclusions Our findings reveal that miRNAs both positively and negatively influence life span. Because several miRNAs upregulated during aging regulate genes in conserved pathways of aging and thereby influence life span in C. elegans, we propose that miRNAs may play important roles in stress response and aging of more complex organisms.
  • Directed Microtubule Growth, +TIPs, and Kinesin-2 Are Required for Uniform Microtubule Polarity in Dendrites
    - curr biol 20(24):2169-2177 (2010)
    Background In many differentiated cells, microtubules are organized into polarized noncentrosomal arrays, yet few mechanisms that control these arrays have been identified. For example, mechanisms that maintain microtubule polarity in the face of constant remodeling by dynamic instability are not known. Drosophila neurons contain uniform-polarity minus-end-out microtubules in dendrites, which are often highly branched. Because undirected microtubule growth through dendrite branch points jeopardizes uniform microtubule polarity, we have used this system to understand how cells can maintain dynamic arrays of polarized microtubules. Results We find that growing microtubules navigate dendrite branch points by turning the same way, toward the cell body, 98% of the time and that growing microtubules track along stable microtubules toward their plus ends. Using RNAi and genetic approaches, we show that kinesin-2, and the +TIPS EB1 and APC, are required for uniform dendrite microtubule polarity. Moreover, the protein-protein interactions and localization of Apc2-GFP and Apc-RFP to branch points suggests that these proteins work together at dendrite branches. The functional importance of this polarity mechanism is demonstrated by the failure of neurons with reduced kinesin-2 to regenerate an axon from a dendrite. Conclusions We conclude that microtubule growth is directed at dendrite branch points and that kinesin-2, APC, and EB1 are likely to play a role in this process. We propose that kinesin-2 is recruited to growing microtubules by +TIPS and that the motor protein steers growing microtubules at branch points. This represents a newly discovered mechanism for maintaining polarized arrays of microtubules.
  • Centrioles Regulate Centrosome Size by Controlling the Rate of Cnn Incorporation into the PCM
    - curr biol 20(24):2178-2186 (2010)
    Background Centrosomes are major microtubule organizing centers in animal cells, and they comprise a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Centrosome size is tightly regulated during the cell cycle, and it has recently been shown that the two centrosomes in certain stem cells are often asymmetric in size. There is compelling evidence that centrioles influence centrosome size, but how centrosome size is set remains mysterious. Results We show that the conserved Drosophila PCM protein Cnn exhibits an unusual dynamic behavior, because Cnn molecules only incorporate into the PCM closest to the centrioles and then spread outward through the rest of the PCM. Cnn incorporation into the PCM is driven by an interaction with the conserved centriolar proteins Asl (Cep152 in humans) and DSpd-2 (Cep192 in humans). The rate of Cnn incorporation into the PCM is tightly regulated during the cell cycle, and this rate influences the amount of Cnn in the PCM, which in turn is an important determinant of overall centrosome size. Intriguingly, daughter centrioles in syncytial embryos only start to incorporate Cnn as they disengage from their mothers; this generates a centrosome size asymmetry, with mother centrioles always initially organizing more Cnn than their daughters. Conclusions Centrioles can control the amount of PCM they organize by regulating the rate of Cnn incorporation into the PCM. This mechanism can explain how centrosome size is regulated during the cell cycle and also allows mother and daughter centrioles to set centrosome size independently of one another.
  • Cnn Dynamics Drive Centrosome Size Asymmetry to Ensure Daughter Centriole Retention in Drosophila Neuroblasts
    - curr biol 20(24):2187-2192 (2010)
    Centrosomes comprise a pair of centrioles surrounded by an amorphous network of pericentriolar material (PCM). In certain stem cells, the two centrosomes differ in size, and this appears to be important for asymmetric cell division [[1] and [2]]. In some cases, centrosome asymmetry is linked to centriole age because the older, mother centriole always organizes more PCM than the daughter centriole, thus ensuring that the mother centriole is always retained in the stem cell after cell division [3]. This has raised the possibility that an "immortal" mother centriole may help maintain stem cell fate [[4] and [5]]. It is unclear, however, how centrosome size asymmetry is generated in stem cells. Here we provide compelling evidence that centrosome size asymmetry in Drosophila neuroblasts is generated by the differential regulation of Cnn incorporation into the PCM at mother and daughter centrioles. Shortly after centriole separation, mother and daughter centrioles organi! ze similar amounts of PCM, but Cnn incorporation is then rapidly downregulated at the mother centriole, while it is maintained at the daughter centriole. This ensures that the daughter centriole maintains its PCM and so its position at the apical cortex. Thus, the daughter centriole, rather than an "immortal" mother centriole, is ultimately retained in these stem cells.
  • A Molecular Switch for Photoperiod Responsiveness in Mammals
    - curr biol 20(24):2193-2198 (2010)
    Seasonal synchronization based on day length (photoperiod) allows organisms to anticipate environmental change. Photoperiodic decoding relies on circadian clocks, but the underlying molecular pathways have remained elusive [1]. In mammals and birds, photoperiodic responses depend crucially on expression of thyrotrophin β subunit RNA (TSHβ) in the pars tuberalis (PT) of the pituitary gland [[2], [3] and [4]]. Now, using our well-characterized Soay sheep model [2], we describe a molecular switch governing TSHβ transcription through the circadian clock. Central to this is a conserved D element in the TSHβ promoter, controlled by the circadian transcription factor thyrotroph embryonic factor (Tef). In the PT, long-day exposure rapidly induces expression of the coactivator eyes absent 3 (Eya3), which synergizes with Tef to maximize TSHβ transcription. The pineal hormone melatonin, secreted nocturnally, sets the phase of rhythmic Eya3 expression in the PT to peak 12 hr ! after nightfall. Additionally, nocturnal melatonin levels directly suppress Eya3 expression. Together, these effects form a switch triggering a strong morning peak of Eya3 expression under long days. Species variability in the TSHβ D element influences sensitivity to TEF, reflecting species variability in photoperiodic responsiveness. Our findings define a molecular pathway linking the circadian clock to the evolution of seasonal timing in mammals.
  • Acute Induction of Eya3 by Late-Night Light Stimulation Triggers TSHβ Expression in Photoperiodism
    - curr biol 20(24):2199-2206 (2010)
    Living organisms detect seasonal changes in day length (photoperiod) [[1], [2] and [3]] and alter their physiological functions accordingly to fit seasonal environmental changes. TSHβ, induced in the pars tuberalis (PT), plays a key role in the pathway that regulates vertebrate photoperiodism [[4] and [5]]. However, the upstream inducers of TSHβ expression remain unknown. Here we performed genome-wide expression analysis of the PT under chronic short-day and long-day conditions in melatonin-proficient CBA/N mice, in which the photoperiodic TSHβ expression response is preserved [6]. This analysis identified "short-day" and "long-day" genes, including TSHβ, and further predicted the acute induction of long-day genes by late-night light stimulation. We verified this by advancing and extending the light period by 8 hr, which induced TSHβ expression within one day. In the following genome-wide expression analysis under this acute long-day condition, we searched! for candidate upstream genes by looking for expression that preceded TSHβ's, and we identified the Eya3 gene. We demonstrated that Eya3 and its partner Six1 synergistically activate TSHβ expression and that this activation is further enhanced by Tef and Hlf. These results elucidate the comprehensive transcriptional photoperiodic response in the PT, revealing the complex regulation of TSHβ expression and unexpectedly rapid response to light changes in the mammalian photoperiodic system.
  • Social Bonds Enhance Reproductive Success in Male Macaques
    - curr biol 20(24):2207-2210 (2010)
    For animals living in mixed-sex social groups, females who form strong social bonds with other females live longer and have higher offspring survival [[1], [2] and [3]]. These bonds are highly nepotistic, but sometimes strong bonds may also occur between unrelated females if kin are rare [[2] and [3]] and even among postdispersal unrelated females in chimpanzees and horses [[4] and [5]]. Because of fundamental differences between the resources that limit reproductive success in females (food and safety) and males (fertilizations), it has been predicted that bonding among males should be rare and found only for kin and among philopatric males [6] like chimpanzees [[7], [8] and [9]]. We studied social bonds among dispersing male Assamese macaques (Macaca assamensis) to see whether males in multimale groups form differentiated social bonds and whether and how males derive fitness benefits from close bonds. We found that strong bonds were linked to coalition formation, whi! ch in turn predicted future social dominance, which influenced paternity success. The strength of males' social bonds was directly linked to the number of offspring they sired. Our results show that differentiated social relationships exert an important influence on the breeding success of both sexes that transcends contrasts in relatedness.
  • The Habenula Prevents Helpless Behavior in Larval Zebrafish
    - curr biol 20(24):2211-2216 (2010)
    Animals quickly learn to avoid predictable danger. However, if pre-exposed to a strong stressor, they do not display avoidance even if this causes continued contact with painful stimuli [[1] and [2]]. In rodents, lesioning the habenula, an epithalamic structure that regulates the monoaminergic system, has been reported to reduce avoidance deficits caused by inescapable shock [3]. This is consistent with findings that inability to overcome a stressor is accompanied by an increase in serotonin levels [4]. However, other studies conclude that habenula lesions cause avoidance deficits [[5] and [6]]. These contradictory results may be caused by lesions affecting unintended regions [6]. To clarify the role of the habenula, we used larval zebrafish, whose transparency and amenability to genetic manipulation enables more precise disruption of cells. We show that larval zebrafish learn to avoid a light that has been paired with a mild shock but fail to do so when pre-exposed to! inescapable shock. Photobleaching of habenula afferents expressing the photosensitizer KillerRed causes a similar failure in avoidance. Expression of tetanus toxin in dorsal habenula neurons is sufficient to prevent avoidance. We suggest that this region may signal the ability to control a stressor, and that its disruption could contribute to anxiety disorders.
  • Multigene Phylogeny of the Green Lineage Reveals the Origin and Diversification of Land Plants
    - curr biol 20(24):2217-2222 (2010)
    The Viridiplantae (green plants) include land plants as well as the two distinct lineages of green algae, chlorophytes and charophytes. Despite their critical importance for identifying the closest living relatives of land plants, phylogenetic studies of charophytes have provided equivocal results [[1], [2], [3], [4] and [5]]. In addition, many relationships remain unresolved among the land plants, such as the position of mosses, liverworts, and the enigmatic Gnetales. Phylogenomics has proven to be an insightful approach for resolving challenging phylogenetic issues, particularly concerning deep nodes [[6], [7] and [8]]. Here we extend this approach to the green lineage by assembling a multilocus data set of 77 nuclear genes (12,149 unambiguously aligned amino acid positions) from 77 taxa of plants. We therefore provide the first multigene phylogenetic evidence that Coleochaetales represent the closest living relatives of land plants. Moreover, our data reinforce the ! early divergence of liverworts and the close relationship between Gnetales and Pinaceae. These results provide a new phylogenetic framework and represent a key step in the evolutionary interpretation of developmental and genomic characters in green plants.
  • Arabidopsis thaliana Leaf Form Evolved via Loss of KNOX Expression in Leaves in Association with a Selective Sweep
    - curr biol 20(24):2223-2228 (2010)
    Morphological diversity is often caused by altered gene expression of key developmental regulators. However, the precise developmental trajectories through which morphologies evolved remain poorly understood. It is also unclear to what degree genetic changes contributing to morphological divergence were fixed by natural selection. Here we investigate these problems in the context of evolutionary developmental transitions that produced the simple unlobed leaf of the model species Arabidopsis thaliana. We demonstrate that A. thaliana leaf shape likely derived from a more complex lobed ancestral state that persists in extant Arabidopsis species. We also show that evolution of the unlobed leaf form in A. thaliana involved loss of expression of the knotted1-like homeobox gene SHOOTMERISTEMLESS (STM) in leaves and that cis-regulatory divergence contributed to this process. Further, we provide evidence for a selective sweep at the A. thaliana STM locus, indicating that loss o! f STM expression in A. thaliana leaves may have been fixed by positive selection. In summary, our data provide key information as to when and how the characteristic leaf form of A. thaliana evolved.
  • Reinforcement Can Overcome Gene Flow during Speciation in Drosophila
    - curr biol 20(24):2229-2233 (2010)
    Reinforcement, the strengthening of prezygotic reproductive isolation by natural selection in response to maladaptive hybridization [[1], [2] and [3]], is one of the few processes in which natural selection directly favors the evolution of species as discrete groups (e.g., [[4], [5], [6] and [7]]). The evolution of reproductive barriers via reinforcement is expected to evolve in regions where the ranges of two species overlap and hybridize as an evolutionary solution to avoiding the costs of maladaptive hybridization [[2], [3] and [8]]. The role of reinforcement in speciation has, however, been highly controversial because population-genetic theory suggests that the process is severely impeded by both hybridization [[8], [9], [10] and [11]] and migration of individuals from outside the contact zone [[12] and [13]]. To determine whether reinforcement could strengthen the reproductive barriers between two sister species of Drosophila in the face of these impediments, I i! nitiated experimental populations of these two species that allowed different degrees of hybridization, as well as migration from outside populations. Surprisingly, even in the face of gene flow, reinforcement could promote the evolution of reproductive isolation within only five generations. As theory predicts, high levels of hybridization (and/or strong selection against hybrids) and migration impeded this evolution. These results suggest that reinforcement can help complete the process of speciation.
  • Parallel Reinforcement Pathways for Conditioned Food Aversions in the Honeybee
    - curr biol 20(24):2234-2240 (2010)
    Avoiding toxins in food is as important as obtaining nutrition. Conditioned food aversions have been studied in animals as diverse as nematodes and humans [[1] and [2]], but the neural signaling mechanisms underlying this form of learning have been difficult to pinpoint. Honeybees quickly learn to associate floral cues with food [3], a trait that makes them an excellent model organism for studying the neural mechanisms of learning and memory. Here we show that honeybees not only detect toxins but can also learn to associate odors with both the taste of toxins and the postingestive consequences of consuming them. We found that two distinct monoaminergic pathways mediate learned food aversions in the honeybee. As for other insect species conditioned with salt or electric shock reinforcers [[4], [5], [6] and [7]], learned avoidances of odors paired with bad-tasting toxins are mediated by dopamine. Our experiments are the first to identify a second, postingestive pathway f! or learned olfactory aversions that involves serotonin. This second pathway may represent an ancient mechanism for food aversion learning conserved across animal lineages.

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