Hot off the presses! Mar 08 curr biol

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Latest Articles Include:

• The benefits of bottlenecks
  - curr biol 21(5):R171-R173 (2011)
  Population bottlenecks are commonly thought to be disadvantageous because they deplete genetic variation. But they can be advantageous too, in particular for biological invaders like the harlequin ladybird. Florian Maderspacher reports.
• Drosophila embryonic hemocytes
  - curr biol 21(5):R173-R174 (2011)
  
• Alexander Borst
  - curr biol 21(5):R175-R176 (2011)
  
• Sexual selection in plants
  - curr biol 21(5):R176-R182 (2011)
  Darwin's theory of evolution by natural selection provided an immediately convincing explanation for the close fit between form and function in nature that had previously only been explicable in terms of supernatural design. Traits evolved in a way that improved their bearer's chances of survival and its success at producing offspring. But what could be said of exaggerated ornamental traits such as the long and lurid tail feathers of many male birds and the ferocious looking mandibles and horned protuberances of various male insects, which were almost certain to compromise their bearer's survival? To explain these traits, Darwin proposed the theory of sexual selection, first in 'Origin of Species' and then, at greater length, in 'The Descent of Man'. In a nutshell, he argued that certain traits (secondary sex characters) will be favoured not because they improve survivorship or fecundity (i.e., by natural selection), but because they improve an individual's mat! ing success. This basic idea has been broadly accepted by zoologists, but it has been contentious when applied to plants, not least because they are often hermaphrodites. In this Primer, we explain the application of sexual-selection ideas to both dioecious and hermaphroditic plants. We point out that, far from being irrelevant to their study, sexual selection to increase male mating success can be interpreted as a major selective force in the evolution of floral diversity (Figure 1).
• Wake deterioration and sleep restoration of human learning
  - curr biol 21(5):R183-R184 (2011)
  While the benefit of sleep after learning in offline consolidation is established [1], a role for sleep before learning in promoting initial memory formation remains largely uncharacterized. Existing theoretical frameworks speculate that accrued time awake, associated with ongoing experience, decreases learning capacity, while specific non-rapid-eye-movement (NREM) oscillations support restoration of learning ability [1] and [2]. Despite these model predictions, it remains untested whether episodic learning capacity remains stable across the day, or is progressively compromised by continued time awake. Furthermore, it is similarly unclear whether the presence, rather than the detrimental absence, of sleep restores efficient learning ability, and if so, what aspect(s) of sleep physiology support such reinstatement [3] and [4]. We have tested these related hypotheses, and report here a learning interaction, such that episodic encoding capacity deteriorates across a dayti! me waking interval, but sleep and associated NREM spindle oscillations restore efficient learning ability.
• Cell Cycle: Who Turns the Crank?
  - curr biol 21(5):R185-R187 (2011)
  The oscillating activity of a single CDK–cyclin fusion protein can drive the orderly progression of yeast cells through DNA replication, mitosis and cell division.
• Structural Colour: Elusive Iridescence Strategies Brought to Light
  - curr biol 21(5):R187-R189 (2011)
  Understanding structural colours in nature requires the right set of optical experiments: this is illustrated by a new study on iridescent bird of paradise feathers, which suggests the potential behavioural importance of dynamic colour changes.
• Cognitive Neuroscience: Distinguishing Self from Other
  - curr biol 21(5):R189-R190 (2011)
  Neurons in medial frontal cortex have been found to distinguish between whether an animal or its partner is responding on a turn-taking task, but are they really the basis of a social learning mechanism?
• Climate: Baselines for the Biological Effects of Environmental Change
  - curr biol 21(5):R190-R192 (2011)
  Establishing biological baselines requires access to organisms which lived earlier in, or before, the present episode of anthropogenic change. Specimens of a bryozoan collected on Scott's Antarctic expeditions, and subsequently, provide clear evidence of recent increases in growth rate after 80 years of constancy.
• Host–Pathogen Interactions: Cheating the Host by Making New Connections
  - curr biol 21(5):R192-R194 (2011)
  Dynamic signaling networks are required to perform complex cellular processes. Structural and functional data now indicate the intriguing possibility that extracellular bacterial pathogens use catalytic scaffolds to assemble unique supramolecular signaling networks that effectively subvert key cellular processes in the host.
• Cooperative Breeding: A Question of Climate?
  - curr biol 21(5):R195-R197 (2011)
  In some species, including humans, parents receive help with offspring care. A new comparative study suggests that birds breed cooperatively when environmental conditions vary. Further empirical and theoretical work will be required to understand the evolutionary significance of this insight.
• Cell Polarity: PIN It Down!
  - curr biol 21(5):R197-R199 (2011)
  How do plants create and maintain cell polarity? Recent studies reveal a plant-specific mechanism, which links the static cellulose-based extracellular matrix to the dynamic localization of PIN auxin carrier proteins.
• Climbing Plants: Attachment and the Ascent for Light
  - curr biol 21(5):R199-R201 (2011)
  How do vines climb upward and harvest sunlight? A detailed study of one species (Galium aparine L.) shows that leaf hairs (trichomes) provide one solution that solves both tasks simultaneously.
• Plant Stem Cell Signaling Involves Ligand-Dependent Trafficking of the CLAVATA1 Receptor Kinase
  - curr biol 21(5):345-352 (2011)
  Background Cell numbers in above-ground meristems of plants are thought to be maintained by a feedback loop driven by perception of the glycopeptide ligand CLAVATA3 (CLV3) by the CLAVATA1 (CLV1) receptor kinase and the CLV2/CORYNE (CRN) receptor-like complex [1]. CLV3 produced in the stem cells at the meristem apex limits the expression level of the stem cell-promoting homeodomain protein WUSCHEL (WUS) in the cells beneath, where CLV1 and WUS RNA are localized. WUS downregulation nonautonomously reduces stem cell proliferation. Overexpression of CLV3 eliminates the stem cells, causing meristem termination [2], and loss of CLV3 function allows meristem overproliferation [3]. There are many questions regarding the CLV3/CLV1 interaction, including where in the meristem it occurs, how it is regulated, and how it is that a large range of CLV3 concentrations gives no meristem size phenotype [4]. Results Here we use genetics and live imaging to examine the cell biology of CLV1 in Arabidopsis meristematic tissue. We demonstrate that plasma membrane-localized CLV1 is reduced in concentration by CLV3, which causes trafficking of CLV1 to lytic vacuoles. We find that changes in CLV2 activity have no detectable effects on CLV1 levels. We also find that CLV3 appears to diffuse broadly in meristems, contrary to a recent sequestration model [5]. Conclusions This study provides a new model for CLV1 function in plant stem cell maintenance and suggests that downregulation of plasma membrane-localized CLV1 by its CLV3 ligand can account for the buffering of CLV3 signaling in the maintenance of stem cell pools in plants.
• Regulation of Response Properties and Operating Range of the AFD Thermosensory Neurons by cGMP Signaling
  - curr biol 21(5):353-362 (2011)
  Background The neuronal mechanisms that encode specific stimulus features in order to elicit defined behavioral responses are poorly understood. C. elegans forms a memory of its cultivation temperature (Tc) and exhibits distinct behaviors in different temperature ranges relative to Tc. In particular, C. elegans tracks isotherms only in a narrow temperature band near Tc. Tc memory is in part encoded by the threshold of responsiveness (T*AFD) of the AFD thermosensory neuron pair to temperature stimuli. However, because AFD thermosensory responses appear to be similar at all examined temperatures above T*AFD, the mechanisms that generate specific behaviors in defined temperature ranges remain to be determined. Results Here, we show that the AFD neurons respond to the sinusoidal variations in thermal stimuli followed by animals during isothermal tracking (IT) behavior only in a narrow temperature range near Tc. We find that mutations in the AFD-expressed gcy-8 receptor guanylyl cyclase (rGC) gene result in defects in the execution of IT behavior and are associated with defects in the responses of the AFD neurons to oscillating thermal stimuli. In contrast, mutations in the gcy-18 or gcy-23 rGCs alter the temperature range in which IT behavior is exhibited. Alteration of intracellular cGMP levels via rGC mutations or addition of cGMP analogs shift the lower and upper ranges of the temperature range of IT behavior in part via alteration in T*AFD. Conclusions Our observations provide insights into the mechanisms by which a single sensory neuron type encodes features of a given stimulus to generate different behaviors in defined zones.
• A Ventral Visual Stream Reading Center Independent of Visual Experience
  - curr biol 21(5):363-368 (2011)
  The visual word form area (VWFA) is a ventral stream visual area that develops expertise for visual reading [[1], [2] and [3]]. It is activated across writing systems and scripts [[4] and [5]] and encodes letter strings irrespective of case, font, or location in the visual field [1] with striking anatomical reproducibility across individuals [6]. In the blind, comparable reading expertise can be achieved using Braille. This study investigated which area plays the role of the VWFA in the blind. One would expect this area to be at either parietal or bilateral occipital cortex, reflecting the tactile nature of the task and crossmodal plasticity, respectively [[7] and [8]]. However, according to the metamodal theory [9], which suggests that brain areas are responsive to a specific representation or computation regardless of their input sensory modality, we predicted recruitment of the left-hemispheric VWFA, identically to the sighted. Using functional magnetic resonance im! aging, we show that activation during Braille reading in blind individuals peaks in the VWFA, with striking anatomical consistency within and between blind and sighted. Furthermore, the VWFA is reading selective when contrasted to high-level language and low-level sensory controls. Thus, we propose that the VWFA is a metamodal reading area that develops specialization for reading regardless of visual experience.
• Kinetic Analysis Reveals the Fate of a MicroRNA following Target Regulation in Mammalian Cells
  - curr biol 21(5):369-376 (2011)
  Considerable details about microRNA (miRNA) biogenesis and regulation have been uncovered, but little is known about the fate of the miRNA subsequent to target regulation. To gain insight into this process, we carried out kinetic analysis of a miRNA's turnover following termination of its biogenesis and during regulation of a target that is not subject to Ago2-mediated catalytic cleavage. By quantitating the number of molecules of the miRNA and its target in steady state and in the course of its decay, we found that each miRNA molecule was able to regulate at least two target transcripts, providing in vivo evidence that the miRNA is not irreversibly sequestered with its target and that the nonslicing pathway of miRNA regulation is multiple-turnover. Using deep sequencing, we further show that miRNA recycling is limited by target regulation, which promotes posttranscriptional modifications to the 3′ end of the miRNA and accelerates the miRNA's rate of decay. These stu! dies provide new insight into the efficiency of miRNA regulation that help to explain how a miRNA can regulate a vast number of transcripts and that identify one of the mechanisms that impart specificity to miRNA decay in mammalian cells.
• A Sensory Code for Host Seeking in Parasitic Nematodes
  - curr biol 21(5):377-383 (2011)
  Parasitic nematode species often display highly specialized host-seeking behaviors that reflect their specific host preferences. Many such behaviors are triggered by host odors, but little is known about either the specific olfactory cues that trigger these behaviors or the underlying neural circuits. Heterorhabditis bacteriophora and Steinernema carpocapsae are phylogenetically distant insect-parasitic nematodes whose host-seeking and host-invasion behavior resembles that of some devastating human- and plant-parasitic nematodes. We compare the olfactory responses of Heterorhabditis and Steinernema infective juveniles (IJs) to those of Caenorhabditis elegans dauers, which are analogous life stages [1]. The broad host range of these parasites results from their ability to respond to the universally produced signal carbon dioxide (CO2), as well as a wide array of odors, including host-specific odors that we identified using thermal desorption-gas chromatography-mass spec! troscopy. We find that CO2 is attractive for the parasitic IJs and C. elegans dauers despite being repulsive for C. elegans adults [[2], [3] and [4]], and we identify a sensory neuron that mediates CO2 response in both parasitic and free-living species, regardless of whether CO2 is attractive or repulsive. The parasites' odor response profiles are more similar to each other than to that of C. elegans despite their greater phylogenetic distance, likely reflecting evolutionary convergence to insect parasitism.
• The Formin DAD Domain Plays Dual Roles in Autoinhibition and Actin Nucleation
  - curr biol 21(5):384-390 (2011)
  Formins are a large family of actin assembly-promoting proteins with many important biological roles [[1], [2] and [3]]. However, it has remained unclear how formins nucleate actin polymerization. All other nucleators are known to recruit actin monomers as a central part of their mechanisms [[3], [4] and [5]]. However, the actin-nucleating FH2 domain of formins lacks appreciable affinity for monomeric actin [[6] and [7]]. Here, we found that yeast and mammalian formins bind actin monomers but that this activity requires their C-terminal DAD domains. Furthermore, we observed that the DAD works in concert with the FH2 to enhance nucleation without affecting the rate of filament elongation. We dissected this mechanism in mDia1, mapped nucleation activity to conserved residues in the DAD, and demonstrated that DAD roles in nucleation and autoinhibition are separable. Furthermore, DAD enhancement of nucleation was independent of contributions from the FH1 domain to nucleati! on [8]. Together, our data show that (1) the DAD has dual functions in autoinhibition and nucleation; (2) the FH1, FH2, and DAD form a tripartite nucleation machine; and (3) formins nucleate by recruiting actin monomers and therefore are more similar to other nucleators than previously thought.
• Direct Binding of Cenp-C to the Mis12 Complex Joins the Inner and Outer Kinetochore
  - curr biol 21(5):391-398 (2011)
  Kinetochores are proteinaceous scaffolds implicated in the formation of load-bearing attachments of chromosomes to microtubules during mitosis. Kinetochores contain distinct chromatin- and microtubule-binding interfaces, generally defined as the inner and outer kinetochore, respectively (reviewed in [1]). The constitutive centromere-associated network (CCAN) and the Knl1-Mis12-Ndc80 complexes (KMN) network are the main multisubunit protein assemblies in the inner and outer kinetochore, respectively. The point of contact between the CCAN and the KMN network is unknown. Cenp-C is a conserved CCAN component whose central and C-terminal regions have been implicated in chromatin binding and dimerization [[2], [3], [4], [5], [6], [7], [8], [9] and [10]]. Here, we show that a conserved motif in the N-terminal region of Cenp-C binds directly and with high affinity to the Mis12 complex. Expression in HeLa cells of the isolated N-terminal motif of Cenp-C prevents outer kinetocho! re assembly, causing chromosome missegregation. The KMN network is also responsible for kinetochore recruitment of the components of the spindle assembly checkpoint, and we observe checkpoint impairment in cells expressing the Cenp-C N-terminal segment. Our studies unveil a crucial and likely universal link between the inner and outer kinetochore.
• CENP-C Is a Structural Platform for Kinetochore Assembly
  - curr biol 21(5):399-405 (2011)
  Centromeres provide a region of chromatin upon which kinetochores are assembled in mitosis [[1] and [2]]. Centromeric protein C (CENP-C) is a core component of this centromeric chromatin [[3] and [4]] that, when depleted, prevents the proper formation of both centromeres and kinetochores [[5], [6], [7], [8], [9] and [10]]. CENP-C localizes to centromeres throughout the cell cycle via its C-terminal part [[6] and [8]], whereas its N-terminal part appears necessary for recruitment of some but not all components of the Mis12 complex of the kinetochore [8]. We now find that all kinetochore proteins belonging to the KMN (KNL1/Spc105, the Mis12 complex, and the Ndc80 complex) network [1] bind to the N-terminal part of Drosophila CENP-C. Moreover, we show that the Mis12 complex component Nnf1 interacts directly with CENP-C in vitro. To test whether CENP-C's N-terminal part was sufficient to recruit KMN proteins, we targeted it to the centrosome by fusing it to a domain of Plk! 4 kinase [11]. The Mis12 and Ndc80 complexes and Spc105 protein were then all recruited to centrosomes at the expense of centromeres, leading to mitotic abnormalities typical of cells with defective kinetochores. Thus, the N-terminal part of Drosophila CENP-C is sufficient to recruit core kinetochore components and acts as the principal linkage between centromere and kinetochore during mitosis.
• Long- and Short-Range Transcriptional Repressors Induce Distinct Chromatin States on Repressed Genes
  - curr biol 21(5):406-412 (2011)
  Transcriptional repression is essential for establishing precise patterns of gene expression during development [1]. Repressors governing early Drosophila segmentation can be classified as short- or long-range factors based on their ranges of action, acting either locally to quench adjacent activators or broadly to silence an entire locus [2]. Paradoxically, these repressors recruit common corepressors, Groucho and CtBP, despite their different ranges of repression [[3], [4], [5], [6] and [7]]. To reveal the mechanisms underlying these two distinct modes of repression, we performed chromatin analysis using the prototypical long-range repressor Hairy and the short-range repressor Knirps. Chromatin immunoprecipitation and micrococcal nuclease mapping studies reveal that Knirps causes local changes of histone density and acetylation, and the inhibition of activator recruitment, without affecting the recruitment of basal transcriptional machinery. In contrast, Hairy induce! s widespread histone deacetylation and inhibits the recruitment of basal machinery without inducing chromatin compaction. Our study provides detailed mechanistic insight into short- and long-range repression on selected endogenous target genes and suggests that the transcriptional corepressors can be differentially deployed to mediate chromatin changes in a context-dependent manner.
• Social Group Size Predicts the Evolution of Individuality
  - curr biol 21(5):413-417 (2011)
  Discriminating among individuals is a critical social behavior in humans and many other animals [[1], [2] and [3]] and is often required for offspring and mate recognition, territorial or coalitional behaviors, signaler reliability assessment, and social hierarchies [[4], [5], [6], [7], [8] and [9]]. Being individually discriminated is more difficult in larger groups, and large group size may select for increased individuality–signature information–in social signals, to facilitate discrimination [[4], [10], [11], [12], [13] and [14]]. Small-scale studies suggest that more social species have greater individuality in their social signals, such as contact calls [[4], [12] and [13]]. However, this relationship has not been evaluated in a broader-scale evolutionary context or in social signals other than contact calls. It is not yet known whether social group size may be viewed as a general evolutionary driver of individuality. Here we show a strong positive evolutiona! ry link between social group size in sciurid rodents and individuality in their social alarm calls. Social group size explained over 88% of the variation in vocal individuality in phylogenetic independent contrasts. Species living in larger groups, but not in more complex groups, had more signature information in their calls. Our results suggest that social group size may promote the evolution of individual signatures and that the sociality-individuality relationship may be a general phenomenon in nature.
• Magnetoreception in an Avian Brain in Part Mediated by Inner Ear Lagena
  - curr biol 21(5):418-423 (2011)
  Many animals use the Earth's geomagnetic field for orientation and navigation, but the neural mechanisms underlying that ability remain enigmatic [[1] and [2]]. Support for at least two avian magnetoreceptors exists, including magnetically activated photochemicals in the retina [[3] and [4]] and ferrimagnetic particles in the beak [[5] and [6]]. The possibility of a third magnetoreceptor in the inner ear lagena organs has been suggested [7]. The brain must process magnetic receptor information to derive constructs representing directional heading and geosurface location. Here, we used the c-Fos transcription factor, a marker for activated neurons [8], to discover where in the brain computations related to a specific set of magnetic field stimulations occur. We found that neural activations in discrete brain loci known to be involved in orientation, spatial memory, and navigation may constitute a major magnetoreception pathway in birds. We also found, through ablation s! tudies, that much of the observed pathway appears to receive magnetic information from the pigeon lagena receptor organs.
• Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis
  - curr biol 21(5):424-427 (2011)
  Bottlenecks in population size reduce genetic diversity and increase inbreeding, which can lead to inbreeding depression [1]. It is thus puzzling how introduced species, which typically pass through bottlenecks, become such successful invaders [2]. However, under certain theoretical conditions, bottlenecks of intermediate size can actually purge the alleles that cause inbreeding depression [3]. Although this process has been confirmed in model laboratory systems [4], it has yet to be observed in natural invasive populations. We evaluate whether such purging could facilitate biological invasions by using the world-wide invasion of the ladybird (or ladybug) Harmonia axyridis. We first show that invasive populations endured a bottleneck of intermediate intensity. We then demonstrate that replicate introduced populations experience almost none of the inbreeding depression suffered by native populations. Thus, rather than posing a barrier to invasion as often assumed, bottl! enecks, by purging deleterious alleles, can enable the evolution of invaders that maintain high fitness even when inbred.
• Identification of a Polo-like Kinase 4-Dependent Pathway for De Novo Centriole Formation
  - curr biol 21(5):428-432 (2011)
  Supernumerary centrosomes are a key cause of genomic instability in cancer cells [1]. New centrioles can be generated by duplication with a mother centriole as a platform or, in the absence of preexisting centrioles, by formation de novo [2]. Polo-like kinase 4 (Plk4) regulates both modes of centriole biogenesis, and Plk4 deregulation has been linked to tumor development [[1] and [3]]. We show that Plx4, the Xenopus homolog of mammalian Plk4 and Drosophila Sak, induces de novo centriole formation in vivo in activated oocytes and in egg extracts, but not in immature or in vitro matured oocytes. Both kinase activity and the polo-box domain of Plx4 are required for de novo centriole biogenesis. Polarization microscopy in "cycling" egg extracts demonstrates that de novo centriole formation is independent of Cdk2 activity, a major difference compared to template-driven centrosome duplication that is linked to the nuclear cycle and requires cyclinA/E/Cdk2. Moreover, we s! how that the Mos-MAPK pathway blocks Plx4-dependent de novo centriole formation before fertilization, thereby ensuring paternal inheritance of the centrosome. The results define a new system for studying the biochemical and molecular basis of de novo centriole formation and centriole biogenesis in general.
• Single-Locus Recessive Inheritance of Asexual Reproduction in a Parasitoid Wasp
  - curr biol 21(5):433-437 (2011)
  The evolutionary maintenance of sex is one of the big unresolved puzzles in biology [1]. All else being equal, all-female asexual populations should enjoy a two-fold reproductive advantage over sexual relatives consisting of male and female individuals [1]. However, the "all else being equal" assumption rarely holds in real organisms because asexuality tends to be confounded with altered genomic constitutions such as hybridization [2] and polyploidization [3] or to be associated with parthenogenesis-inducing microbes [[4] and [5]]. This limits the ability to draw general conclusions from any particular system. Here we describe a new system that permits unbiased comparisons of sexual and asexual reproduction: the parasitic wasp Lysiphlebus fabarum. Crossing experiments demonstrated that asexual reproduction has a simple genetic basis in this species and is consistently inherited as a single-locus recessive trait. We further show that the asexuality-inducing allele e! xhibits complete linkage to a specific allele at a microsatellite marker: all asexual lines in the field were homozygous for this allele, and the allele cosegregated perfectly with asexual reproduction in our experimental crossings. This novel system of contagious asexuality allows the production of closely related individuals with different reproductive modes, as well as the monitoring of the asexuality-inducing allele in natural and experimental populations.
• Environmental Uncertainty and the Global Biogeography of Cooperative Breeding in Birds
  - curr biol 21(5):438 (2011)