Hot off the presses! Aug 23 Curr Biol

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Latest Articles Include:

• What exactly is synthetic biology?
  - Curr Biol 21(16):R611-R614 (2011)
  Having spent the 20th century taking living things apart, biologists are now beginning to put pieces together in new forms. Synthetic biology emerges as a new discipline with a few success stories, but no clear direction yet. Michael Gross reports.
• The Lombard effect
  - Curr Biol 21(16):R614-R615 (2011)
  
• Coprinopsis cinerea
  - Curr Biol 21(16):R616-R617 (2011)
  
• Rebecca Safran
  - Curr Biol 21(16):R617-R618 (2011)
  
• Neural basis of mathematical cognition
  - Curr Biol 21(16):R618-R621 (2011)
  The human brain has remarkable capabilities for encoding and manipulating information about quantities. Understanding how the brain carries out such number and quantity processing is a problem not just for those interested in numerical cognition: it raises important questions that are relevant to understanding development, action, vision, language, executive function and cortical organisation. It is also a clear case of research into a core human psychological function having indisputable everyday relevance; hence the emphasis in early education on numeracy and later on mathematics.
• Genetic detection of mislabeled fish from a certified sustainable fishery
  - Curr Biol 21(16):R621-R622 (2011)
  The decline and collapse of many of the world's fisheries has led to the implementation of social marketing that promotes the consumption of sustainably harvested seafood [1] and [2]. Because the success of this strategy depends on supply chain integrity, we investigated the accuracy of eco-labels for Patagonian toothfish, marketed as 'Chilean sea bass' (Dissostichus eleginoides), by genetically analyzing retail fish bearing certification labels from the Marine Stewardship Council (MSC). For Chilean sea bass, MSC certification labels indicate that fish were harvested from the only sustainable fishery [3] and [4], a population in waters surrounding the sub-Antarctic island of South Georgia and the nearby plateau at Shag Rocks [3]. We found that not all MSC-certified fish were Chilean sea bass from the certified stock: some were simply not D. eleginoides, but among those that were, we found significant genetic differences between the retail sample of fish and the cer! tified stock population. Uncertified fish may not necessarily resemble stocks closest to their country of origin because capture and processing often occur at different places. However, significant differences between MSC-certified Chilean sea bass and the sole certified fishery for this species indicate that uncertified fish were inserted into the MSC supply chain.
• Endosymbiosis: Bacteria Sharing the Load
  - Curr Biol 21(16):R623-R624 (2011)
  A nested set of bacterial endosymbionts within mealybug cells collectively provides amino acids to their host, but their genomes show that some pathways are distributed between both endosymbionts, while other essential proteins are missing altogether. The possibility that additional functions are shared between partners warrants comparisons with organelles.
• Metabolism: What Causes the Gut's Circadian Instincts?
  - Curr Biol 21(16):R624-R626 (2011)
  What mechanisms control circadian rhythms in the gastrointestinal tract and how does this impact nutrient metabolism? The deadenylase and leucine zipper protein Nocturnin is now shown to play a central role.
• Yeast Mating: Trying Out New Pickup Lines
  - Curr Biol 21(16):R626-R628 (2011)
  Mating of Ascomycete fungi involves chemically distinct pheromones; one partner makes a lipid-modified peptide, the other partner a simple peptide. A new study has now found that this inherent asymmetry may not be necessary.
• Bacterial Cell Wall: Thinking Globally, Actin Locally
  - Curr Biol 21(16):R628-R630 (2011)
  The bacterial actin-like protein MreB is thought to form a continuous helical polymer at the membrane to confer rod shape. Two new studies now show that MreB forms discrete dynamic patches that travel circumferentially.
• Oogenesis: Active Heterochromatin
  - Curr Biol 21(16):R630-R632 (2011)
  The genome of Drosophila is protected from DNA damage during oogenesis by a mechanism involving short RNAs. Surprisingly transcription of these RNAs requires that their DNA is associated with a histone modification usually associated with gene silencing.
• Sensory Transduction: The 'Swarm Intelligence' of Auditory Hair Bundles
  - Curr Biol 21(16):R632-R634 (2011)
  In vertebrate hair cells, the hair bundle is responsible for the conversion of mechanical vibrations into electrical signals. In a combined experimental and computational tour de force, a group of researchers now presents a quantitative model that explains how the bundle's specific microarchitecture gives rise to its exquisite mechanosensory properties.
• Chromosome Segregation: Monopolin Attracts Condensin
  - Curr Biol 21(16):R634-R636 (2011)
  To segregate chromosomes properly, the cell must prevent merotely, an error that occurs when a single kinetochore is attached to microtubules emanating from both spindle poles. Recent evidence suggests that cooperation between Pcs1/Mde4 and condensin complexes plays an important role in preventing merotely.
• Circadian Rhythms: FLOWERING LOCUS T Extends Opening Hours
  - Curr Biol 21(16):R636-R638 (2011)
  Plants are more sensitive to light in the day than at night due to the circadian clock. The protein that acts downstream from the clock to modulate blue light signalling in stomata comes as a surprise; it is FT, which is thought to be the long-distance regulator of flowering.
• DNA Replication: Mammalian Treslin–TopBP1 Interaction Mirrors Yeast Sld3–Dpb11
  - Curr Biol 21(16):R638-R640 (2011)
  There are many parallels between DNA replication in yeast and humans. Now, two recent studies extend this relationship by dissecting key conserved interactions necessary for initiation of the replisome.
• Asymmetry in Sexual Pheromones Is Not Required for Ascomycete Mating
  - Curr Biol 21(16):1337-1346 (2011)
  Background We investigated the determinants of sexual identity in the budding yeast Saccharomyces cerevisiae. The higher fungi are divided into the ascomycetes and the basidiomycetes. Most ascomycetes have two mating types: one (called α in yeasts and MAT1-1 in filamentous fungi) produces a small, unmodified, peptide pheromone, and the other (a in yeasts and MAT1-2 in filamentous fungi) produces a peptide pheromone conjugated to a C-terminal farnesyl group that makes it very hydrophobic. In the basidiomycetes, all pheromones are lipid-modified, and this difference is a distinguishing feature between the phyla. We asked whether the asymmetry in pheromone modification is required for successful mating in ascomycetes. Results We cloned receptor and pheromone genes from a filamentous ascomycete and a basidiomycete and expressed these in the budding yeast, Saccharomyces cerevisiae, to generate novel, alternative mating pairs. We find that two yeast cells can mate even when both cells secrete a-like or α-like peptides. Importantly, this is true regardless of whether the cells express the a- or α-mating-type loci, which control the expression of other, sex-specific genes, in addition to the pheromones and pheromone receptors. Conclusions We demonstrate that the asymmetric pheromone modification is not required for successful mating of ascomycete fungi and confirm that, in budding yeast, the primary determinants of mating are the specificity of the receptors and their corresponding pheromones.
• Nocturnin Regulates Circadian Trafficking of Dietary Lipid in Intestinal Enterocytes
  - Curr Biol 21(16):1347-1355 (2011)
  Background Efficient metabolic function in mammals depends on the circadian clock, which drives temporal regulation of metabolic processes. Nocturnin is a clock-regulated deadenylase that controls its target mRNA expression posttranscriptionally through poly(A) tail removal. Mice lacking nocturnin (Noc−/− mice) are resistant to diet-induced obesity and hepatic steatosis yet are not hyperactive or hypophagic. Results Here we show that nocturnin is expressed rhythmically in the small intestine and is induced by olive oil gavage and that the Noc−/− mice have reduced chylomicron transit into the plasma following the ingestion of dietary lipids. Genes involved in triglyceride synthesis and storage and chylomicron formation have altered expression, and large cytoplasmic lipid droplets accumulate in the apical domains of the Noc−/− enterocytes. The physiological significance of this deficit in absorption is clear because maintenance of Noc−/− mice on diets that challenge the chylomicron synthesis pathway result in significant reductions in body weight, whereas diets that bypass this pathway do not. Conclusions Therefore, we propose that nocturnin plays an important role in the trafficking of dietary lipid in the intestinal enterocytes by optimizing efficient absorption of lipids.
• A Complex of Kif18b and MCAK Promotes Microtubule Depolymerization and Is Negatively Regulated by Aurora Kinases
  - Curr Biol 21(16):1356-1365 (2011)
  Introduction Spindle assembly requires tight control of microtubule (MT) dynamics. This is dependent on a variety of MT binding proteins and their upstream regulators. The Aurora kinases have several well-described functions during cell division, but it remains unclear whether they control global spindle microtubule dynamics. Results Here, we find that simultaneous inhibition of Aurora A and B results in a dramatic decrease in spindle MT stability, and we identify the uncharacterized kinesin-8 Kif18b as a mediator of this effect. In interphase, Kif18b is nuclear, but upon nuclear envelope breakdown, Kif18b binds to astral MT plus ends through an interaction with EB1. Surprisingly, Kif18b also binds to the kinesin-13 motor MCAK, and this interaction is required for robust MT depolymerization. Furthermore, the Kif18b-MCAK interaction is negatively regulated by Aurora kinases through phosphorylation of MCAK, indicating that Aurora kinases regulate MT plus-end stability in mitosis through control of Kif18b-MCAK complex formation. Conclusion Together, these results uncover a novel role for Aurora kinases in regulating spindle MT dynamics through Kif18b-MCAK and suggest that the Kif18b-MCAK complex constitutes the major MT plus-end depolymerizing activity in mitotic cells.
• An Interdependent Metabolic Patchwork in the Nested Symbiosis of Mealybugs
  - Curr Biol 21(16):1366-1372 (2011)
  Highly reduced genomes of 144–416 kilobases have been described from nutrient-provisioning bacterial symbionts of several insect lineages [[1], [2], [3], [4] and [5]]. Some host insects have formed stable associations with pairs of bacterial symbionts that live in specialized cells and provide them with essential nutrients; genomic data from these systems have revealed remarkable levels of metabolic complementarity between the symbiont pairs [[3], [4], [6] and [7]]. The mealybug Planococcus citri (Hemiptera: Pseudococcidae) contains dual bacterial symbionts existing with an unprecedented organization: an unnamed gammaproteobacteria, for which we propose the name Candidatus Moranella endobia, lives inside the betaproteobacteria Candidatus Tremblaya princeps [8]. Here we describe the complete genomes and metabolic contributions of these unusual nested symbionts. We show that whereas there is little overlap in retained genes involved in nutrient production between symbi! onts, several essential amino acid pathways in the mealybug assemblage require a patchwork of interspersed gene products from Tremblaya, Moranella, and possibly P. citri. Furthermore, although Tremblaya has the smallest cellular genome yet described, it contains a genomic inversion present in both orientations in individual insects, starkly contrasting with the extreme structural stability typical of highly reduced bacterial genomes [[4], [9] and [10]].
• piRNA Production Requires Heterochromatin Formation in Drosophila
  - Curr Biol 21(16):1373-1379 (2011)
  Protecting the genome from transposable element (TE) mobilization is critical for germline development. In Drosophila, Piwi proteins and their bound small RNAs (piRNAs) provide a potent defense against TE activity. TE targeting piRNAs are processed from TE-dense heterochromatic loci termed piRNA clusters. Although piRNA biogenesis from cluster precursors is beginning to be understood, little is known about piRNA cluster transcriptional regulation. Here, we show that deposition of histone 3 lysine 9 by the methyltransferase dSETDB1 (egg) is required for piRNA cluster transcription. In the absence of dSETDB1, cluster precursor transcription collapses in germline and somatic gonadal cells and TEs are activated, resulting in germline loss and a block in germline stem cell differentiation. We propose that heterochromatin protects the germline by activating the piRNA pathway.
• Spatiotopic Visual Maps Revealed by Saccadic Adaptation in Humans
  - Curr Biol 21(16):1380-1384 (2011)
  Saccadic adaptation [1] is a powerful experimental paradigm to probe the mechanisms of eye movement control and spatial vision, in which saccadic amplitudes change in response to false visual feedback. The adaptation occurs primarily in the motor system [[2] and [3]], but there is also evidence for visual adaptation, depending on the size and the permanence of the postsaccadic error [[4], [5], [6] and [7]]. Here we confirm that adaptation has a strong visual component and show that the visual component of the adaptation is spatially selective in external, not retinal coordinates. Subjects performed a memory-guided, double-saccade, outward-adaptation task designed to maximize visual adaptation and to dissociate the visual and motor corrections. When the memorized saccadic target was in the same position (in external space) as that used in the adaptation training, saccade targeting was strongly influenced by adaptation (even if not matched in retinal or cranial position)! , but when in the same retinal or cranial but different external spatial position, targeting was unaffected by adaptation, demonstrating unequivocal spatiotopic selectivity. These results point to the existence of a spatiotopic neural representation for eye movement control that adapts in response to saccade error signals.
• Regenerant Arabidopsis Lineages Display a Distinct Genome-Wide Spectrum of Mutations Conferring Variant Phenotypes
  - Curr Biol 21(16):1385-1390 (2011)
  Multicellular organisms can be regenerated from totipotent differentiated somatic cell or nuclear founders [[1], [2] and [3]]. Organisms regenerated from clonally related isogenic founders might a priori have been expected to be phenotypically invariant. However, clonal regenerant animals display variant phenotypes caused by defective epigenetic reprogramming of gene expression [2], and clonal regenerant plants exhibit poorly understood heritable phenotypic ("somaclonal") variation [[4], [5], [6] and [7]]. Here we show that somaclonal variation in regenerant Arabidopsis lineages is associated with genome-wide elevation in DNA sequence mutation rate. We also show that regenerant mutations comprise a distinctive molecular spectrum of base substitutions, insertions, and deletions that probably results from decreased DNA repair fidelity. Finally, we show that while regenerant base substitutions are a likely major genetic cause of the somaclonal variation of regenerant ! Arabidopsis lineages, transposon movement is unlikely to contribute substantially to that variation. We conclude that the phenotypic variation of regenerant plants, unlike that of regenerant animals, is substantially due to DNA sequence mutation.
• Expansion-Repression Mechanism for Scaling the Dpp Activation Gradient in Drosophila Wing Imaginal Discs
  - Curr Biol 21(16):1391-1396 (2011)
  Maintaining a proportionate body plan requires the adjustment or scaling of organ pattern with organ size. Scaling is a general property of developmental systems, yet little is known about its underlying molecular mechanisms. Using theoretical modeling, we examine how the Dpp activation gradient in the Drosophila wing imaginal disc scales with disc size. We predict that scaling is achieved through an expansion-repression mechanism [1] whose mediator is the widely diffusible protein Pentagone (Pent). Central to this mechanism is the repression of pent expression by Dpp signaling, which provides an effective size measurement, and the Pent-dependent expansion of the Dpp gradient, which adjusts the gradient with tissue size. We validate this mechanism experimentally by demonstrating that scaling requires Pent and further, that scaling is abolished when pent is ubiquitously expressed. The expansion-repression circuit can be readily implemented by a variety of molecular inte! ractions, suggesting its general utilization for scaling morphogen gradients during development.
• Differential Prefrontal White Matter Development in Chimpanzees and Humans
  - Curr Biol 21(16):1397-1402 (2011)
  A comparison of developmental patterns of white matter (WM) within the prefrontal region between humans and nonhuman primates is key to understanding human brain evolution. WM mediates complex cognitive processes and has reciprocal connections with posterior processing regions [[1] and [2]]. Although the developmental pattern of prefrontal WM in macaques differs markedly from that in humans [3], this has not been explored in our closest evolutionary relative, the chimpanzee. The present longitudinal study of magnetic resonance imaging scans demonstrated that the prefrontal WM volume in chimpanzees was immature and had not reached the adult value during prepuberty, as observed in humans but not in macaques. However, the rate of prefrontal WM volume increase during infancy was slower in chimpanzees than in humans. These results suggest that a less mature and more protracted elaboration of neuronal connections in the prefrontal portion of the developing brain existed in t! he last common ancestor of chimpanzees and humans, and that this served to enhance the impact of postnatal experiences on neuronal connectivity. Furthermore, the rapid development of the human prefrontal WM during infancy may help the development of complex social interactions, as well as the acquisition of experience-dependent knowledge and skills to shape neuronal connectivity.
• Speech Facilitation by Left Inferior Frontal Cortex Stimulation
  - Curr Biol 21(16):1403-1407 (2011)
  Electrophysiological studies in humans and animals suggest that noninvasive neurostimulation methods such as transcranial direct current stimulation (tDCS) can elicit long-lasting [1], polarity-dependent [2] changes in neocortical excitability. Application of tDCS can have significant and selective behavioral consequences that are associated with the cortical location of the stimulation electrodes and the task engaged during stimulation [[3], [4], [5], [6], [7] and [8]]. However, the mechanism by which tDCS affects human behavior is unclear. Recently, functional magnetic resonance imaging (fMRI) has been used to determine the spatial topography of tDCS effects [[9], [10], [11], [12] and [13]], but no behavioral data were collected during stimulation. The present study is unique in this regard, in that both neural and behavioral responses were recorded using a novel combination of left frontal anodal tDCS during an overt picture-naming fMRI study. We found that tDCS had! significant behavioral and regionally specific neural facilitation effects. Furthermore, faster naming responses correlated with decreased blood oxygen level-dependent (BOLD) signal in Broca's area. Our data support the importance of Broca's area within the normal naming network and as such indicate that Broca's area may be a suitable candidate site for tDCS in neurorehabilitation of anomic patients, whose brain damage spares this region.
• Voice Cells in the Primate Temporal Lobe
  - Curr Biol 21(16):1408-1415 (2011)
  Communication signals are important for social interactions and survival and are thought to receive specialized processing in the visual and auditory systems. Whereas the neural processing of faces by face clusters and face cells has been repeatedly studied [[1], [2], [3], [4] and [5]], less is known about the neural representation of voice content. Recent functional magnetic resonance imaging (fMRI) studies have localized voice-preferring regions in the primate temporal lobe [[6] and [7]], but the hemodynamic response cannot directly assess neurophysiological properties. We investigated the responses of neurons in an fMRI-identified voice cluster in awake monkeys, and here we provide the first systematic evidence for voice cells. "Voice cells" were identified, in analogy to "face cells," as neurons responding at least 2-fold stronger to conspecific voices than to "nonvoice" sounds or heterospecific voices. Importantly, whereas face clusters are thought to ! contain high proportions of face cells [4] responding broadly to many faces [[1], [2], [4], [5], [8], [9] and [10]], we found that voice clusters contain moderate proportions of voice cells. Furthermore, individual voice cells exhibit high stimulus selectivity. The results reveal the neurophysiological bases for fMRI-defined voice clusters in the primate brain and highlight potential differences in how the auditory and visual systems generate selective representations of communication signals.
• A Bias Caused by Ectopic Development Produces Sexually Dimorphic Sperm in Nematodes
  - Curr Biol 21(16):1416-1420 (2011)
  Self-fertile hermaphrodites have evolved independently several times in the genus Caenorhabditis [[1] and [2]]. These XX hermaphrodites make smaller sperm than males [[3] and [4]], which they use to fertilize their own oocytes. Because larger sperm outcompete smaller sperm in nematodes [[3], [4] and [5]], it had been assumed that this dimorphism evolved in response to sperm competition. However, we show that it was instead caused by a developmental bias. When we transformed females of the species Caenorhabditis remanei into hermaphrodites [6], their sperm were significantly smaller than those of males. Because this species never makes hermaphrodites in the wild, this dimorphism cannot be due to selection. Instead, analyses of the related nematode Caenorhabditis elegans suggest that this dimorphism might reflect the development of sperm within the distinct physiological environment of the hermaphrodite gonad. These results reveal a new mechanism for some types of develo! pmental bias—the effects of a novel physical location alter the development of ectopic cells in predictable ways.