Current Biology Vol 19 No 10 R404 performance. An absence of What are the implications of these 4. Merker, B. (2000). Synchronous chorusing and human origins. In The Origins of entrainment in such species would animal findings for research on human Music, N.L. Wallin, B. Merker, and S. Brown, raise the question of what else is music and its evolution? The first is that eds. (Cambridge, Mass.: The MIT required for entrainment. we now have animal models to further Press), pp. 315–327. 5. Patel, A.D., Iversen, J.R., Bregman, M.R., and One possibility is the propensity to explore the neural and genetic basis for Schulz, I. (2009). Experimental evidence for engage in joint social action. A recent entrainment. The second illustrates the synchronization to a musical beat in a nonhuman animal. Curr. Biol. 19, study of entrainment in human children fundamental point that evolutionary 827–830. [11] showed that young children find convergence or ‘analogy’ allows us to 6. Schachner, A., Brady, T.F., Pepperberg, I.M., it difficult to entrain to a purely test evolutionary hypotheses, such as and Hauser, M.D. (2009). Entrainment to music requires vocal mimicry: Evidence from non- auditory stimulus (a disembodied the vocal mimicry hypothesis [14]. human animals. Curr. Biol. 19, 831–836. metronomic beat), or to a visible Homologous traits represent a single 7. Fitch, W.T. (2006). The biology and evolution of music: A comparative perspective. Cognition drumbeating robot. They nonetheless evolutionary event, and count 100, 173–215. entrain with a human adult in a socially- statistically as a single datapoint. In 8. Patel, A.D. (2006). Musical rhythm, linguistic engaged game-playing context. contrast, when different clades evolve rhythm, and human evolution. Music Perception 24, 99–104. Perhaps a similar propensity for social the same trait convergently, these 9. Patel, A.D. (2008). Music, Language, and the engagement underlies the apparent constitute statistically independent Brain (New York: Oxford University Press). 10. Janik, V.M., and Slater, P.B. (1997). Vocal capacity for parrots, but not other events [15], allowing us to test learning in mammals. Adv. Stud. Behav. 26, birds, to entrain to a beat? hypotheses about the evolution of 59–99. Parrots are long-lived, group-living human music or language that might 11. Kirschner, S., and Tomasello, M. (2009). Joint drumming: Social context facilitates birds, and their open-ended learning otherwise remain ‘just-so-stories’. synchronization in preschool children. J. Exp. abilities are sometimes employed At both mechanistic and functional Child Psychol. 102, 299–314. 12. Farabaugh, S.M., Linzenbold, A., and to develop vocal ‘badges’ of group levels, then, the discovery of parrot Dooling, R.J. (1994). Vocal plasticity in membership [12]. Although the entrainment provides a rich foundation budgerigars (Melopsittacus undulatus): adaptive functions of parrot for further advances in understanding evidence for social factors in the learning of contact calls. J. Comp. Psychol. 108, 81–92. vocalizations remain poorly the biology and evolution of human 13. Rendell, L., and Whitehead, H. (2001). Culture in understood, they appear to be more music. whales and dolphins. Behav. Brain Sci. 24, 309–324. group-oriented than the mostly 14. Fitch, W.T. (2006). On the biology and evolution individual territorial and courtship of music. Music Perception 24, 85–88. displays that typify songbirds, and this 15. Harvey, P.H., and Pagel, M.D. (1991). The References Comparative Method in Evolutionary Biology may be one factor explaining the 1. Clayton, N.S., Bussey, T.J., and Dickinson, A. (Oxford: Oxford University Press). (2003). Can animals recall the past and plan complete dominance of parrot species for the future? Nat. Rev. Neurosci. 4, in the YouTube sample. This 685–691. School of Psychology, University hypothesis also suggests, given the 2. Kuhl, P.K., and Miller, J.D. (1978). Speech of St. Andrews, St. Andrews, perception by the chinchilla: Identification Scotland, KY6 9JP, UK. capacity of dolphins to engage in functions for synthetic VOT stimuli. J. Acoustic. imitation and joint action [13], that the Soc. Am. 63, 905–917. E-mail: [email protected] 3. Wallin, N.L., Merker, B., and Brown, S. (2000). potential for entrainment in this species The Origins of Music (Cambridge, Mass: The deserves a closer experimental look. MIT Press). DOI: 10.1016/j.cub.2009.04.004

Chromosome Segregation: Ndc80 can directly bind microtubules in vitro Can Carry the Load (reviewed in [1]). In budding yeast, the Ndc80 complex is thought to partner with the Dam1 complex to form kinetochore–microtubule attachments, Dynamic attachments between kinetochores and spindle microtubules are perhaps through direct interaction [2]. required for chromosome bi-orientation in mitosis. A new study provides Dam1 complexes are able to form biophysical insight into how the Ndc80 complex may contribute to the rings and non-ring oligomers on formation of these attachments. microtubules in vitro [3,4], and both forms of assemblages can generate Ajit P. Joglekar1 and the constant gain and loss of tubulin load-bearing attachments to growing Jennifer G. DeLuca2,* subunits at the microtubule plus-ends. and shortening microtubule plus-ends The search for molecular components [5,6]. Although much evidence How cells generate the kinetochore– that serve as load-bearing couplers suggests that Dam1 complexes may microtubule attachments required to between kinetochores and serve as kinetochore–microtubule drive chromosome movements in microtubules is active. Much effort has couplers, no convincing Dam1 mitosis has long puzzled cell biologists. focused on the Ndc80 complex — homologs have surfaced in higher This problem is an interesting one, composed of Ndc80 (Hec1 in humans), eukaryotes. as attachments must be strong to Nuf2, Spc24, and Spc25 — since it is This raises the question: can generate forces for chromosome essential for efficient kinetochore– the Ndc80 complex alone form movements, yet flexible to allow for microtubule attachment in cells and it load-bearing attachments to dynamic Dispatch R405 microtubules? A recent report by Powers and colleagues [7] suggests Ndc80- Ndc80 complex: that it can, and they argue it does so Ndc80 complex- Spc24 using a biased-diffusion mechanism. complex-coated coated Spc25 Hill originally proposed biased bead bead Nuf2 diffusion as a mechanism to explain Bead Hec1 chromosome movement coupled with microtubule depolymerization [8]. This mechanism relies on multiple weak interactions of binding proteins with Diffusion rate = 2.3 ± 0.4µm2/sec the plus-end of a microtubule to produce depolymerization-dependent force. Hill modeled the microtubule- binding site at the kinetochore as a sleeve with regularly spaced weak Tension interaction sites covering its inner applied surface based on the electron microscopy structure of the Bead tracks Bead tracks kinetochore available at the time. depolymerizing polymerizing Thermal diffusion of this sleeve microtubule microtubule (facilitated by weak individual interactions) maintains the most favorable position of the sleeve in relation to the plus-end — one that maximizes the number of interactions Current Biology between the sleeve and the microtubule lattice, minimizing the Figure 1. The Ndc80 complex can form load-bearing attachments to microtubules using free energy of the system. a biased-diffusion-based mechanism. Depolymerization at the microtubule Powers et al. [7] demonstrate that beads coated with Ndc80 complexes (top right) bind and plus-end biases the diffusion of the diffuse rapidly along microtubules (top left). Ndc80-complex-coated beads can track depoly- sleeve, and the sleeve then follows the merizing microtubule plus-ends (bottom left) and, in the presence of tension, can track poly- receding plus-end to attain minimum merizing microtubule plus-ends (bottom right). In the experiments described by Powers et al. [7], beads were coated evenly with Ndc80 complexes; however, only a subset is shown in the free energy. This arrangement can figure for clarity. generate a significant force (approximately 12–13 pN) in a microtubule-depolymerization- a catastrophe were able to persistently competent, on its own, to serve as dependent fashion. Although the track depolymerizing plus-ends over a biased-diffusion-based microtubule specific geometry of the binding site long (and physiologically relevant) coupler at the kinetochore. assumed by Hill may not be present distances (average of 4.8 mm). These How does the Ndc80 complex in cells, biased diffusion of the findings are consistent with a recent interface the microtubule lattice to kinetochore, facilitated by multiple study by McIntosh et al. [9] in which facilitate dynamic coupling? X-ray weak interactions with the microtubule beads coated with purified crystallographic data indicate that lattice, remains a compelling force Caenorhabditis elegans Ndc80 both Hec1 and Nuf2 contain calponin generation mechanism. Experiments complex bound microtubules in vitro, homology (CH) domains, which have carried out by Powers et al. [7] and a subset were able to track been implicated in direct microtubule demonstrate that the Ndc80 complex depolymerizing microtubule ends. In binding [12]. The CH domains in Hec1 meets the requirements of the biased the Powers et al. [7] experiments, when and Nuf2 contain prominent positive diffusion mechanism and suggest force was applied to the beads to ridges that have been proposed to that it can serve as a kinetochore– simulate a load, the beads also tracked interface the negatively charged microtubule coupler in cells. polymerizing microtubule plus-ends carboxy-terminal tails of tubulin that Powers et al. [7] simulated without dissociating. This finding is extend from the microtubule lattice interactions between microtubules significant, as during chromosome [12]. Supporting this notion, single and kinetochores by incubating bi-orientation in cells, kinetochores point mutations in Hec1 or Nuf2 that polymerized microtubules with must remain bound to both reduce the positive charge in the ridge polystyrene microbeads coated with depolymerizing and polymerizing domain result in a 30–40-fold decrease purified budding yeast or human Ndc80 microtubules. Furthermore, Ndc80- in Ndc80 complex microtubule-binding complexes (Figure 1). When such complex-coated beads remained activity in vitro [12]. However, the CH beads were captured with a laser trap bound to microtubules under a load domains of Hec1 and Nuf2 alone are and held near the microtubules, they of up to 2.5 pN, which the authors not sufficient to bind microtubules. readily bound and rapidly diffused estimate to be near the force generated Deletion of the positively charged along the microtubule lattice, unbiased at kinetochores during chromosome amino-terminal 80 amino-acid tail in either direction, at a rate of 2.3 6 bi-orientation (0.4–8.0 pN) in cells domain of Hec1, which is not involved 0.4 mm2/sec. Beads bound to [10,11]. The authors argue from these in CH domain folding, significantly microtubules that underwent studies that the Ndc80 complex is impairs kinetochore–microtubule Current Biology Vol 19 No 10 R406 attachment in cells [13,14] and reduces microtubule has been reported for increases the fidelity of Ndc80- the affinity of the Ndc80 complex for various yeasts [16], and approximately mediated kinetochore–microtubule microtubules by approximately 100- 30 complexes have been reported per attachment. In support of this, Powers fold in vitro [12]. Modifying the charge kinetochore–microtubule in Xenopus et al. [7] demonstrated that soluble of the Hec1 tail domain also prevents egg extracts [17]. Using soluble Ndc80 complexes rarely exhibited stable kinetochore–microtubule (non-bead-associated) Ndc80 microtubule binding in buffers of attachment in cells [13], presumably complexes at very low concentrations, physiological ionic strength (lowering by altering electrostatic interactions Powers et al. [7] demonstrated that the ionic strength was required to between the oppositely charged tail complexes bound to microtubules measure diffusion rates), suggesting domains of Hec1 and tubulin. Binding singly or in pairs but did not exhibit the presence of cellular factors that mediated through such interactions fits tip-tracking behavior. Addition of increase the ability of the Ndc80 well into the model of diffusion-based antibodies to induce Ndc80 complex complex to bind and diffuse along coupling, which relies on multiple weak oligomerization restored the ability of microtubules. Future studies probing interactions to function [8]. In support the complexes to track the tips of the relationship between the two of the role of electrostatic interactions, depolymerizing microtubules. This complexes will be key in understanding Powers et al. [7] demonstrate that observation indicates that multiple how cells generate the dynamic increasing the ionic strength of their Ndc80 complex molecules are interface between kinetochores and assay buffer increased the dissociation necessary for stable kinetochore– microtubules. rate of Ndc80 complexes from microtubule attachment. The need microtubules. Rather than relying on for aggregation is also important to References a single binding site, it is likely that consider when interpreting data from 1. Cheeseman, I.M., and Desai, A. (2008). multiple contact points exist between experiments using Ndc80 mutants Molecular architecture of the kinetochore- microtubule interface. Nat. Rev. Mol. Cell the Ndc80 complex and microtubules, that prevent kinetochore–microtubule Biol. 9, 33–46. as suggested in a recent study by attachment in cells or direct 2. Shang, C., Hazbun, T.R., Cheeseman, I.M., Wilson-Kubalek and colleagues [15]. microtubule binding in vitro. Removal Aranda, J., Fields, S., Drubin, D.G., and Barnes, G. (2003). Kinetochore protein They combined cryo-electron or mutation of certain domains may interactions and their regulation by the Aurora microscopy of Ndc80-complex- not disrupt the Ndc80–lattice interface kinase Ipl1p. Mol. Biol. Cell 14, 3342–3355. 3. Miranda, J.J., De Wulf, P., Sorger, P.K., and decorated microtubules with published but, instead, may prevent proper Harrison, S.C. (2005). The yeast DASH complex Ndc80 complex crystallographic data oligomerization of the complexes. For forms closed rings on microtubules. Nat. to model the interaction. Fitting the example, the Nuf2 CH domain may not Struct. Mol. Biol. 12, 138–143. 4. Westermann, S., Avila-Sakar, A., Wang, H.W., crystal structure of the globular amino be required for microtubule binding, Niederstrasser, H., Wong, J., Drubin, D.G., termini of Hec1 and Nuf2 into their but for oligomerization of the Nogales, E., and Barnes, G. (2005). Formation of a dynamic kinetochore-microtubule interface electron density maps of Ndc80- complexes, explaining the low affinity through assembly of the Dam1 ring complex. complex-decorated microtubules led of a CH domain Nuf2 mutant for Mol. Cell 17, 277–290. to a model in which the CH domain and microtubules in vitro [12] and 5. Grishchuk, E.L., Spiridonov, I.S., Volkov, V.A., Efremov, A., Westermann, S., Drubin, D., the 80 amino-acid tail domain of Hec1 accounting for the inability to model Barnes, G., Ataullakhanov, F.I., and contact the microtubule directly. Nuf2 as a direct binder of the McIntosh, J.R. (2008). Different assemblies of the DAM1 complex follow shortening Interestingly, their data did not fit microtubule lattice via cryo-electron microtubules by distinct mechanisms. Proc. a model in which Nuf2 directly binds the microscopy [15]. Natl. Acad. Sci. USA 105, 6918–6923. microtubule lattice, suggesting the Powers and colleagues [7] 6. Gestaut, D.R., Graczyk, B., Cooper, J., Widlund, P.O., Zelter, A., Wordeman, L., Ndc80 complex diffusion observed by demonstrate that the Ndc80 complex Asbury, C.L., and Davis, T.N. (2008). Powers et al. [7] is primarily mediated alone can serve as a load-bearing Phosphoregulation and depolymerization- driven movement of the Dam1 complex do through Hec1. coupler between microtubules and not require ring formation. Nat. Cell Biol. 10, As expected for a Hill-type biased- kinetochores. But does the Ndc80 407–414. diffusion-based coupler, Powers et al. complex really go it alone in cells? A 7. Powers, A.F., Franck, A.D., Gestaut, D.R., Cooper, J., Gracyzk, B., Wei, R.R., [7] demonstrate that multiple Ndc80 recent study by Welburn et al. [18] Wordeman, L., Davis, T.N., and Asbury, C.L. complexes per microtubule are suggests it may not, and kinetochores (2009). The Ndc80 kinetochore complex forms load-bearing attachments to dynamic required for generating load-bearing may also rely on the Ska1 complex microtubule tips via biased diffusion. attachments. When at least 360 to form load-bearing attachments Cell 136, 865–875. Ndc80 complexes were bound per to microtubules. The authors 8. Hill, T.L. (1985). Theoretical problems related to the attachment of microtubules to bead, attachment to microtubules demonstrate that the Ska1 complex kinetochores. Proc. Natl. Acad. Sci. USA persisted during cycles of growth (containing Ska1, Ska2, and the newly 82, 4404–4408. 9. McIntosh, J.R., Grishchuk, E.L., and shortening, while decreasing the identified Rama1 protein) can form Morphew, M.K., Efremov, A.K., Zhudenkov, K., amount of Ndc80 complexes per bead oligomeric assemblages on Volkov, V.A., Cheeseman, I.M., Desai, A., significantly increased the detachment microtubules and can couple bead Mastronarde, D.N., and Ataullakhanov, F.I. (2008). Fibrils connect microtubule tips with rate. The authors estimate, based movement along depolymerizing kinetochores: A mechanism to couple tubulin on bead surface geometry, that 360 microtubules in vitro. The Ska1 dynamics to chromosome motion. Cell 135, 322–333. complexes per bead correspond to components bind kinetochores in 10. Nicklas, R.B. (1988). The forces that move 14 6 5 complexes interacting with human cultured cells [18,19] and, chromosomes in mitosis. Annu. Rev. Biophys. a microtubule. Interestingly, this is similar to the Dam1 complex in budding Biophys. Chem. 17, 431–449. 11. Pearson, C.G., Maddox, P.S., Salmon, E.D., and close to previous estimates of Ndc80 yeast, kinetochore binding is Bloom, K. (2001). Budding yeast chromosome complexes per kinetochore– dependent on both microtubules and structure and dynamics during mitosis. J. Cell Biol. 152, 1255–1266. microtubule in vivo: a range of 6–8 the Ndc80 complex. A testable 12. 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Oxygen Sensation: Into Thick Air the two groups identified the glb-5 locus as an additional factor required for the different behavioral responses. Two recent studies show that a neural globin tunes oxygen responses in the The glb-5 gene encodes a protein nematode Caenorhabditis elegans. Analysis of wild C. elegans strains suggests with a globin domain and is a member that the commonly used Bristol strain may have adapted to life in the laboratory. of a large superfamily (including hemoglobin and myoglobin) of Mark J. Alkema oxygen. The C. elegans laboratory heme-binding proteins that play roles strain N2, originally isolated from in oxygen storage, transport and Each spring bar-headed geese leave a compost heap in Bristol England, is sensation. In strains carrying both the their closely-related greylag geese relatively indifferent to high oxygen Hawaiian npr-1(215F) and glb-5 alleles, behind at their winter feeding grounds concentrations as long as food oxygen downshifts led to a marked in the lowlands of India to fly to their is plentiful. The Hawaiian strain increase in turning rate and a reduction nesting grounds in Tibetan highlands. CB4856, which was isolated from of locomotion rate. Strains carrying Bar-headed geese are the world’s a pineapple field, avoids high oxygen the Hawaiian npr-1(215F) allele and highest-flying migrants. They have concentrations in the presence of the Bristol glb-5 allele showed only been spotted flying over the top of food, and tends to aggregate and minimal changes in turning and Mount Everest where the oxygen forage at the border of bacterial lawns. locomotion rate. concentration is one third of that at Polymorphisms in the neuropeptide The Bristol glb-5 locus contains sea level. The hemoglobin of the receptor, npr-1, are largely responsible an exon duplication that gives rise to bar-headed goose has a much higher for differences in the behavioral splice variants encoding truncated affinity for oxygen than that of their responses between Bristol and versions of GLB-5. The Bristol glb-5 lowland relatives allowing the bar- Hawaiian strains [5]. The Bristol strain allele is recessive to the Hawaiian glb-5 headed geese to cope with life and has a high activity npr-1(215V) allele allele, suggesting that the duplication flight at high altitudes in the Himalayas resulting in a weak response to reduces gene function. Absorption [1]. Two papers recently published in oxygen and aversion to carbon spectra of recombinant GLB-5 protein Neuron [2] and Nature [3] show that dioxide, whereas the Hawaiian strain indicate that it can reversibly bind globin polymorphisms also shape has a low activity npr-1(215F) allele oxygen, like the human neuroglobins. behavioral responses to oxygen in the resulting in an aversion to high oxygen The glb-5 gene is expressed in the nematode Caenorhabditis elegans. levels and indifference to carbon URX, AQR/PQR and the BAG sensory The favorite breeding grounds of dioxide. neurons, which have been identified as C. elegans are found in the soil, Changing oxygen concentrations the oxygen and carbon dioxide sensing compost and rotting fruit, where alters locomotion rate and turning neurons of the worm. A GFP-tagged oxygen concentrations can vary from behavior of the worm, both of which version of GLB-5 is highly enriched to near hypoxia to atmospheric levels; are likely to allow the animal to find its the sensory ending of these neurons, the worm avoids both hypoxia and preferred concentration on an oxygen suggesting a direct role in sensory hyperoxia and seems to prefer oxygen gradient. McGrath et al. [2] and Persson transduction. To analyze how glb-5 concentrations ranging from 5 to 12% et al. [3] found that the Hawaiian strain affects URX responses, both groups [4]. Because C. elegans lacks increased their locomotion and turning [2,3] used genetically encoded calcium a specialized respiratory system or rate in response to small increases in indicators to measure intracellular circulatory organs, it must rely on the oxygen concentration (19/20% to calcium in the URX neurons. These diffusion for gaseous exchange. 21%), while the Bristol strain were analyses showed that changes in Previous work showed that wild relatively unresponsive to small shifts oxygen produced larger responses in C. elegans strains collected from in oxygen concentration. Using the URX neurons of animals carrying different locations around the world recombinant inbred strains between the Hawaiian glb-5 allele than in those have markedly different responses to the Hawaiian and Bristol relatives, carrying Bristol glb-5. High GLB-5