|
ScienceWeek
SCIENCEWEEK
February 23, 2007
Vol. 11 - Number 8
--------------------------------
To receive the complete text of ScienceWeek each week by Email via the Science and Politics discussion list, send Email to contents@scienceweek.com with subject line "CONTENTS", and we will subscribe you and add your name to the distribution list.
You can make relevant comments or queries to the entire distribution list of ScienceWeek and the Science and Politics discussion list simply by sending Email to scipol@googlegroups.com with an appropriate subject line. Please do not make any posts not relevant to science or the interface between science and politics. Announcements of conferences, symposia, new books, etc. are acceptable.
--------------------------------
What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school... It is my task to convince you not to turn away because you don't understand it. You see my physics students don't understand it... That is because I don't understand it. Nobody does.
-- Richard P. Feynman (1918-1988)
--------------------------------
Contents (full text below):
1. Oceans: Climate Drives Sea Change
2. Astronomy: Nebulae Around Evolved Stars
3. Cell Biology: On Networks
4. Animal Behaviour: On Planning in Animals
5. Neuroscience: Kiwis Forego Vision at Night
6. Insect Biology: On Honey Bee Response to Waggle Dance Sounds
7. Neuroscience: Culture of Functional Mammalian Retinas
--------------------------------
1. Science 23 February 2007: Vol. 315. no. 5815, pp. 1084 - 1085 DOI: 10.1126/science.1136495
Perspectives
OCEANS: Climate Drives Sea Change
Charles H. Greene and Andrew J. Pershing
Ecosystems can shift rapidly from one state to another as a result of natural environmental variability, human activities (such as overfishing or human-induced climate change), or both. Recently, Frank et al. reported such an ecosystem regime shift in the northwest Atlantic during the early 1990s (1). To understand the likely causes for this shift, we here consider changes in the climate system that occurred at the same time.
Changes in climate beginning in the late 1980s resulted in an enhanced outflow of low-salinity waters from the Arctic (2) and a general freshening of shelf waters from the Labrador Sea to the Mid-Atlantic Bight (3-5). This freshening altered circulation and stratification patterns on the shelf and has been linked to changes in the abundances and seasonal cycles of phytoplankton, zooplankton, and fish populations (6, 7).
In recent decades, the Arctic has experienced a period of historically unprecedented changes (2, 8). In 1987, atmospheric pressure at the sea surface began to decline in the central Arctic. Two years later, this sea level pressure dropped precipitously, and a strongly cyclonic atmospheric regime emerged. This cyclonic regime increases the delivery of warmer, higher-salinity Atlantic water into the Arctic Ocean, mainly via the Barents Sea (see the first figure).
Associated with these changes in the atmosphere and Atlantic water inflow, circulation in the upper layers of the Arctic Ocean changed substantially between the late 1980s and early 1990s (see the first figure). From an Atlantic perspective, the most important consequence of these changes was a redirection of the shallow outflow from the Arctic Ocean. Instead of entering the North Atlantic mainly via Fram Strait as before, much of this low-salinity outflow began to exit the Canadian Basin and enter the Labrador Sea via the Canadian Archipelago.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
2. Science 23 February 2007: Vol. 315. no. 5815, pp. 1086 - 1087 DOI: 10.1126/science.1139937
Perspectives
ASTRONOMY: Nebulae Around Evolved Stars
Noam Soker
Despite many observations of impressive nebular structures around evolved stars, there is no acceptable theory that explains their formation. Such structures include multiple-ring systems, hourglass-shaped nebulae, and butterfly-type morphologies, and they are found around quietly dying Sun-like stars as well as around massive stars that explode as supernovae (see the figure). The formation mechanism of these structures is connected to the most puzzling questions regarding stellar evolution: What is the role of rotation as matter collapses to form a star, and how does stellar rotation evolve from the birth to the death of stars?
As reported on page 1103 of this issue, Morris and Podsiadlowski (1) propose a model for the formation of the triple-ring system of supernova (SN) 1987A, a supernova whose explosion was detected 20 years ago. The model is based on fast rotation of the progenitor star, and the authors nicely pinpoint the crucial role of stellar rotation. In particular, they attribute the fast rotation of the progenitor of SN 1987A to a stellar companion that was swallowed by the progenitor about 20,000 years before the explosion.
Knowledge of stellar processes does not come easily. For about a hundred years, from the middle of the 19th century to the middle of the 20th century, the major question in the field of stellar structure and evolution was the energy source of stars. Fierce arguments raged between scientists who thought that gravitational energy powered the stars and those who thought another process was at work. At the beginning of the 20th century it became clear that nuclear reactions are the source of energy in the Sun and other stars. It took another 50 years and the efforts of great scientists such as Arthur Eddington and Hans Bethe to identify the major nuclear reactions in stars.
In the past 40 years, the major open questions in the field of stellar evolution have been related to the role and evolution of rotation (or angular momentum) in stars and in gaseous disks around stars. Are magnetic fields required to transport angular momentum in gaseous disks around stars? How can two oppositely directed and well-collimated jets be emitted by such disks? How are jets responsible for the gamma-ray burst phenomena formed from collapsing rotating massive stars? What is the mechanism that shapes gaseous nebulae around evolved stars?
Morris and Podsiadlowski address this last question. The gaseous nebulae around evolved stars are made of gas that once was part of the envelope of the central star. This gas was expelled via a strong wind from the stellar surface when the star was a red giant star close to the end of its nuclear activity. At this stage, the star was several hundred times the present size of the Sun and its surface temperature was a few thousand kelvin. With a huge luminosity and low surface gravity, these stars eject mass at a very high rate.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
3. Nature 445, 823 (22 February 2007) | doi:10.1038/445823a; Published online 21 February 2007
Cell Biology: On Networks
John J. Tyson
John Tyson is university distinguished professor of biological sciences at Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
Open any issue of Nature and you will find a diagram illustrating the molecular interactions purported to underlie some behaviour of a living cell. The accompanying text explains how the link between molecules and behaviour is thought to be made. For the simplest connections, such stories may be convincing, but as the mechanisms become more complex, intuitive explanations become more error prone and harder to believe.
A better way to build bridges from molecular biology to cell physiology is to recognize that a network of interacting genes and proteins is a dynamic system evolving in space and time according to fundamental laws of reaction, diffusion and transport. These laws govern how a regulatory network, confronted by any set of stimuli, determines the appropriate response of a cell. This information-processing system can be described in precise mathematical terms, and the resulting equations can be analysed and simulated to provide reliable, testable accounts of the molecular control of cell behaviour. To make these ideas clear, I will use a simplified example.
Sometimes the best response a cell can make is suicide. Programmed cell death involves activation of proteases, called caspases, that disassemble the doomed cell in a tidy fashion, avoiding the inflammatory consequences of cell breakdown. Cells stockpile caspases in precursor forms and activate the precursors under carefully regulated conditions. The network that activates caspase is extremely complex, with many levels of control to keep caspase activity switched off in healthy cells and to turn it on in cells that need to die.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. Nature 445, 825-826 (22 February 2007) | doi:10.1038/445825a; Published online 21 February 2007
Animal Behaviour: Planning for breakfast
Sara J. Shettleworth
Can it ever be said that animals plan ahead? Animals do show behaviour that prepares them for the future, but in general that behaviour reflects unlearned or conditioned responses to predictive cues. For example, a swallow flying south or a marmot entering hibernation is reacting to a cue that has foretold the seasons for its ancestors. A hungry rat pressing a lever that provides food in ten seconds, rather than a lever providing food later, does so because rewards are more effective after short than after long delays. Two requirements1 for genuine future planning are that the behaviour involved should be a novel action, or combination of actions (thus ruling out migrating and hibernating), and that it should be appropriate to a motivational state other than the one the animal is in at that moment (thus ruling out the rat's lever pressing). In their report of two experiments with western scrub-jays (page 919 of this issue2), Raby et al. describe the first observations that unambiguously fulfil both requirements.
The scrub-jay (Fig. 1) naturally caches food. In Raby and colleagues' research, jays were first allowed to acquire information about where food would be available in the morning. Then, in a test in the evening, the authors found that the birds behaved as if they were planning for breakfast by caching food items in the place where the food was most likely to be needed. The birds lived in large cages with three compartments (rooms) (see Fig. 1 of the paper on page 919). In the first experiment, each evening they ate powdered pine nuts, food they were unable to cache, in the central room. Then the next morning each bird was confined to one of the end rooms for two hours. In the 'breakfast room', a bird was always fed, whereas in the 'no-breakfast room' no food was given.
The test of planning came after several cycles of this treatment. For the first time, whole pine nuts were provided in the central room in the evening, along with sand-filled trays for caching in the two end rooms. The authors found that the birds cached three times as many pine nuts in the no-breakfast room as in the breakfast room. Importantly, all the data came from this one test: learning how their choices determined the next day's breakfast could not have influenced the jays' behaviour.
In the second experiment, the birds learned to expect breakfast in both rooms, peanuts only in one and dog kibble only in the other. On their first opportunity to cache peanuts and dog kibble in the evening, they distributed their caches so as to provide each room with the kind of food it usually lacked.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. Citation: Martin GR, Wilson K, Wild JM, Parsons S, Kubke MF, et al. (2007) Kiwi Forego Vision in the Guidance of Their Nocturnal Activities. PLoS ONE 2(2): e198. doi:10.1371/journal.pone.0000198
Neuroscience: Kiwi Forego Vision in the Guidance of Their Nocturnal Activities
Graham R. Martin1*, Kerry-Jayne Wilson2, J. Martin Wild3, Stuart Parsons4, M. Fabiana Kubke3, Jeremy Corfield3,4
1 Centre for Ornithology, School of Biosciences, University of Birmingham, Edgbaston, United Kingdom, 2 Bio-Protection and Ecology Division, Lincoln University, Lincoln, New Zealand, 3 Department of Anatomy, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand, 4 School of Biological Sciences, University of Auckland, Auckland, New Zealand Background
In vision, there is a trade-off between sensitivity and resolution, and any eye which maximises information gain at low light levels needs to be large. This imposes exacting constraints upon vision in nocturnal flying birds. Eyes are essentially heavy, fluid-filled chambers, and in flying birds their increased size is countered by selection for both reduced body mass and the distribution of mass towards the body core. Freed from these mass constraints, it would be predicted that in flightless birds nocturnality should favour the evolution of large eyes and reliance upon visual cues for the guidance of activity. Methodology/Principal Findings
We show that in Kiwi (Apterygidae), flightlessness and nocturnality have, in fact, resulted in the opposite outcome. Kiwi show minimal reliance upon vision indicated by eye structure, visual field topography, and brain structures, and increased reliance upon tactile and olfactory information. Conclusions/Significance
This lack of reliance upon vision and increased reliance upon tactile and olfactory information in Kiwi is markedly similar to the situation in nocturnal mammals that exploit the forest floor. That Kiwi and mammals evolved to exploit these habitats quite independently provides evidence for convergent evolution in their sensory capacities that are tuned to a common set of perceptual challenges found in forest floor habitats at night and which cannot be met by the vertebrate visual system. We propose that the Kiwi visual system has undergone adaptive regressive evolution driven by the trade-off between the relatively low rate of gain of visual information that is possible at low light levels, and the metabolic costs of extracting that information.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. Citation: Tsujiuchi S, Sivan-Loukianova E, Eberl DF, Kitagawa Y, Kadowaki T (2007) Dynamic Range Compression in the Honey Bee Auditory System toward Waggle Dance Sounds. PLoS ONE 2(2): e234. doi:10.1371/journal.pone.0000234
Insect Biology: Dynamic Range Compression in the Honey Bee Auditory System toward Waggle Dance Sounds
Seiya Tsujiuchi1*, Elena Sivan-Loukianova2, Daniel F. Eberl2, Yasuo Kitagawa1*, Tatsuhiko Kadowaki1*
1 Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya, Japan, 2 Department of Biological Sciences, University of Iowa, Iowa City, Iowa, United States of America
Honey bee foragers use a “waggle dance” to inform nestmates about direction and distance to locations of attractive food. The sound and air flows generated by dancer's wing and abdominal vibrations have been implicated as important cues, but the decoding mechanisms for these dance messages are poorly understood. To understand the neural mechanisms of honey bee dance communication, we analyzed the anatomy of antenna and Johnston's organ (JO) in the pedicel of the antenna, as well as the mechanical and neural response characteristics of antenna and JO to acoustic stimuli, respectively. The honey bee JO consists of about 300–320 scolopidia connected with about 48 cuticular “knobs” around the circumference of the pedicel. Each scolopidium contains bipolar sensory neurons with both type I and II cilia. The mechanical sensitivities of the antennal flagellum are specifically high in response to low but not high intensity stimuli of 265–350 Hz frequencies. The structural characteristics of antenna but not JO neurons seem to be responsible for the non-linear responses of the flagellum in contrast to mosquito and fruit fly. The honey bee flagellum is a sensitive movement detector responding to 20 nm tip displacement, which is comparable to female mosquito. Furthermore, the JO neurons have the ability to preserve both frequency and temporal information of acoustic stimuli including the “waggle dance” sound. Intriguingly, the response of JO neurons was found to be age-dependent, demonstrating that the dance communication is only possible between aged foragers. These results suggest that the matured honey bee antennae and JO neurons are best tuned to detect 250–300 Hz sound generated during “waggle dance” from the distance in a dark hive, and that sufficient responses of the JO neurons are obtained by reducing the mechanical sensitivity of the flagellum in a near-field of dancer. This nonlinear effect brings about dynamic range compression in the honey bee auditory system.
The honey bee (Apis mellifera) uses various chemical and physical stimuli for communication. One of the best-characterized forms of honey bee communication is the forager's “waggle dance”, which informs nestmates about the direction and distance to locations of attractive food [1]. This dance consists of a series of alternating left-hand and right-hand loops, interspersed by a phase in which a dancer waggles her abdomen. The duration of the waggle run represents the distance to the food location. The direction of the waggle run relative to gravity corresponds to the direction with respect to the sun's azimuth. During the waggle run, the dancer waggles her abdomen while vibrating her wings, thereby generating various sounds and air flows. In addition, other signals such as temperature [2], odor [1], tactile contact [3], and comb vibration [4] have been suggested to assist followers to find, orient towards, and follow the waggle dancer. Airborne signals emitted by the dancer have been extensively studied. They consist of roughly 30 pulses per second, each pulse with a duration of about 20 ms and a carrier frequency of 265 Hz, air flows of a carrier frequency of 12–15 Hz, and jet flows [5]–[8]. Behavioral experiments demonstrated that honey bees can hear near-field sounds by detecting air-particle movements with Johnston's organ (JO) located at the second segment (pedicel) of the antenna [9]–[11]. This hearing mechanism is ideal for the followers located only millimeters away from a dancer. Furthermore, these experiments suggested that honey bees can learn, and discriminate between, different sound frequencies. JO is an antennal chordotonal organ, specialized for hearing in some insects, as best characterized in two Dipterans, mosquito and Drosophila melanogaster [12]. It consists of hundreds to thousands of scolopidial units, each composed of 2–3 neurons and several support cells [13]–[15]. JO transduces the mechanical vibration of the flagellum (the third antennal segment) into the electrophysiological activation of chordotonal neurons. These neurons project to the antennal mechanosensory region of the brain for further auditory processing.
Recent studies have shown that honey bees estimate the distance flown by optic flow and translates it to the duration of the “waggle dance” [16], [17]. The followers then need to perceive the duration of waggle run and translate it to the distance they are expected to fly. These results thus suggest neural templates for the amount of image motion and time period in the honey bee brain. To understand the mechanism of honey bee dance communication from detection to interpretation of dance messages at the neuronal level, we first started to characterize the anatomy and mechanical response characteristics of the antenna, and electrophysiological recordings of the antennal nerve in response to various near-field acoustic stimuli.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
7. Citation: Koizumi A, Zeck G, Ben Y, Masland RH, Jakobs TC (2007) Organotypic Culture of Physiologically Functional Adult Mammalian Retinas. PLoS ONE 2(2): e221. doi:10.1371/journal.pone.0000221
Neuroscience: Organotypic Culture of Physiologically Functional Adult Mammalian Retinas
Amane Koizumi1, Günther Zeck2, Yixin Ben3, Richard H. Masland1, Tatjana C. Jakobs1*
1 Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, United States of America, 2 Systems and Computational Biology, Max Planck Institute of Neurobiology, Martinsried, Germany, 3 Burke Institute, White Plains, New York, United States of America Background
The adult mammalian retina is an important model in research on the central nervous system. Many experiments require the combined use of genetic manipulation, imaging, and electrophysiological recording, which make it desirable to use an in vitro preparation. Unfortunately, the tissue culture of the adult mammalian retina is difficult, mainly because of the high energy consumption of photoreceptors. Methods and Findings
We describe an interphase culture system for adult mammalian retina that allows for the expression of genes delivered to retinal neurons by particle-mediated transfer. The retinas retain their morphology and function for up to six days— long enough for the expression of many genes of interest—so that effects upon responses to light and receptive fields could be measured by patch recording or multielectrode array recording. We show that a variety of genes encoding pre- and post-synaptic marker proteins are localized correctly in ganglion and amacrine cells. Conclusions
In this system the effects on neuronal function of one or several introduced exogenous genes can be studied within intact neural circuitry of adult mammalian retina. This system is flexible enough to be compatible with genetic manipulation, imaging, cell transfection, pharmacological assay, and electrophysiological recordings.
Several features make the retina an appealing model of the mammalian central nervous system. It is the most accessible part of the CNS, and its thin, layered structure can be regarded as a stack of two dimensional arrays. The flow of information is essentially unidirectional toward the ganglion cells, the only projection neurons in the system. Most of the diverse functional cell types in the mammalian retina have been identified at least morphologically, and in some cases also physiologically [1], [2].
An approach to this highly complex system would be to selectively alter the function of identified neurons within the functional neural network. It would in principle be possible to create transgenic mice to perform such studies. An alternative is to inject viral vectors carrying the gene of interest into the eye. Though these are well-established technologies, drawbacks are relatively high cost and the long time required for each experiment. Here we sought to develop a more flexible and quickly responsive system.
The system combines features used for short-term recording from the retina with those used for longer term maintenance of brain slices. Retinas are difficult to culture, mainly because of the exceptionally high metabolism of the photoreceptors [3]. Yet, preparations for recording from rabbit retinas in vitro have been established that can be kept for over fifty hours [4]. These methods rely on constant superfusion of the tissue with oxygenated medium, and therefore require constant monitoring. On the other hand, brain slices in interphase chambers can be successfully maintained and manipulated in vitro for weeks [5]–[7]. Limited survival of retinas under relatively simple culture conditions has been demonstrated [8]–[13], most notably in elegant developmental studies by Wong and co-workers [14]. However, most of these experiments used neonatal or very young retinas, which are immature in most mammalian species and, because the photoreceptors are not functional, have low metabolic demands. In addition, most studies have been limited to incubations lasting less than 24 h. In order to provide a platform for experiments on the microcircuitry of the mature adult retina, we have established a hybrid interphase/perfusion system. We show that it is possible to record responses to light from rabbit retinas for up to six days in culture. This time is sufficient to allow for the expression of genes that have been introduced into individual retinal neurons. We have expressed a wide spectrum of proteins in retinas incubated in this way. As an illustration, we show here that fusion proteins of synaptic markers with GFP can be expressed within this time frame and appropriately localize within the dendritic tree.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
|