Retinotopic order in the absence of axon competition

The retinotectal projection has long been studied experimentally and theoretically, as a model for the formation of topographic brain maps. Neighbouring retinal ganglion cells (RGCs) project their axons to neighbouring positions in the optic tectum, thus re-establishing a continuous neural representation of visual space. Mapping along this axis requires chemorepellent signalling from tectal cells, expressing ephrin-A ligands, to retinal growth cones, expressing EphA receptors. High concentrations of ephrin A, increasing from anterior to posterior, prevent temporal axons from invading the posterior tectum. However, the force that drives nasal axons to extend past the anterior tectum and terminate in posterior regions remains to be identified. We tested whether axon-axon interactions, such as competition, are required for posterior tectum innervation. By transplanting blastomeres from a wild-type (WT) zebrafish into a lakritz (lak) mutant, which lacks all RGCs, we created chimaeras with eyes that contained single RGCs. These solitary RGCs often extended axons into the tectum, where they branched to form a terminal arbor. Here we show that the distal tips of these arbors were positioned at retinotopically appropriate positions, ruling out an essential role for competition in innervation of the ephrin-A-rich posterior tectum. However, solitary arbors were larger and more complex than under normal, crowded conditions, owing to a lack of pruning of proximal branches during refinement of the retinotectal projection. We conclude that dense innervation is not required for targeting of retinal axons within the zebrafish tectum but serves to restrict arbor size and shape.     

Deactivation of macrophages by transforming growth factor-beta

Macrophage activation--enhanced capacity to kill, in a cell that otherwise mostly scavenges--is essential for host survival from infection and contributes to containment of tumours. Both microbes and tumour cells, therefore, may be under pressure to inhibit or reverse the activation of macrophages. This reasoning led to the demonstration of macrophage deactivating factors from both microbes and tumour cells. In some circumstances the host itself probably requires the ability to deactivate macrophages. Macrophages are essential to the healing of wounds and repair of tissues damaged by inflammation. Yet the cytotoxic products of the activated macrophages can damage endothelium, fibroblasts, smooth muscle and parenchymal cells (reviewed in ref. 6). Thus, after an inflammatory site has been sterilized, the impact of macrophage activation on the host might shift from benefit to detriment. These concepts led us to search for macrophage deactivating effects among polypeptide growth factors that regulate angiogenesis, fibrogenesis and other aspects of tissue repair. Among 11 such factors, two proteins that are 71% similar proved to be potent macrophage deactivators: these are transforming growth factor-beta 1 (TGF-beta 1) and TGF-beta 2.     

A study and critique of the teaching of the history of science and technology. Interim report by the committee on undergraduate education of the history of science society (U.S.A.)

A great deal is known about the technical issues surrounding the introduction of Hugo De Vries's mutation theory and the subsequent development of the modern genetical theory of natural selection. But so far little has been done to relate these events to the wider issues of the time. This article suggests that extra-scientific factors played a significant role, and substantiates this by comparing De Vries's respect for the original Darwinian spirit with Thomas Hunt Morgan's use of the mutation theory as part of an attack on the whole philosophy of Darwinism. In particular, it is argued that Morgan's attitude was dictated by his moral objections to the picture of a world dominated by struggle.     

Water stress and hybridization between Quercus gambelii and Quercus grisea

The historical and environmental forces involved in determining the geographic location of plant hybrid zones have long been of interest. Often hybrid zones appear to be intimately tied to the environment, yet because many abiotic factors covary it is often difficult to understand which are truly the most important in maintaining the position and structure of a hybrid zone. This study uses empirical data and modeling analyses to examine whether abiotic factors are responsible for the location and structure of an oak ( Quercus ) hybrid zone and, if so, which factors are the most important. Specifically, leaf trait measurements and ecological niche models were used to test the hypothesis that water availability plays a primary role in promoting and maintaining the location of hybridization between 2 species. Leaf trait analyses and ecological niche models both supported the hypothesis that water availability determines the location and structure of the hybrid zone. These findings lend support for the general importance of environmental factors in determining hybrid zone location and structure. Furthermore, they demonstrate how functional trait analyses and predictive ecological niche models can be used in future hybrid zone research.     

Nectar bat stows huge tongue in its rib cage

Bats of the subfamily Glossophaginae (family Phyllostomidae) are arguably the most specialized of mammalian nectarivores, and hundreds of neotropical plants rely on them for pollination. But flowers pollinated by bats are not known to specialize for bat subgroups (unlike flowers that have adapted to the length and curvature of hummingbird bills, for example), possibly because the mouthparts of bats do not vary much compared with the bills of birds or the probosces of insects. Here I report a spectacular exception: a recently-described nectar bat that can extend its tongue twice as far as those of related bats and is the sole pollinator of a plant with corolla tubes of matching length.     

Atmospheric science: stratospheric cooling and the troposphere

Satellite observations of tropospheric temperatures seem to show less warming than surface temperatures, contrary to physical predictions. Fu et al. show that statistical correction for the effect of stratospheric cooling brings the satellite-based estimates of tropospheric warming into closer agreement with observations of surface warming. Here we apply the method of Fu et al. to output from a state-of-the-art coupled climate model and show that simulated tropospheric temperature trends are consistent with those observed and that their method is robust.