A receptor kinase gene regulating symbiotic nodule development

Leguminous plants are able to establish a nitrogen-fixing symbiosis with soil bacteria generally known as rhizobia. Metabolites exuded by the plant root activate the production of a rhizobial signal molecule, the Nod factor, which is essential for symbiotic nodule development. This lipo-chitooligosaccharide signal is active at femtomolar concentrations, and its structure is correlated with host specificity of symbiosis, suggesting the involvement of a cognate perception system in the plant host. Here we describe the cloning of a gene from Medicago sativa that is essential for Nod-factor perception in alfalfa, and by genetic analogy, in the related legumes Medicago truncatula and Pisum sativum. The identified 'nodulation receptor kinase', NORK, is predicted to function in the Nod-factor perception/transduction system (the NORK system) that initiates a signal cascade leading to nodulation. The family of 'NORK extracellular-sequence-like' (NSL) genes is broadly distributed in the plant kingdom, although their biological function has not been previously ascribed. We suggest that during the evolution of symbiosis an ancestral NSL system was co-opted for transduction of an external ligand, the rhizobial Nod factor, leading to development of the symbiotic root nodule.     

Immunotherapy against antigenic tumors: a game with a lot of players

Our understanding of how immune responses are generated and regulated drives the design of possible immunotherapies for cancer patients. For that reason, we first describe briefly the actual immunological theories and their common perspectives about cancer vaccine development. Second, we describe cancer vaccines that are able to induce tumor-specific immune responses in cancer patients. However, these responses are not always followed by tumor rejection. At the end of the review, we discuss two possible reasons that might explain this dichotomy of cancer immunology. First, the immune response generated, although detectable, may not be quantitatively sufficient to reject the tumor. Second, the tumor microenvironment may modulate tumor cell susceptibility to the systemic immune response induced by the immunization. Finally, we discuss what, in our opinion, might be the best way to improve cancer vaccine strategies and how the relationship between the tumor and its surroundings might be studied in more details. Received 21 June 2001; received after revision 15 August 2001; accepted 15 August 2001     

Future projections for Mexican faunas under global climate change scenarios

Global climates are changing rapidly, with unexpected consequences. Because elements of biodiversity respond intimately to climate as an important driving force of distributional limitation, distributional shifts and biodiversity losses are expected. Nevertheless, in spite of modelling efforts focused on single species or entire ecosystems, a few preliminary surveys of fauna-wide effects, and evidence of climate change-mediated shifts in several species, the likely effects of climate change on species' distributions remain little known, and fauna-wide or community-level effects are almost completely unexplored. Here, using a genetic algorithm and museum specimen occurrence data, we develop ecological niche models for 1,870 species occurring in Mexico and project them onto two climate surfaces modelled for 2055. Although extinctions and drastic range reductions are predicted to be relatively few, species turnover in some local communities is predicted to be high (>40% of species), suggesting that severe ecological perturbations may result.     

Recombinational DNA double-strand breaks in mice precede synapsis

In Saccharomyces cerevisiae, meiotic recombination is initiated by Spo11-dependent double-strand breaks (DSBs), a process that precedes homologous synapsis. Here we use an antibody specific for a phosphorylated histone (gamma-H2AX, which marks the sites of DSBs) to investigate the timing, distribution and Spo11-dependence of meiotic DSBs in the mouse. We show that, as in yeast, recombination in the mouse is initiated by Spo11-dependent DSBs that form during leptotene. Loss of gamma-H2AX staining (which in irradiated somatic cells is temporally linked with DSB repair) is temporally and spatially correlated with synapsis, even when this synapsis is 'non-homologous'.     

Gate-induced superconductivity in a solution-processed organic polymer film

The electrical and optical properties of conjugated polymers have received considerable attention in the context of potentially low-cost replacements for conventional metals and inorganic semiconductors. Charge transport in these organic materials has been characterized in both the doped-metallic and the semiconducting state, but superconductivity has not hitherto been observed in these polymers. Here we report a distinct metal-insulator transition and metallic levels of conductivity in a polymer field-effect transistor. The active material is solution-cast regioregular poly(3-hexylthiophene), which forms relatively well ordered films owing to self-organization, and which yields a high charge carrier mobility (0.05-0.1 cm2 V(-1) s(-1)) at room temperature. At temperatures below approximately 2.35 K with sheet carrier densities exceeding 2.5 x 10(14) cm(-2), the polythiophene film becomes superconducting. The appearance of superconductivity seems to be closely related to the self-assembly properties of the polymer, as the introduction of additional disorder is found to suppress superconductivity. Our findings therefore demonstrate the feasibility of tuning the electrical properties of conjugated polymers over the largest range possible-from insulating to superconducting.     

Neanderthal cranial ontogeny and its implications for late hominid diversity

Homo neanderthalensis has a unique combination of craniofacial features that are distinct from fossil and extant 'anatomically modern' Homo sapiens (modern humans). Morphological evidence, direct isotopic dates and fossil mitochondrial DNA from three Neanderthals indicate that the Neanderthals were a separate evolutionary lineage for at least 500,000 yr. However, it is unknown when and how Neanderthal craniofacial autapomorphies (unique, derived characters) emerged during ontogeny. Here we use computerized fossil reconstruction and geometric morphometrics to show that characteristic differences in cranial and mandibular shape between Neanderthals and modern humans arose very early during development, possibly prenatally, and were maintained throughout postnatal ontogeny. Postnatal differences in cranial ontogeny between the two taxa are characterized primarily by heterochronic modifications of a common spatial pattern of development. Evidence for early ontogenetic divergence together with evolutionary stasis of taxon-specific patterns of ontogeny is consistent with separation of Neanderthals and modern humans at the species level.     

Phylogeography of crossbills, bullfinches, grosbeaks, and rosefinches

Mitochondrial cytochrome b (cyt b) from 24 Carduelini species including crossbills, bullfinches, grosbeaks, rosefinches, and other related, but not conclusively classified species, was sequenced. These sequences were also compared with all the available sequences from the genera Carduelis, Serinus, and Passer. Phylogenetic analyses consistently gave the same groups of finches and the calculated divergence times suggest that speciation of the studied species occurred between 14 and 3 million years ago (Miocene-Pliocene), appearing before the Passer, Carduelis, and Serinus genera. Pleistocene glaciations may have been important in sub-speciation. Crossbills are integrated within the genus Carduelis, and within redpolls; the common crossbill shows subspeciation with Loxia japonica in the Pleistocene epoch. Pinicola enucleator groups together with bullfinches and is probably the ancestor of the group. Hawfinch is only distantly related to the studied groups, and might either represent an isolated genus or be related to the New World genus Hesperiphona. The grosbeak genera Eophona and Mycerobas are clearly sister groups, and species belonging to the former might have given rise to Mycerobas species. The isolated (in classification) Uragus sibiricus and Haematospiza sipahi are included within the genus Carpodacus (rosefinches); Carpodacus nipalensis is outside the genus Carpodacus in the molecular analyses and might be an isolated species or related to the genus Montifringilla.     

The bacterial conjugation protein TrwB resembles ring helicases and F1-ATPase

The transfer of DNA across membranes and between cells is a central biological process; however, its molecular mechanism remains unknown. In prokaryotes, trans-membrane passage by bacterial conjugation, is the main route for horizontal gene transfer. It is the means for rapid acquisition of new genetic information, including antibiotic resistance by pathogens. Trans-kingdom gene transfer from bacteria to plants or fungi and even bacterial sporulation are special cases of conjugation. An integral membrane DNA-binding protein, called TrwB in the Escherichia coli R388 conjugative system, is essential for the conjugation process. This large multimeric protein is responsible for recruiting the relaxosome DNA-protein complex, and participates in the transfer of a single DNA strand during cell mating. Here we report the three-dimensional structure of a soluble variant of TrwB. The molecule consists of two domains: a nucleotide-binding domain of alpha/beta topology, reminiscent of RecA and DNA ring helicases, and an all-alpha domain. Six equivalent protein monomers associate to form an almost spherical quaternary structure that is strikingly similar to F1-ATPase. A central channel, 20 A in width, traverses the hexamer.