蹄盖蕨科的亚科划分的修订

蹄盖蕨科Athyriaceae是蕨类植物中复杂的大科,分子系统学的研究证据表明它是一个自然类群。前人根据染色体的基数,将蹄盖蕨科划分为3个亚科,但没有得到分子证据的支持;本文依据分子系统学的研究结果,再结合形态特征,将该科重新划分为5个亚科：冷蕨亚科、蹄盖蕨亚科、对囊蕨亚科、双盖蕨亚科和轴果蕨亚科。     

A novel, 11 nucleotide variant of chi, chi*: one of a class of sequences defining the Escherichia coli recombination hotspot chi

In wild-type Escherichia coli, recognition of the recombination hotspot, chi (5'-GCTGGTGG-3'), by the RecBCD enzyme is central to homologous recombination. However, in the recC* class of RecBCD mutants, stimulation of recombination by the canonical chi sequence is not detectable, but the levels of homologous recombination are nearly wild-type. In vivo studies demonstrate that a member of this class of mutants, the recC1004 allele, encodes an enzyme that responds to a novel variant of chi, termed chi* (5'-GCTGGTGCTCG-3'). Here, we establish that, in vitro, the chi* sequence is recognized more efficiently by the RecBC(1004)D enzyme than is the wild-type chi. This is manifest by both a greater modification of nuclease activity and a higher stimulation of RecA protein-mediated joint molecule formation at chi* than at chi. Sequencing of the recC1004 gene revealed that it contains a frameshift mutation, which results in a replacement of nine of the wild-type amino acid residues by eight in the mutant protein, and defines a locus that is important for the specificity of chi-recognition. In addition, we show that this novel, 11 nucleotide chi* sequence also regulates the wild-type RecBCD enzyme, supporting the notion that variants of the canonical chi constitute a class of sequences that regulate the recombination function of RecBCD enzyme.     

A note on permutation tests for variance components in multilevel generalized linear mixed models

In many applications of generalized linear mixed models to multilevel data, it is of interest to test whether a random effects variance component is zero. It is well known that the usual asymptotic chi-square distribution of the likelihood ratio and score statistics under the null does not necessarily hold. In this note we propose a permutation test, based on randomly permuting the indices associated with a given level of the model, that has the correct Type I error rate under the null. Results from a simulation study suggest that it is more powerful than tests based on mixtures of chi-square distributions. The proposed test is illustrated using data on the familial aggregation of sleep disturbance.     

A note: gut bacteria produce components of a locust cohesion pheromone

AIMS: Faecal pellets from germ-free locusts were used as culture media to determine the ability of locust gut bacteria to synthesize phenolic components of the locust cohesion pheromone. METHODS AND RESULTS: Inoculation of germ-free faecal pellets with Pantoea agglomerans, a species commonly isolated from locusts, resulted in the release of large amounts of guaiacol and small amounts of phenol, both of which are components of the locust cohesion pheromone. Two other locust-derived species, Klebsiella pneumoniae pneumoniae and Enterobacter cloacae, also produced guaiacol from germ-free faecal pellets, but the opportunistic locust pathogen, Serratia marcescens, did not. The most likely precursor for guaiacol is the plant-derived vanillic acid, which is present in large amounts in the faeces of both conventional and germ-free locusts. CONCLUSIONS: These observations are consistent with previous ones, that locust gut bacteria are responsible for the production of components of the locust cohesion pheromone. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings illustrate how an insect can adapt to make use of a common bacterial metabolite produced by one or more of its indigenous gut bacterial species. This observation has implications for our appreciation of insect gut microbiota interactions.     

A note on the horopter

By assuming the fixity (but not the symmetry) of corresponding points on the two retinae, it is possible to derive the equation of any horopter when one is known. In particular when, as experiment shows, one horopter is linear, then all horopters must be conics. These have the form given by Ogle, but whereas Ogle leaves one parameter undetermined at each fixation, on our assumption the only arbitrary parameter is determined by the position of the linear horopter.     

A note on Ca2+ binding to calmodulin

Binding of Ca2+ to calmodulin has been simulated on the basis of a model that assumes two classes, two sites in each class, of Ca2+ binding sites. With properly chosen values of binding constants for the two classes of sites, and with the assumption that certain degree of positive cooperativity exists between the two sites in each class, the overall binding isotherm can be generated so that it appears to be a single-transition, non-cooperative binding curve of four equivalent sites. Thus this model offers a resolution for some of the discrepancies among Ca2+ binding studies of calmodulin.