Wettable and unsinkable: the hydrodynamics of saccate pollen grains in relation to the pollination mechanism in the two New Zealand species of Prumnopitys Phil. (Podocarpaceae)

The pollination mechanism of most genera of the Podocarpaceae involves inverted ovules, a pollination drop and bisaccate pollen grains. Saccate grains have sometimes been referred to as 'non-wettable' due to their buoyant properties, while non-saccate pollen grains have been described as 'wettable'. The hydrodynamic properties of saccate pollen grains of seven podocarp species in five genera, Dacrydium Sol. ex G. Forst., Dacrycarpus (Endl.) de Laub., Manoao Molloy, Podocarpus L'Hér. ex Pers. and Prumnopitys Phil. have been tested in water, together with saccate and non-saccate pollen of four other conifer genera, Cedrus Trew (Pinaceae), Cephalotaxus Siebold & Zucc. ex Endl. (Cephalotaxaceae), Cupressus L. (Cupressaceae) and Phyllocladus Rich. ex Mirb. (Phyllocladaceae), and spores of three fern species and one lycopod species. All four spore types studied were non-wettable, whereas the bisaccate and trisaccate pollen types, like all other conifer pollen types, were wettable, enabling the grains to cross the surface tension barrier of water. Once past this barrier, grain behaviour was governed by presence or absence of sacci. Non-saccate and vestigially saccate grains sank, whereas saccate grains behaved like air bubbles, floating up to the highest point. In addition, the grains were observed to float in water with sacci uppermost, consistent with the suggestion that distally placed sacci serve to orientate the germinal furrow of the pollen grain towards the nucellus of an inverted ovule. Observations of pollen grains in the pollen chambers of naturally pollinated Prumnopitvs ovules confirmed this. The combination of buoyancy and wettability in saccate pollen has implications for the efficiency of the typical podocarp pollination mechanism.     

Applicability of a model for non-pathogenic Escherichia coli for predicting the growth of pathogenic Escherichia coli

A model was developed for the temperature dependence of growth rate of a non-pathogenic Escherichia coli strain. The suitability of that model for predicting the growth rate of pathogenic E. coli strains was assessed. Growth rates of pathogenic strains were found to be adequately described by the model. Model predictions were also found to describe sufficiently well-published growth rate data for non-pathogenic E. coli on mutton carcase surfaces and E. coli O157:H7 in ground roasted beef, milk, and on cantaloupes and water melons. In addition, E. coli O157:H7 was found to grow in the region of 44–45·5 °C.     

Appropriate in vivo expression of a muscle-specific promoter by using avian retroviral vectors for gene transfer [corrected]

The promoter regions of the chicken skeletal muscle alpha-actin (alpha sk-actin) and the cytoplasmic beta-actin genes were linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. Replication-competent retroviral vectors were used to introduce these two actin/CAT cassettes into the chicken genome. Chickens infected with retroviruses containing the alpha sk-actin promoter expressed high levels of CAT activity in striated muscle (skeletal muscle and heart); much lower levels of CAT activity were produced in the other nonmuscle tissues. In contrast, chickens infected with retroviruses containing the beta-actin promoter linked to the CAT gene expressed low levels of CAT activity in many different tissue types and with no discernible tissue specificity. Data are presented to demonstrate that the high levels of CAT activity that were detected in the skeletal muscle of chickens infected with the retrovirus containing the alpha sk-actin promoter/CAT cassette were not due to preferential infectivity, integration, or replication of the retrovirus vector in the striated muscles of these animals.     

A comparison of methods for estimating the transition:transversion ratio from DNA sequences

Estimation of the ratio of the rates of transitions to transversions (TI:TV ratio) for a collection of aligned nucleotide sequences is important because it provides insight into the process of molecular evolution and because such estimates may be used to further model the evolutionary process for the sequences under consideration. In this paper, we compare several methods for estimating the TI:TV ratio, including the pairwise method [TREE 11 (1996) 158], a modification of the pairwise method due to Ina [J. Mol. Evol. 46 (1998) 521], a method based on parsimony (TREE 11 (1996) 158), a method due to Purvis and Bromham [J. Mol. Evol. 44 (1997) 112] that uses phylogenetically independent pairs of sequences, the maximum likelihood method, and a Bayesian method [Bioinformatics 17 (2001) 754]. We examine the performance of each estimator under several conditions using both simulated and real data.     

Domain-specific purinergic signaling in polarized rat cholangiocytes

In cholangiocytes, adenine nucleotides function as autocrine/paracrine signals that modulate ductular ion transport by activation of purinergic receptors. The purpose of these studies was to identify cellular signals that modulate ATP release and nucleotide processing in polarized normal rat cholangiocytes. In Ussing chamber studies, selective exposure of the apical and basolateral membranes to ATP or adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) stimulated increases in short-circuit current. Apical purinergic receptor agonist preference was consistent with the P2Y(2) subtype. In contrast, basolateral ADP was more potent in stimulating transepithelial currents, consistent with the expression of different basolateral P2 receptor(s). Luminometric analysis revealed that both membranes exhibited constitutive ATP efflux. Hypotonic exposure enhanced ATP release in both compartments, whereas decreases in ATP efflux during hypertonicity were more prominent at the apical membrane. Increases in intracellular cAMP, cGMP, and Ca(2+) also increased ATP permeability, but selective effects on apical and basolateral ATP release differed. Finally, the kinetics of ATP degradation in apical and basolateral compartments were distinct. These findings suggest that there are domain-specific signaling pathways that contribute to purinergic responses in polarized cholangiocytes.     

Stochastic search strategy for estimation of maximum likelihood phylogenetic trees

The maximum likelihood (ML) method of phylogenetic tree construction is not as widely used as other tree construction methods (e.g., parsimony, neighbor-joining) because of the prohibitive amount of time required to find the ML tree when the number of sequences under consideration is large. To overcome this difficulty, we propose a stochastic search strategy for estimation of the ML tree that is based on a simulated annealing algorithm. The algorithm works by moving through tree space by way of a "local rearrangement" strategy so that topologies that improve the likelihood are always accepted, whereas those that decrease the likelihood are accepted with a probability that is related to the proportionate decrease in likelihood. Besides greatly reducing the time required to estimate the ML tree, the stochastic search strategy is less likely to become trapped in local optima than are existing algorithms for ML tree estimation. We demonstrate the success of the modified simulated annealing algorithm by comparing it with two existing algorithms (Swofford's PAUP* and Felsenstein's DNAMLK) for several theoretical and real data examples.     

Role of intercellular adhesion molecule 1 in indomethacin-induced ileitis

Adhesion molecules have been implicated in the pathogenesis of inflammatory bowel diseases. We investigated their expression and contribution to leukocyte recruitment in experimental intestinal inflammation. Ileitis was induced in Sprague-Dawley rats by two injections of indomethacin (7.5 mg/kg), given 24 h apart. Endothelial intercellular adhesion molecule-1 (ICAM-1) expression was quantified using the dual radiolabeled monoclonal antibody technique and Mac-1 (CD11b/CD18) expression on leukocytes by flow cytometry. Leukocyte infiltration was monitored by tissue myeloperoxidase (MPO) activity. The first indomethacin injection induced a time- and site-dependent increase of ICAM-1 expression in ileal mucosa and muscularis. The second injection resulted in a reduction of ICAM-1 expression below constitutive levels whereas Mac-1 was upregulated. MPO changes paralleled lesion development over 48 h. ICAM-1 and MPO values were correlated for the first 24 h. Immunoneutralization of either ICAM-1 or Mac-1 attenuated mucosal injury. We conclude that (i) indomethacin-induced ileitis is associated with a temporally disassociated upregulation of ICAM-1 and (ii) despite a reduction in ICAM-1 after 24 h, ICAM-1, in concert with Mac-1, contributes to mucosal injury and leukocyte infiltration elicited by indomethacin.     

CAKbeta/Pyk2 kinase is a signaling link for induction of long-term potentiation in CA1 hippocampus

Long-term potentiation (LTP) is an activity-dependent enhancement of synaptic efficacy, considered a model of learning and memory. The biochemical cascade producing LTP requires activation of Src, which upregulates the function of NMDA receptors (NMDARs), but how Src becomes activated is unknown. Here, we show that the focal adhesion kinase CAKbeta/Pyk2 upregulated NMDAR function by activating Src in CA1 hippocampal neurons. Induction of LTP was prevented by blocking CAKbeta/Pyk2, and administering CAKbeta/Pyk2 intracellularly mimicked and occluded LTP. Tyrosine phosphorylation of CAKbeta/Pyk2 and its association with Src was increased by stimulation that produced LTP. Finally, CAKbeta/Pyk2-stimulated enhancement of synaptic AMPA responses was prevented by blocking NMDARS, chelating intracellular Ca(2+), or blocking Src. Thus, activating CAKbeta/Pyk2 is required for inducing LTP and may depend upon downstream activation of Src to upregulate NMDA receptors.