INVERTEBRATE‐MEDIATED NUTRIENT LOADING INCREASES GROWTH OF AN INTERTIDAL MACROALGA1

Even in nitrogen‐replete ecosystems, microhabitats exist where local‐scale nutrient limitation occurs. For example, coastal waters of the northeastern Pacific Ocean are characterized by high nitrate concentrations associated with upwelling. However, macroalgae living in high‐zone tide pools on adjacent rocky shores are isolated from this upwelled nitrate for extended periods of time, leading to nutrient limitation. When high‐intertidal pools are isolated during low tide, invertebrate‐excreted ammonium accumulates, providing a potential nitrogen source for macroalgae. I quantified the influence of mussels (Mytilus californianus Conrad) on ammonium accumulation rates in tide pools. I then evaluated the effects of ammonium loading by mussels on nitrogen assimilation and growth rates of Odonthalia floccosa (Esp.) Falkenb., a common red algal inhabitant of pools on northeastern Pacific rocky shores. Odonthalia was grown in artificial tide pool mesocosms in the presence and absence of mussels. Mesocosms were subjected to a simulated tidal cycle mimicking emersion and immersion patterns of high‐intertidal pools on the central Oregon coast. In the presence of mussels, ammonium accumulated more quickly in the mesocosms, resulting in increased rates of nitrogen assimilation into algal tissues. These increased nitrogen assimilation rates were primarily associated with higher growth rates. In mesocosms containing mussels, Odonthalia individuals added 41% more biomass than in mesocosms without mussels. This direct positive effect of mussels on macroalgal biomass represents an often overlooked interaction between macroalgae and invertebrates. In nutrient‐limited microhabitats, such as high‐intertidal pools, invertebrate‐excreted ammonium is likely an important local‐scale contributor to macroalgal productivity.     

Does Weeding Promote Regeneration of an Indigenous Tree Community in Felled Pine Plantations in Uganda?

The use of plantations to manage extensive tracks of deforested lands in the tropics is a conservation strategy that has recently received considerable attention. Plantation trees can promote seed dispersal by attracting dispersers and creating favorable site conditions, leading to increased germination and establishment of indigenous trees. Subsequently, plantation trees can be harvested for profit or left to senesce, leaving a native tree community. We evaluated the effect of vine, grass, and shrub cutting (weeding) over a 3‐year period on regeneration of indigenous trees subsequent to the removal of plantation softwoods in Kibale National Park, Uganda. Counter to what would be expected if weeding released trees from competition, we found no difference in the total number of stems or in the stems greater than 10 cm diameter at breast height between control and weeded plots; there were more stems greater than 1 cm diameter at breast height in the control plots. For species found in both control and weeded plots, the maximum size of individuals did not differ. At the end of the study, 61 species were found in the control plots and 43 species were found in the weeded plots, and in both types of plots the three most abundant species were the same. The number of species and stems classified as early or middle successional species did not differ between weeded and control plots. The fact that weeding did not promote regeneration of indigenous trees after the removal of plantation trees illustrates the importance of evaluating and field‐testing potential management options.     

Cholesteryl ester transfer protein is secreted by Hep G2 cells and contains asparagine-linked carbohydrate and sialic acid

A cholesteryl ester transfer protein (CETP) of apparent Mr 74,000 has recently been purified from human plasma. Cholesteryl ester transfer activity was found to accumulate in the medium of cultured Hep G2 cells. The transfer activity was removed by immunoprecipitation with specific antibodies to the plasma CETP. Sodium dodecyl sulfate gel electrophoresis of immunoprecipitates prepared from the medium of cells pulsed with [35S]methionine revealed a broad specific band of protein of Mr 72,000 to 76,000; by contrast, immunoprecipitates of cellular homogenates showed a sharp specific band of Mr 58,000. The Mr 72,000 to 76,000 band disappears, concomitant with the appearance of lower Mr products, upon neuraminidase or glycopeptidase F treatment of medium immunoprecipitates or of purified CETP. The results indicate that liver cells have the capacity to synthesize and secrete CETP. The CETP peptide acquires asparagine-linked carbohydrate and sialic acid during intracellular processing.     

Mitochondrial DNA of the extinct quagga: Relatedness and extent of postmortem change

Sequences are reported for portions of two mitochondrial genes from a domestic horse and a plains zebra and compared to those published for a quagga and a mountain zebra. The extinct quagga and plains zebra sequences are identical at all silent sites, whereas the horse sequence differs from both of them by 11 silent substitutions. Postmortem changes in quagga DNA may account for the two coding substitutions between the quagga and plains zebra sequences. The hypothesis that the closest relative of the quagga is the domestic horse receives no support from these data. From the extent of sequence divergence between horse and zebra mitochondrial DNAs (mtDNAs), as well as from information about the fossil record, we estimate that the mean rate of mtDNA divergence in Equus is similar to that in other mammals, i.e., roughly 2% per million years.     

Relationships ofLomatium among other genera ofApiaceae

Lomatium is the largest North American genus ofApiaceae (=Umbelliferae). Relations amongLomatium and other genera have not been well understood. TheApiaceae-Araliaceae lineage and genera of North AmericanApiaceae were analyzed phenetically and cladistically in turn, and seven phenetic alliances of genera are identified. Data on frequencies of character-state combinations were assembled and usingSporne's notions of maximal and minimal co-variation of states, two character-states were identified that are derived among Umbellifer genera. Using cladistic analysis of the alliance consisting ofLomatium and six other genera we identified three sister groups toLomatium:Polytaenia alone;Polytaenia withPseudocymopterus, and a group of four genera. Not all genera in the group of seven withLomatium are supported by autapomorphies impeding phylogenetic reconstructions. Both phenetics and cladistics have helped to suggest and evaluate relationships; to call attention to groups requiring further study, and in particular to identify derived character-states.     

In vivo depletion of mannose receptor bearing cells from rat liver by ricin A chain: effects on clearance of beta-glucuronidase

Ricin A chain has previously been shown to intoxicate macrophages in vitro following binding and endocytosis by the macrophage mannose receptor. In this report it is demonstrated that the intravenous injection of ricin A chain in nephrectomized rats leads to a prolonged plasma half-life for [125I]beta-glucuronidase, a ligand for the mannose receptor. Clearance of [125I]asialofetuin, a ligand for the galactose receptor of hepatocytes, was unaffected by injection of A chain. Microscopic examination of the livers of A chain-treated animals revealed a loss of phagocytic cells from the liver sinusoids. These results suggest that ricin A chain may be useful as a toxin specific for mannose receptor bearing cells of the reticuloendothelial system.     

The Influence of Nonautonomous P Elements on Hybrid Dysgenesis in Drosophila melanogaster

An inbred line of the M' strain Muller-5 Birmingham was studied for its abilities to affect P-M hybrid dysgenesis. This strain possesses 57 P elements, all of which are apparently defective in the production of the P transposase. In combination with transposase-producing elements, these nonautonomous elements can enhance or diminish the incidence of hybrid dysgenesis, depending on the trait that is studied. Dysgenic flies that have one or more paternally-derived chromosomes with these elements partially repress the instability of the P element insertion mutation, snw; however, such flies have elevated frequencies of another dysgenic trait, GD sterility, and also show distorted segregation ratios. An explanation is presented in which all of these phenomena are unified as manifestations of the kinetics of P element activation in the germ line. The progeny of Muller-5 Birmingham females exhibit partial repression of both snw instability and GD sterility. This repression appears to involve a factor that can be transmitted maternally through at least two generations. This mode of repression therefore conforms to the pattern of inheritance of the P cytotype, the condition that brings about nearly total repression of P element activity in some strains. Models in which this repression could arise from the nonautonomous P elements of Muller-5 Birmingham are discussed.     

Regulation of intracellular pH by a neuronal homolog of the erythrocyte anion exchanger

We have isolated AE3, a novel gene expressed primarily in brain neurons and in heart. The predicted AE3 polypeptide shares a high degree of identity with the anion exchange and cytoskeletal binding domains of the erythrocyte band 3 protein. Expression of AE3 cDNA in COS cells leads to chronic cytoplasmic acidification and to chloride- and bicarbonate-dependent changes in intracellular pH, confirming that this gene product is an anion exchanger. Characterization of an AE3 mutant lacking the NH2-terminal 645 amino acids demonstrates that the COOH-terminal half of the polypeptide is both necessary and sufficient for correct insertion into the plasma membrane and for anion exchange activity. The NH2-terminal domain may play a role in regulating the activity of the exchanger and may be involved in the structural organization of the cytoskeleton in neurons.