Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.     

Ten different Polycomb group genes are required for spatial control of the abdA and AbdB homeotic products.

Mutations in genes of the Polycomb (Pc) group cause abnormal segmental development due to ectopic expression of the homeotic products of the Antennapedia and bithorax complexes. Here the requirements for Pc group genes in controlling the abdA and AbdB products of the bithorax complex are described. Embryos containing mutations in the genes Polycomb (Pc), extra sex combs (esc), Enhancer of zeste [E(z)], polyhomeotic (ph), Sex comb on midleg (Scm), Polycomb-like (Pcl), Sex comb extra (Sce), Additional sex combs (Asx), Posterior sex combs (Psc) and pleiohomeotic (pho) were examined. In every case, both abdA and AbdB are expressed outside of their normal domains along the anterior-posterior (A-P) axis, consistent with these Pc group products acting in a single pathway or molecular complex. The earliest detectable ectopic expression is highest in the parasegments immediately adjacent to the normal expression boundary. Surprisingly, in the most severe Pc group mutants, the earliest ectopic AbdB is distributed in a pair-rule pattern. At all stages, ectopic abdA in the epidermis is highest along the anterior edges of the parasegments, in a pattern that mimics the normal abdA cell-specific pattern. These examples of highly patterned mis-expression show that Pc group mutations do not cause indiscriminate activation of homeotic products. We suggest that the ectopic expression patterns result from factors that normally activate abdA and AbdB only in certain parasegments, but that in Pc group mutants these factors gain access to regulatory DNA in all parasegments.     

Isolation and physiological study of an amylolytic strain of Lactobacillus plantarum

Summary An amylolytic lactic acid bacterium identified as Lactobacillus plantarum was isolated from cassava roots (Manihot esculenta var. Ngansa) during reting. The amylolytic enzyme synthesized was an extracellular -amylase with an optimum pH of 5.0 and an optimum temperature of 55° C. Cultured on starch, the strain displayed a growth rate of 0.43 h^–1, a biomass yield of 0.19 g·g^–1 and a lactate yield of 0.81 g·g^–1. The growth kinetics were similar on starch and glucose. Sufficient enzyme was synthesized and starch hydrolysis was not a limiting factor for growth. Biosynthesis of the enzyme was observed when the glucose concentration was less than 6.7 g·l^–1 and reached up to 4 IU·ml^–1 at the end of the fermentation. Offprint requests to: M. Raimbault     

Nucleosomal organization of chromatin in sperm nuclei of the bivalve mollusc Aulacomya ater

The sperm nuclei of Aulacomya ater, family Mitylidae, contain three proteins (X, Aa5 and Aa6) which are specific to this cell type coexisting with a set of five somatic-type histones. Information about the chromatin structure resulting from this kind of association is scarce. Therefore, we have probed the structure of this sperm chromatin through digestion with micrococcal nuclease in combination with salt fractionation. The data obtained have allowed us to propose a nucleosomal arrangement for this chromatin. However, two types of nucleosomes would be present in agreement with their protein components.     

Electrotransformation of Streptococcus pyogenes with plasmid and linear DNA

Electrotransformation was used to introduce both plasmid and linear DNA into Streptococcus pyogenes. The method was optimized using strain NZ131, for which transformation frequencies up to 10(7) per micrograms of plasmid DNA were obtained. A linear fragment of DNA, containing the streptokinase gene (ska) in which an internal fragment had been replaced with an erythromycin resistance gene (erm), was transformed into strain NZ131 with a frequency of 10(3) per micrograms DNA. The introduction of linear DNA into S. pyogenes by electrotransformation should be useful for future genetic analyses as well as targeted gene replacement.