Neural crest cell behavior in white and dark embryos of Ambystoma mexicanum: Epidermal inhibition of pigment cell migration in the white axolotl

Melanophores in larvae of the white (dd) strain of the Mexican axolotl (Ambystoma mexicanum) are confined to the dorsal midline of the trunk and dorsal posterior part of the head, whereas those in dark larvae (D_-) are distributed over the flank as well. Our results show that this phenotype of white larvae is the result of the failure of the melanophores or their neural crest precursor cells to migrate laterally due to an inhibition of or a failure in the support of their migration in the subepidermal space by the overlying epidermis. Correlated light and scanning electron microscopy of dissected larvae showed melanophores occupying the subepidermal space on the flank of dark larvae, whereas these cells were restricted to the dorsal midline of white larvae. Grafting experiments in which patches of epidermis, the underlying mesoderm, or both, were exchanged between dark and white embryos suggested that white epidermis alone can prevent the integration of pigment cells on the flank of dark larvae and, conversely, that grafts of dark epidermis alone can support their migration on the flank of white larvae. Mesoderm, when grafted alone, could not be shown to have similar effects.     

Characterization of a pilot plant airlift tower loop bioreactor: II. Evaluation of global mixing properties of the gas phase during yeast cultivation

Saccharomyces cerevisiae was cultivated in a 4-m(3) pilot plant airlift tower loop reactor with a draft tube in batch and continuous operations and for comparison in a laboratory airlift tower loop reactor of 0.08 m(3) volume. The reactors were characterized during and after the cultivation by measuring the distributions of the residence times of the gas phase with pseudostochastic tracer signals and mass spectrometer and by evaluating the mixing in the liquid phase with a pulse-shaped volatile tracer signal and mass spectrometer as a detector. The mean residence times and the intensities of the axial mixing in the riser and downcomer, the circulation times of the gas phase, and the fraction of the recirculated gas phase were evaluated and compared.     

Interaction of invertase with polyelectrolytes

In connection with our work on polyelectrolyte complex formation with polyampholytes, the interaction between invertase and several linear polyelectorlytes has been investigated by means of turbidimetry, light scattering measurements, and determination of the enzyme activity. Polyelectrolyte complex formation of invertase was shown to occur with cationic polyelectrolytes only. The light-scattering data yield information on aggregation and desegregation processes in complex formation. As indicated by our results, only a part of the protein molecules is engaged in this Coulombic interaction, and this part shows a rather small enzyme activity only. Thus, a direct interaction between invertase and a cationic polyelectrolyte is no effective approach to enzyme binding, but a complete immobilization of invertase can be achieved via an "inclusion flocculation" with a symplex formed by interaction between an anionic and a cationic linear polyelectrolyte or via immobilization in symplex microcapsules.     

Parental genomes are separated throughout the cell cycle in a plant hybrid

The positions of the genomes originating from each parent were analysed in root-tip nuclei of the mature, sexual F₁ hybrid plant Hordeum vulgare (barley) x Secale africanum (a wild rye). The two genomes of the hybrid were identified in both spread and sectioned material by non-radioactive DNA:DNA in situ hybridization using labelled total genomic DNA from one parent as a proble and unlabelled total genomic DNA from the other parent to block non-specific hybridization. Complete three-dimensional reconstructions of sets of labelled sections enabled detailed analysis of the position of the genomes at interphase. The parental genomes lay in various non-intermixed configurations, including lateral and concentric arrangements. On spread preparations, the two parental genomes were found to be spatially separated throughout the cell cycle; the genome originating from H. vulgare tended to be located more centrally than that from S. africanum. This results show that the nucleus is spatially organized above the level of the DNA and chromosome at the genome level.by M.F. Trendelenburg     

Phytoplankton biomass and chlorophyll-a in some shallow lakes in central Europe

Based on 388 parallel data of phytoplankton biomass and chlorophyll-a of two shallow lakes and two ponds, the following results were obtained:

1. Relative chlorophyll-a content of phytoplankton biomass varied between 0.08–1.88%; chlorophyll-a concentration showed great differences among lakes.
2. Significant correlations (r = 0.68–0.92) were established between phytoplankton biomass and chlorophyll-a concentration. The regression in the artificial ponds was non-linear.
3. In parallel with the increase of average cell volume, a decrease in relative chlorophyll-a content was observed. A significant correlation (r = + 0.83) between the two variables was found. Relative chlorophyll-a content of phytoplankton is a log-function of average cell volume.

     

The diet of two species of Isoperla (Plecoptera: Perlodidae) in relation to season,site, and sympatry

The seasonal change in gut contents of nymphs of Isoperla grammatica and I. difformis from six streams in southern Sweden was analysed. Both species had ingested a variety of benthic prey and vegetable matter, predominantly diatoms. Some seasonality was evident with high percentages of diatoms in spring in I. grammatica, and in autumn in I. difformis. The scope of food was larger in the latter species which contained about equal proportions of vegetable matter, chironomids, mayfly, stonefly, and black fly larvae. In I. grammatica plant matter and chironomids dominated strongly, comprising > 50% of the gut contents on an annual basis, > 90 % in spring. While small nymphs of I. difformis contained a low proportion of animal matter, only gradually increasing with size, the nymphs of I. grammatica were carnivorous from very early instars. Both species switched to a temporarily strong utilization of algae in spring. This differed among sites, and appeared to reflect differences in insolation and thus the availability of algae. There was a significant negative relationship between the mean densities of Isoperla nymphs and the proportion of animal material found in the guts of I. grammatica (R ² = 0.86). Considering the density of I. grammatica alone, the relationship was weaker (R ² = 0.56). A positive correlation between predator and prey size was observed. With chironomid prey the size range increased with predator size. With simuliid prey, however, prey size increased with predator size in such a way that it suggests selection rather than just an expanding prey size range. Correlations were stronger and regression coefficients significantly higher for I. grammatica than for I. difformis. We suggest that I. grammatica, which ingests a much wider size range of prey might choose prey of optimal sizes more readily than the more synchronously developing I. difformis. Although the life cycles of the two species are staggered, overlap in size distribution indicates that competition for food could be important in spring. However, observed differences in diet should facilitate coexistence. Gut content differences might in turn be accomplished through microhabitat segregation.     

Codon swapping as a possible evolutionary mechanism

Summary It is apparent in the genetic code that amino acids of similar chemical nature have similar codons. I show how through successive codon captures (multiple rounds of Osawa-Jukes type reassignments), complete codon swappings in an unfavorable genetic code are evolutionarily feasible. This mechanisms could have complemented the ambiguity reduction and the vocabulary extension processes of codon-amino acid assignments. Evolution of wobble rules is implied. Transfer RNA molecules and synthetases may still carry memories of it.     

Nano- and picoplankton growth and production in the Bay of Villefranche sur Mer (N.W. Mediterranean)

Plankton production in the Bay of Villefranche was relatively constant during March and April 1986 but the particle size at which the production occurred was more variable. At the beginning of the study, production was dominated by the larger (ca. 6 m) flagellates but towards the end it was more or less equally divided between the nano- and picoplankton. There were considerable differences in the estimates of population growth rates, depending on the methods used, but on average the population doubling times were close to 12 hours for autotrophs and 24 hours for heterotrophs. As autotrophs do not grow during the night, each population was therefore doubling once per day. It seemed that each of the nanoor picoplankton populations could adversely affect the growth of the others. This could be either by simple predation or by some form of inhibition. Although nutrient levels in the bay were uniformly low, the addition of nutrients did not always stimulate algal growth. The plankton populations seemed to be both in a state of equilibrium and intense ecological competition.     

Queen polymorphism in the North American harvester ant,Ephebomyrmex imberbiculus

Summary In the North American harvester antEphebomyrmex imberbiculus, in addition to dealate queens, wingless female reproductives occur that have greatly reduced ocelli and thoracic sutures (intermorphic queens). Both queen types are equivalent in function, and do not differ in ovarian morphology. Colonies may contain several inseminated and egg-laying intermorphic queens. We discuss queen polymorphism in respect to the biology of this desert-dwelling species.