When the feasibility of an ecosystem is sufficient for global stability?

We show via a Liapunov function that in every model ecosystem governed by generalized Lotka-Volterra equations, a feasible steady state is globally asymptotically stable if the number of interaction branches equals n-1, where n is the number of species. This means that the representative graph for which the theorem holds is a 'tree' and not only an alimentary chain. Our result is valid also in the case of non-homogeneous systems, which model situations in which input fluxes are present.

On the informational content of overlapping genes in prokaryotic and eukaryotic viruses.

In genetic language a peculiar arrangement of biological information is provided by overlapping genes in which the same region of DNA can code for functionally unrelated messages. In this work, the informational content of overlapping genes belonging to prokaryotic and eukaryotic viruses was analyzed. Using information theory indices, we identified in the regions of overlap a first pattern, exhibiting a more uniform base composition and more severe constraints in base ordering with respect to the nonoverlapping regions. This pattern was found to be peculiar to coliphage, avian hepatitis B virus, human lentivirus, and plant luteovirus families. A second pattern, characterized by the occurrence of similar compositional constraints in both types of coding regions, was found to be limited to plant tymoviruses. At the level of codon usage, a low degree of correlation between overlapping and nonoverlapping coding regions characterized the first pattern, whereas a close link was found in tymoviruses, indicating a fine adaptation of the overlapping frame to the original codon choice of the virus. As a result of codon usage correlation analysis, deductions concerning the origin and evolution of several overlapping frames were also proposed. Comparison of amino acid composition revealed an increased frequency of amino acid residues with a high level of degeneracy (arginine, leucine, and serine) in the proteins encoded by overlapping genes; this peculiar feature of overlapping genes can be viewed as a way with which they may expand their coding ability and gain new, specialized functions.

Mitochondrial activity and development: energy carrier selection in a controlled reaction mechanism.

There is evidence of an important role carried out by mitochondria and by proteins of mitochondrial origin during key moments of development. We assume that it is possible to represent the energy activity of the cell through the level of ATP production and we propose a simple model system for the ATP/ADP cycle. The model takes into account the basic aspect of ATP dynamics in macromolecular synthesis and involves the possibility of selection between states of high and low protein production. A simple extension of the model leads to the creation of a mechanism enabling the establishment of self-sustained oscillations.

A bifurcation analysis of pattern formation in a diffusion governed morphogenetic field.

We discuss from an analytical point of view the mechanism of pre-pattern formation in a diffusion governed morphogenetic field. The model here considered is a normalized form of one of the models, proposed by Gierer and Meinhardt, based on the general principle of lateral inhibition. The results, obtained in the framework of bifurcation theory, shows that there is a spontaneous formation of a gradient both for activator and inhibitor concentrations if the ratio between the mean squares of the diffusion ranges of inhibitor and activator is greater than a well defined critical value.