Toxicity in animals. Trends in evolution?

Animals acquire toxicity either by metabolic synthesis of toxins (secondary metabolites), by expression of toxin genes or by the uptake, storage and s equestration of toxins produced by other organisms, i.e., microbes, plants or other animals. Variability of toxin structure and function is high. Pept ide toxins in particular, although relying on a limited number of structura l Frameworks, often exhibit considerable structural hypervariability. An ac celerated rate of evolution in the toxin gene structure (conserved introns, but high substitution rates in the exons) leads to the functional diversit y of these peptides or proteins. The selective forces which may drive toxin evolution an unknown. Venomousness or the possession of toxins can be esse ntial for survival, but the advantage of toxin biosynthesis may also be of minor importance or has been lost during evolution. (C) 2000-Elsevier Scien ce Ltd. All rights reserved.