One of these molecules, kahalalide F, is a potent cytotoxin and has been evaluated clinically as an anticancer agent. Both organisms are chemically defended against predators by a diverse library of lipopeptide toxins, the kahalalides, but the details of kahalalide production and diversification are unknown (see the figure). and its predator, the mollusk Elysia rufescens. RATIONALE In this work, we studied toxin production in the Hawaiian marine alga Bryopsis sp. Elucidating the molecular basis of toxin production in chemically defended organisms is important for a complete understanding of their ecological interactions. The actual source of these molecules may be the organism itself, as observed in marine algae a microbial symbiont, as commonly seen in marine sponges and tunicates or diet, as in several marine mollusks. In some cases, the same defensive molecules are shared by taxonomically distant organisms, raising questions about their molecular origin. INTRODUCTION Chemical defense strategies, in which organisms use toxic molecules for protection against pathogens or predators, are widespread in the marine environment. 1034 A bacterial endosymbiont of a marine alga generates a library of toxins that protect the alga from predation as well as the mollusk that eats it. It seems that the sea slug not only tolerates the toxins but, to protect itself from being eaten by fish, grazes on the alga to accumulate kahalalide. The authors elucidated the pathways for generating this chemical diversity. Within the alga, a species of bacterium with a very reduced genome was discovered to be a factory for the nonribosomal assembly of a family of kahalalides. wondered if a third party was involved in toxin production (see the Perspective by Mascuch and Kubanek). The alga defends itself from predators using peptide toxins decorated with fatty acids, called kahalalides. In a wider context, these findings highlight the worth of chemoinformatic approaches to a better understanding the evolution of metabolism.A little help from a friend The Hawaiian sea slug Elysia rufescens grazes on an alga called Bryopsis sp. Our results clearly show that sterols, the primal steroids that first appeared, have more conserved properties and that, from then on, more complex compounds with increasingly diverse properties have emerged, suggesting that chemical diversification parallels the expansion of biological complexity. In this work, we selected steroids as an ancient family of metabolites widely distributed in all eukaryotes and applied unsupervised machine learning techniques to reveal the traits that natural selection has imprinted on molecular properties throughout the evolutionary process. These studies, however, are still mainly centered on genes and the proteins they encode, somehow neglecting the small organic chemicals that support life processes. Modern statistical bioinformatic approaches targeted to the comparative analysis of genomes are being used to detect signatures of natural selection at the gene and population level, as an attempt to understand the origin of primordial metabolism and its expansion. Evolution of metabolism is a longstanding yet unresolved question, and several hypotheses were proposed to address this complex process from a Darwinian point of view.
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