Catálogo de publicaciones - libros

Insects are analytical chemists . They perceive the world through semiochemicals with inordinate sensitivity. A male moth, for example, can detect a “scent of woman,” i.e., a female-produced sex pheromone, even when the signal-to-noise ratio is very low. In a sense the antennae are “signal translators.” The chemicals signals are “translated” into the language of the brain (nerve impulses or spikes) by an array of sensilla mainly located on the antennae. This information is conveyed to the brain for further processing. Chemical ecologists utilize insect antennae as biosensors for the identification of pheromones and other semiochemicals. The insect olfactory system is also highly selective, able to discriminate natural pheromones from molecules with minimal structural changes. In some cases, one stereoisomer functions as an attractant sex pheromone and its antipode is a behavioral antagonist (inhibitory signal). The specificity of the olfactory system seems to be achieved by two layers of filters. The first level of discrimination is determined by odorant-binding proteins (OBPs) that assist the hydrophobic pheromones to cross an aqueous barrier and reach their receptors. Both OBP and odorant receptor (OR) contribute to the specificity of the cell response and lead to the remarkable selectivity of the insect olfactory system. The members of the OBP-gene family, encoding the encapsulins, form a large group with olfactory and non-olfactory proteins. While the functions of many members of the family are yet to be determined, there is solid evidence for the mode of action of OBPs. Pheromones (and other semiochemicals) enter the sensillar lymph through pore tubules in the cuticle (sensillar wall), are solubilized upon being encapsulated by odorant-binding proteins, and transported to the olfactory receptors. Bound pheromone molecules are protected from odorant-degrading enzymes. Upon interaction with negatively-charged sites at the dendritic membrane, the OBP-ligand complex undergoes a conformational change that leads to the ejection of pheromone. Direct activation of odorant receptors by odorant molecules initiates a cascade of events leading to the generation of spikes. Reverse chemical ecology is a new concept for the screening of attractants based on the binding ability of OBPs to test compounds.

The Heteroptera, or true bugs, comprise a large and widely distributed group of sucking insects, with about 38,000 described species, some of which are important pests of crops. True bugs are characterized by well-developed scent glands, the contents of which provide an effective chemical defense against predation. Typical defensive compounds include short-chain alcohols, aldehydes, and esters, ()-2-alkenals, 4-oxo-()-2-alkenals, alkanes, monoterpenes, and aromatic alcohols and aldehydes. The chemistry of bug pheromones is more complex, and includes simple esters and monoterpenes, linear, monocyclic, and multicyclic sesquiterpenoids, and novel acetogenins. This chapter summarizes the identification and synthesis of these true bug semiochemicals. Overall, the identification and synthesis of chemicals produced by bugs has greatly outpaced our understanding of their specific roles as signals mediating bug behavior.

This chapter reviews chemical structures of biologically active, volatile compounds in beetles. Techniques used for structure elucidation are briefly discussed as well as facts and speculations on the biosynthesis of target compounds. Syntheses of selected substances are cursorily presented. The order of sections follows taxonomic classifications. Depending on the biological significance of relevant compounds in certain taxa, the corresponding sections are again subdivided into “attractive compounds” (mostly intraspecifically active pheromones) and “defensive compounds” (mostly interspecifically active allomones).

Research on the defensive chemistry of insects during the last decade is reviewed, with special emphasis on non-volatile compounds. The isolation and structure determination of defensive chemicals, of glandular and non-glandular origins, are first discussed, followed by an overview of the synthesis and biological/pharmacological activities of some of them. Biosynthesis has been largely omitted since this topic has been addressed in a recent review. During the period covered, beetles (e.g., coccinellids and chrysomelids) and ants have undoubtedly been the most prolific producers of repellent and/or toxic compounds. This survey also shows that alkaloids are the most frequently encountered defensive compounds in insects.

Progress that has been made in research on the chemical aspects of mammalian semiochemistry over the past decade is discussed on the basis of examples from the most topical problem areas. The chemical characterization of the volatile organic constituents of the urine, anal gland secretions and exocrine gland secretions of rodents, carnivores, proboscids, artiodactyls and primates, and their possible role in the semiochemical communication of these mammals are discussed, with particular emphasis on the advances made in the elaboration of the function of proteins as controlled release carrier materials for the semiochemicals of some of these animals.

Populations of bacterial cells often coordinate their responses to changes in their local environmental conditions through “quorum sensing”, a cell-to-cell communication system employing small diffusible signal molecules. While there is considerable diversity in the chemistry of such signal molecules, in different Gram-positive and Gram-negative bacteria they control pathogenicity, secondary metabolite production, biofilm differentiation, DNA transfer and bioluminescence. The development of biosensors for the detection of these signal molecules has greatly facilitated their subsequent chemical analysis which in turn has resulted in significant progress in understanding the molecular basis of quorum sensing-dependent gene expression. Consequently, the discovery and characterisation of natural molecules which antagonize quorum sensing-mediated responses has created new opportunities for the design of novel anti-infective agents which control infection through the attenuation of bacterial virulence.