As typical Staphylinidae, rove beetles of the genera Stenus and Dianous possess a slim body shape and strongly reduced elytra. This body form enhances the flexibility of the abdomen that allows the beetles to better colonize habitats with small interstices, but also significantly increases the susceptibility to predation and parasitism due to the missing protection of the elytra. To compensate for this disadvantage, the Steninae synthesize and store several highly effective defensive compounds in abdominal defense glands. The aim of this dissertation was to gain further insights into the complex chemical ecology of the Steninae. The morphology, ultrastructure and function of the pygidial gland system as well as the biological activity of the defense compounds and secretions were analyzed. Furthermore, hypotheses concerning potential evolutionary trends and phylogenetic relationships within the Steninae were formulated based on analyses of the biosynthesis of the nitrogen containing secretion compounds and their distribution among different species. With regard to the chemical ecology of the Steninae, the pygidial glands are of greatest importance for these beetles. The paired defense glands are located lateral to the gut and dorsal to the genitals in the last three to four segments of the abdomen. They consist of two big reservoirs with a band of secretory tissue, located on the reservoir surface, together with two smaller reservoirs and the corresponding gland tissue situated at the base of the big reservoirs. In both secretory tissues the secretion is produced by gland cells containing an elongated extracellular cavity. The synthesized compounds are secreted via numerous microvilli in the gland cell membrane into the extracellular cavity, where it collects in a cuticular tubulus and is conducted to the corresponding reservoir. External to the gland cell, the tubulus is surrounded by a canal cell, forming together with the gland cell a glandular unit. The structure and function of the big reservoirs and the band of secretory tissue are identic in all Steninae. The small reservoirs, however, as well as their corresponding gland tissues are often more or less reduced and in many species even functionless. The secretions of the big reservoirs consist of the alkaloid compounds stenusine, 3-(2-methyl-1-butenyl)pyridine and cicindeloine and sometimes the nor- or dehydro-derivates of these compounds, whereby the qualitative, as well as the quantitative composition of the secretion is species specific. The small reservoirs contain monoterpenes such as 1,8-cineole, α-pinene and 6-methyl-5-hepten-2-one and related compounds. In bioassays all secreted compounds revealed a significant deterrent activity against the ant Lasius flavus and the fish Xiphophorus helleri. Furthermore, an antimicrobial activity against bacteria and fungi was observed with most secreted compounds. The deterrent, as well as the antimicrobial activity of the alkaloid compounds turned out to be for the most part more effective than the terpenes. In bioassays with two out of four stereoisomers of the chiral compound stenusine ants discriminated between the tested diastereomers, while antibacterial activity was identical with both stenusine isomers. All alkaloids stored in the big reservoirs exhibit the same molecular base structure consisting of an N-heterocyclic six-ring, substituted by a branched alkyl side chain. By feeding deuterated amino acids to the beetles, it was possible to demonstrate a biogenetical relationship between the alkaloids stenusine, 3-(2-methyl-1-butenyl)pyridine and Cicindeloine. In all three compounds the heterocyclic ring structure is synthesized out of L-lysine and the side chain out of L-isoleucine. The biosynthesis of the compounds first follows an identical pathway up to two precursor molecules that can be transformed into the alkaloid products by a few chemical modifications. Concerning the morphology of the pygidial glands, as well as the qualitative and quantitative composition of the Steninae’s pygidial gland secretions, some evolutionary trends can be recognized. The phylogenetically primitive species S. comma and S. biguttatus both possess well developed and completely functional small reservoirs and gland tissue at the base of the big reservoirs. The secretion is made up by stenusine, norstenusine, as well as the terpenes stored in the small reservoirs. During evolution of the Steninae, the small reservoir and the corresponding secretory tissue were reduced and the terpene content of the secretion minimized. Subsequently, the alkaloid repertoire was first extended by 3-(2-methyl-1-butenyl)pyridine and later by cicindeloine, whereas stenusine and norstenusine were step by step replaced by other alkaloids.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
|Subjects:||Biology, Ecology, Entomology, Physiology|
|Keywords:||Rove beetles, Steninae, Elytra|
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