Norsesquiterpenes as defensive toxins of whirligig beetles (Coleoptera: Gyrinidae)

Abstract

By use of a minnow bioassay, toxins were detected in the pygidial secretions of the gyrinid beetlesDineutus assimilis (Kirby) andDineutus nigrior Roberts. The active agents, which may be largely responsible for the relative immunity of the Gyrinidae from predation, were isolated and identified as the norsesquiterpenesgyrinidione [(E)-1-methyl-2-formyl-3-(1′-methylhex-3′-ene-2′,5′-dione)-cyclopentane],gyrinidone [(E,Z)-2-hydroxy-5,9-dimethyl-4-(but-1′-ene-3′-one)-3-oxo-bicyclo[4.3.0]-non-4-ene],gyrinidal [(E,E,E)-3,7-dimethyl-8,11-dioxo-2,6,9-dodecatrienal], andisogyrinidal[(E,E,Z)-3,7-dimethyl-8,11-dioxo-2,6,9-dodecatrienal]. Since gyrinidione and isogyrinidal are being reported for the first time, their physical and chemical properties are presented and biosynthetic relationships of the four norsesquiterpene structures are discussed. About 50% of the beetle defensive material was norsesquiterpenes, 25% polar lipids, and 20% could not be extracted from water into chloroform. As quantified by gas-liquid chromatography,D. assimilis contained 245±73 μg andD. nigrior 144±64 μg norsesquiterpenes per individual. The average relative composition of norsesquiterpenes in the pygidial secretions of both beetle species was constant: isogyrinidal, 6%; gyrinidone, 7%; gyrinidione, 36%; and gyrinidal, 48%. When administered externally in solution to fish, isolated norsesquiterpenes possessed narcotic and toxic activity similar to that of the anesthetic steroids deoxycorticosterone (DOC) and testosterone. Minnow dose-response curves demonstrated that gyrinidione and gyrinidal (LC₁₀₀=ca. 2 μg/ml) were as toxic to fish as was DOC (LC₁₀₀=ca. 3 μg/ml). Gyrinidone was less toxic (LC₁₀₀=ca. 15 μg/ml) while isogyrinidal was relatively inactive (LC₁₀₀=ca. 90 μg/ml).