1ULB, Dpt of Molecular Biology, Unit of Evolutionary Genetics cp300, Rue Jeener and Brachet 12, B-6041 Gosselies, Belgium.
2ULB, Laboratoire de biologie Animale, CP 160/12, Avenue F.D. Roosevelt 50, B-1050 Brussels, Belgium.
Larvae of Chrysomela leaf beetles release for defense volatile compounds belonging to various chemical families. Our study focuses on the mixed defensive strategy based on isobutyric and 2-methylbutyric acids (produced by the insect) and esters of them with a wide variety of alcohols (taken up from the host plant). To date, only two species are known to produce these repellents: C. interrupta associated with Betulaceae, and C. lapponica occuring either on Betulaceae or Salicaceae. In order to investigate whether these compounds occur in other species and how the food plant influences the secretion of these toxins, we used mass spectrometry for detecting iso- and 2-methylbutyric acids and esters of them in the defensive secretions of larvae from Chrysomela species exclusively associated with Betulaceae or Salicaceae. Targeting analyses reveal that the synthesis of these compounds is a character found in all investigated members of the C. interrupta group (sensu Brown, 1956) regardless of the specific host-plant family the larva feeds on. Moreover, the secretions were quantitatively assayed, revealing a chemical plasticity developed by Chrysomela species associated with Salicaceae: the amounts of iso- and 2-methylbutyric acids derivatives and of salicylaldehyde depend on the food plant and, more specifically, on its content in phenolglucosides. In order to provide a phylogenetic hypothesis on the relationships among Chrysomela species, we sequenced a fragment of the COI and COII mitochondrial genes from 19 species belonging to the Chrysomelina subtribe. Our analyses of these molecular data indicate that the defense of mixed origin described above is a synapomorphy for the interrupta group.