Mediation of host-plant use by a glucoside in Callosobruchus maculatus F (Coleoptera: Bruchidae)

Callosobruchus maculatus is a bruchid beetle that develops in the seeds of 14 species of Leguminosae, but the cowpea Vigna unguiculata is its main host plant, The conditions of adaptation for a C. maculatus population originating from West Africa on a new host plant, Vicia faba, were analysed in this study, When the C. maculatus females oviposited on the seeds of V. faba, 2.4% of the larvae penetrated the cotyledons and completed their post-embryonic development, The other larvae died as soon as they began to consume the cotyledons, The presence of a glucoside, vicine, in the seeds of V. faba was the main mortality factor, Vicine is hydrolysed by intestinal beta-glucosidase to a toxic aglycone, divicine, which has adverse effects on larval metabolism, From a pair of C. maculatus that developed in V. faba seeds having a high vicine content, a strain with high larval performance on these seeds was selected, The larvae of this strain could develop in pastilles of cowpea flour containing 0.1-1% vicine, A higher concentration (1.5%) caused significant mortality, Crosses between beetles of the selected and nonselected strains demonstrated that the ability to develop in the V. faba seeds had a genetic basis, and a major autosomal gene was implicated, Larvae completing their post-embryonic development in V. faba seeds were recessive homozygous for this character, The inability to develop in these seeds was due to the presence of a major dominant gene, Biochemical studies showed that midgut beta-glucosidase activity was lower in the selected strain than in the nonselected strain, The ability to develop in seeds with high vicine content is probably due to partial inhibition of a gene coding for a beta-glucosidase that hydrolyses vicine to the toxic form, aglycone, Vicine is transferred from the midgut to the haemolymph without chemical transformation, This adaptation to a new host plant, due to low enzymatic activity preventing the transformation of a glucoside to its toxic aglycone has a low energetic cost. (C) 1997 Elsevier Science Ltd.