MEDIATION OF HOST-PLANT USE BY A GLUCOSIDE IN CALLOSOBRUCHUS-MACULATUS F (COLEOPTERA, BRUCHIDAE)

Callosobruchus maculatus is a bruchid beetle that develops in the seed s of 14 species of Leguminosae, but the cowpea Vigna unguiculata is it s main host plant, The conditions of adaptation for a C. maculatus pop ulation 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 glu coside, vicine, in the seeds of V. faba was the main mortality factor, Vicine is hydrolysed by intestinal beta-glucosidase to a toxic aglyco ne, divicine, which has adverse effects on larval metabolism, From a p air of C. maculatus that developed in V. faba seeds having a high vici ne content, a strain with high larval performance on these seeds was s elected, The larvae of this strain could develop in pastilles of cowpe a flour containing 0.1-1% vicine, A higher concentration (1.5%) caused significant mortality, Crosses between beetles of the selected and no nselected strains demonstrated that the ability to develop in the V. f aba seeds had a genetic basis, and a major autosomal gene was implicat ed, Larvae completing their post-embryonic development in V. faba seed s were recessive homozygous for this character, The inability to devel op in these seeds was due to the presence of a major dominant gene, Bi ochemical studies showed that midgut beta-glucosidase activity was low er in the selected strain than in the nonselected strain, The ability to develop in seeds with high vicine content is probably due to partia l inhibition of a gene coding for a beta-glucosidase that hydrolyses v icine to the toxic form, aglycone, Vicine is transferred from the midg ut to the haemolymph without chemical transformation, This adaptation to a new host plant, due to low enzymatic activity preventing the tran sformation of a glucoside to its toxic aglycone has a low energetic co st. (C) 1997 Elsevier Science Ltd.