Digestion of legume starch granules by larvae of Zabrotes subfasciatus (Coleoptera : Bruchidae) and the induction of alpha-amylases in response to different diets

Zabrotes subfasciatus larvae possess three alpha -amylase isoforms as deter mined by in gel assays following SDS-PAGE. The two minor isoforms present l ower electrophoretic mobility than the major form, and seem to occur as a h eterodimer. When developed inside Vigna unguiculata (cowpea) seeds, fourth instar larvae have minor quantities of the slow-migrating forms, but when r eared on seeds of Phaseolus vulgaris (common bean) or Phaseolus lunatus, th e two slow-migrating forms are expressed in higher amounts, while activity of the major form was independent of the host seed. Larvae developing insid e cowpea seeds at the beginning of the fourth instar were fed on flour from cotyledons of cowpea or common bean. Larvae fed on the common bean flour s tarted to express the dimer in higher amounts when compared with the contro l larvae fed on cowpea flour. In an attempt to correlate differences betwee n starch granules and the induction of alpha -amylases, a detailed study on the digestive process of the granules was conducted. Incorporation of puri fied starch granules into artificial diets did not induce the two minor alp ha -amylases. The in vitro hydrolysis rates of purified granules and the pa ttern of dextrins liberated by the different alpha -amylases were similar f or the two legume species. The starch granules enter the midgut extensively damaged, which may facilitate the access to the more susceptible parts of the granules to enzymatic attack. (C) 2001 Elsevier Science Ltd. All rights reserved.