The making of symbiont capsule in the plataspid stinkbug Megacopta punctatissima

In stinkbugs of the family Plataspidae, adult females deposit small brownish particles, containing specific symbiotic bacteria inside, on the underside of their egg mass. Newborn nymphs ingest the content of the unique structure, called symbiont capsule, whereby vertical transmission of the symbiont occurs. We investigated the fine structure and the formation process of the symbiont capsule in the Japanese common plataspid stinkbug, Megaeopta punctatissima, by using light and electron microscopy. It was demonstrated that (i) the capsule consists of three structural components, namely symbionts, matrix and envelope; (ii) the posterior midgut of adult females is characterized by several specific sections with peculiar anatomical traits, including thin crypt-bearing midgut (TCM) section, swollen crypt-bearing midgut (SCM) section and brownish enlarged midgut (BEM) end section; (iii) the different capsule components, symbionts, matrix and envelope, are produced and/or supplied by the specialized midgut sections, TCM, SCM and BEM, respectively; and (iv) the capsule components are stored in BEM and excreted during oviposition to produce the symbiont capsules. These results strongly suggested that the host insect incurs a substantial cost for the symbiont transmission. Ecological and evolutionary implications of the highly developed, female-specific system for symbiont transmission were discussed. (c) 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.