REVIEW OF GLYCOGEN PLASTID DIFFERENTIATION IN MÜLLERIAN BODY CELLS OF Cecropia peltata

Before describing the anatomical and cytological events leading to glycogen plastid production, I wish to present some background information relevant to the biology of Cecropia. One must appreciate the forces of natural selection in order fully to understand the presence of glycogen in this highly specific tissue. Cecropia (Moraceae) is a commonly occurring, neotropical genus of small trees, which inhabit disturbed, second-growth areas as well as coastal marshlands. The tree is often associated with an ant population that Janzen 1 has shown to be mutualistic and obligatory, at least for the ants. These ants, when present, protect the tree from herbivores by their aggressive stinging behavior, and from overgrowth by other vegetation by chewing and killing the intruding plant material. The plant possesses a number of morphological characteristics that prove necessary to the maintenance of the ant population. The stem is hollow, with nodal diaphragms, thus providing a chambered domicile suitable for a large ant colony, and a food supply for the ants is maintained through the production of Miillerian bodies. Miillerian bodies are small (0.3 mm x 1.0 mm) cellular structures that are produced from the keel-shaped meristematic surface of a leaf petiole-based modification termed a trichilium (FIGURE 1). The trichilium surface is covered by multicellular hairs. The ants continually harvest Mullerian bodies and use them as food tissue for their larvae. No quantitative data are available on the total biomass produced by a Cecropia tree in its natural surroundings, but extrapolation from greenhouse specimens indicates that a mature Cecropia might produce 18-20 g of Miillerian body tissue every two weeks; and that is a conservative estimate. Recently, as part of a long-term study of the anatomical and cytological evolutionary modifications of plant parts toward producing insect it was discovered that plastids in the Miillerian body cells of Cecropia peltata produce and store glycogen rather than starch. This polysaccharide has been analyzed by Marshall, who, using debranching enzymes such as isoamylase and pullulanase, finds the material to be a glycogen.5 Further, analysis in the present author's laboratory shows that 30% of a Miillerian body (wet weight) is glycogen when the polysaccharide is extracted in 0.01 M HgCl and dried to 40" C. It is therefore evident that this carbohydrate plays an important part in maintaining the nutrition of the ant colony. Electron micrographs indicate that a substantial amount of lipid is also present in a Miillerian body cell. All other cell types of the Cecropia plant, such as root, stem, leaf blade, and petiole, contain normal chloroplasts with starch grains included. Except in the matter of long-term evolution, Miillerian bodies are in no