, Volume 99, Issue 2, pp 87-101

Morphological basis of cardiac glycoside sequestration by Oncopeltus fasciatus (Dallas) (Hemiptera: Lygaeidae)

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Summary

The integument of Oncopeltus fasciatus is made up of a vacuolated and a pigmented epidermal cell layer. This double layered integument is present from late embryo to adult in male and female animals reared on milkweed or sunflower seeds. Experiments with a labelled glycoside as well as retrograde ink injections suggest that O. fasciatus concentrates cardiac glycosides, normally derived from the host plants, within the vacuolated epidermal cell layer throughout its life cycle. In the adult, droplets of glycoside-rich fluid appear at precise points along the dorsolateral margins when external pressure is applied to the thorax and abdomen. This pressure causes separation of cuticular flanges in the metathoracic epimeral lobe and rupture of the cuticle in restricted areas in the mesothorax and abdomen. In addition the pigmented epidermal cell layer and the distal membranes of the vacuolated epidermal cell layer rupture with the result that the contents of the vacuolated cell layer are eliminated onto the surface of the animal where they are retained as discrete droplets by the cuticular morphology. Release of cardiac glycosides into the haemolymph is prevented by a thick basal lamina on the haemolymph side of the vacuolated epidermal cells. No specialized muscles involved with fluid release were observed. The vacuolated epidermal cells do not show ultrastructural features characteristic of actively transporting tissues, i.e., abundant mitochondria and elaborate membrane infoldings. This suggests that glycoside sequestration is essentially a passive process and should not be associated with any physiological cost. Large concentration gradients of cardiac glycosides are maintained across the basal lamina, basal plasma and vacuolar membranes of the vacuolated epidermal cell layer. Possible mechanisms by which O. fasciatus is able to concentrate cardiac glycosides as well as the possible function of this phenomenon are discussed.