Ultrastructure of the Encapsulation of Plasmodium cynomolgi (B Strain) on the Midgut of a Refractory Strain of Anopheles gambiae

Abstract

Using transmission electron microscopy, we investigated the encapsulation of the simian malaria parasite, Plasmodium cynomolgi, in a refractory strain of the mosquito, Anopheles gambiae. After the ookinete penetrates the mosquito midgut epithelium and lodges between the basal membrane and the basal lamina, an electron-dense, melanin-like substance begins to coalesce around the parasite. Completely encapsulated parasites were found as early as 16 hr after the blood meal. Granules of the melanin-like substance often appeared to condense onto the parasite from the fluid in the extracellular spaces of the basal membrane labyrinth. Melanin granules also appeared to condense from the hemolymph onto the basal lamina underlying the parasite. In addition, groups of tubules, vesicles, and membranous whorls often were found in midgut cells that were located next to or were enclosing parasites. These structures were unusually electron-dense, and may have been associated with melanization. Hemocytes rarely were observed near completed capsules and neither hemocytes nor their remnants were components of the capsules. During later stages of encapsulation, parasites appeared abnormal and often were infiltrated with melanin. Although late-stage capsules were usually located basally, completed capsules enclosed by membranes were occasionally observed near the apical border of the midgut. Other capsules associated with cellular debris, were found in the lumen of the midgut from 1 to 6 days after the blood meal.

Comparative genomic analysis in the region of a major Plasmodium -refractoriness locus of Anopheles gambiae

“…In this broad context, the biological interaction between the parasite and its vector is of special interest. In the Plasmodium-refractory L3-5 strain of A. gambiae, the mosquito is able to kill the ookinete stage of the parasite by encapsulating it in a proteinaceous envelope cross-linked with melanin, as the parasite completes its transit through the mosquito midgut epithelium (1)(2)(3). Genetic analysis has ascribed melanotic encapsulation to the concerted action of three quantitative trait loci, Pen1, Pen2, and Pen3 (4,5).…”

Phenoloxidase Is an Important Component of the Defense against Aeromonas hydrophila Infection in a Crustacean, Pacifastacus leniusculus

“…In mosquitoes, several transcripts encoding enzymes involved in melanization are induced by a Plasmodium parasite in the midgut of Anopheles gambiae (11), and the resulting induced melanization could kill the ookinetes directly in susceptible mosquitoes and dispose of dead parasites in refractory mosquitoes (12). Phenoloxidase has also been demonstrated to interfere with microbial infection in several insect species (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). In some cases melanization, or rather the activity of the enzyme PO, was found to have no or little impact on the killing/ clearance of parasites (24,25) and the melanization reaction was found not to be required for survival of mosquitoes/flies after certain microbial infections (26,27).…”

Molecular Genetics of Mosquito Resistance to Malaria Parasites

“…However, insect midgut basal lamina is probably nonselectively permeable to molecules below a threshold size (Reddy and Locke 1990), so there is likely passive diffusion of melanization substrates and enzymes into the space occupied by extracellular ookinetes. This view is supported by the observation that capsules were thickest on the side of the parasite facing the hemocoel (Paskewitz et al 1988). Only after the capsule completely surrounded the ookinete did the parasite degenerate ultrastructurally.…”

“…gambiae has been artificially selected against the simian parasite, P. cynomolgi (Collins et al 1986), with resistance manifested as melanotic encapsulation of otherwise ultrastructurally normal parasites after they exit from the midgut cell into the space between the basolateral plasma membrane and the midgut basal lamina, beginning 16 h post bloodmeal (Collins et al 1986;Paskewitz et al 1988). Hemocytes did not appear to be directly involved in the encapsulation process (Paskewitz et al 1988). However, insect midgut basal lamina is probably nonselectively permeable to molecules below a threshold size (Reddy and Locke 1990), so there is likely passive diffusion of melanization substrates and enzymes into the space occupied by extracellular ookinetes.…”