Sporogony, development and ultrastructure of Plasmodium floridense in Culex erraticus

Abstract

The sporogonic stages of Plasmodium floridense were studied for the first time by light and electron microscopy in the midgut and salivary glands of Culex erraticus. The sporogonic cycle of P. floridense was variable, but was usually completed by 11–14 days, at 22°C, after mosquitoes took an infective blood-meal. Melanization of oocysts and degeneration of sporozoites occasionally occurred and may have contributed to the variability in time when sporozoites were observed in the salivary glands. Sporozoites were in the range of 8–22 × 0.8–1.5 μm and were approximately three times the length of sporozoites of two other species of saurian malaria. Plasmodium mexicanum and Plasmodium agamae. Oocysts were usually extracellular, extending into the hemocoel. However, some also protruded into the lumen of the midgut. Early, non-vacuolated oocysts had translucent lipid-like droplets, mitochondria, large nuclei with dense nucleoli, and three types of inclusion bodies: circular granules of moderate density, more dense granules associated with an irregular space of low density, and lamellar forms associated with an irregular space of low density. Differentiation of the oocyst began with internal vacuolization of the sporoblastoid, which later subdivided into several residual bodies. Linear extensions of the internal vacuoles extended to the surface of the sporoblastoid to form subcapsular spaces. Sporozoite formation was initiated by the formation of a pair of convex electron dense membranes along the length of the vacuolar extensions. One or more moderately electron-dense inclusion bodies, a nucleus, and a mitochondrion migrated into the evaginating sporozoites. Rhoptries and micronemes formed as the moderately dense inclusion bodies degenerated prior to the formation of mature sporozoites. Sporozoites were released from the oocyst and then appeared in the salivary glands of C. erraticus. Sporozoites were intracellular and were observed in groups within a vacuolar space in the salivary gland cell. Ten or 11 subpellicular microtubules were arranged asymmetrically in the anterior third of sporozoites within oocysts, two lying dorsolaterally or laterally to the remaining eight or nine arranged along the ventral surface. Microtubules within salivary gland sporozoites were rarely seen, but were clumped together in small groups when visible.