Tokuo Fukuda

Epizootiology and transmission of a newly discovered baculovirus from the mosquitoes Culex nigripalpus and C. quinquefasciatus

Reports of mosquito baculoviruses are extremely uncommon and epizootics in field populations are rarely observed. We describe a baculovirus that was responsible for repeated and extended epizootics in field populations of Culex nigripalpus and C. quinquefasciatus over a 2 year period. These mosquito species are important vectors of St Louis and Eastern equine encephalitis in the United States. Our initial attempts to transmit this baculovirus to mosquitoes in the laboratory were unsuccessful. A salt mixture similar to that found in water supporting infection in the field was used in laboratory bioassays and indicated that certain salts were crucial to transmission of the virus. Further investigations revealed conclusively that transmission is mediated by divalent cations: magnesium is essential, whereas calcium inhibits virus transmission. These findings represent a major advancement in our understanding of the transmission of baculoviruses in mosquitoes and will allow characterization of the virus in the laboratory. In addition, they can explain, in great part, conditions that support epizootics in natural populations of mosquitoes that vector life-threatening diseases of man and animals.

Gametogenesis and plasmogamy in certain species of Microspora

Abstract Gametogenesis and plasmogamy of gametes is documented in light and electron microscopy of Hazardia milleri, Culicospora magna, and an undescribed microsporidium in Aedes aegypti, using three mosquito hosts in experimental infection studies. Events taking place during gametogenesis and plasmogamy were similar in all three species studied. However, merogonial and sporogonial sequences differed among the three species. Gametes of C. magna and a Microsporidium sp. in A. aegypti were structurally different from those of H. milleri in that they contain an unusual nipple-like structure on one end. The function of this structure was not ascertained. C. magna was successfully transmitted to healthy Culex restuans larvae exposed to spores in well water, but infections were higher when larvae were exposed to spores in filtered field water taken from breeding sites. The presented evidence of the production and union of gametes is the first documentation of gametogenesis and plasmogamy in species of microsporidia. The realization of sexual cycles in some species raises important questions concerning the present classification of the Microspora.

Ultrastructure of Culicosporella lunata (Microsporida: Culicosporellidae fam. n.) in the mosquito Culex pilosus (Diptera: Culicidae) with new information on the developmental cycle.

The ultrastructural cytology of Culicosporella lunata is described from larvae of its mosquito host Culex pilosus. Diplokaryotic sporonts entered 1 of 12 fundamentally different sporulation sequences that occurred concurrently, each producing morphologically distinctive spores. One involved abortive meiosis and rarely ended with meiospores; the other, the predominant one, involved diplokaryotic sporogonial plasmodia and produced lanceolate, binucleate spores. A previously unknown sporulation sequence in the adult mosquito was found to result in a second type of binucleate spore. The taxonomic affinities of Culicosporella lunata are discussed and because of its distinctive characters, Culicosporellidae fam. n. is created.

An ultrastructural study of the invasion of Culex quinquefasciatus larvae by Leptolegnia chapmanii (Oomycetes: Saprolegniales).

Transmission electron microscopy of the invasion of Culex quinquefasciatus by Leptolegnia chapmanii confirmed that it is a primary pathogen and revealed several differences between penetration via the gut and penetration via the integument. The latter often involved aggregations of zoospores, appressoria-like swellings of the invasive hyphae, and lateral growth of hyphae between the epicutical and endocuticle. These features were not detected in the case of gut invasion, but hyphal septa at the point of entry were apparently peculiar to this route. There was no evident tissue specificity, and death presumably resulted from generalized destruction of tissues.

Pleistophora chapmani n. sp. (Cnidospora: Microsporida) in Culex territans (Diptera: Culicidae) from Louisiana☆

Abstract A new species of Pleistophora from the larval midgut and gastric caeca of Culex territans is described. This parasite produces unusually heavy-walled cysts which are highly variable in size. Although in many infected larvae a major portion of the cells lining the midgut and gastric caeca are extensively damaged by the parasite, these larvae are nevertheless able to pupate and emerge as apparently healthy adults. Attempts to infect larvae from our laboratory colonies of Culex territans, C. salinarius, C. pipiens quinquefasciatus, Psorophora varipes, P. ferox, Aedes tormentor , and A. triseriatus by feeding them spores and cysts of the parasite all failed. The occurrence and distribution of the microsporidan are discussed.