Regulation of attachment, germination, and appressorium formation by zoospores ofLagenidium giganteum and related oomycetes by chitin, chitosan, and catecholamines

Abstract

Summary. Lagenidium giganteum (Oomycetes: Lagenidiales), a facultative parasite of mosquito larvae, infects the larval stage of most species of mosquitoes and a very limited number of alternate hosts. Host infection by this and other members of Oomycetes is initiated by motile, laterally biflagellate zoospores. Chemical bases for the various degrees of host specificity exhibited by these parasites is not known, but presumably involves receptors on the zoospore surface recognizing compounds either secreted by or on the surface of their hosts. Surface topography had no detectable effect on L. giganteum encystment or appressorium formation. Scanning electron microscopy documented the detachment of flagella during zoospore encystment. Bulbous knobs at the basal end of the detached flagellum were interpreted as encysting zoospores dropping the axoneme and/or the basal body and associated structures to which flagella are attached. Multiple signals appear to be involved in the initial steps ofL. giganteum host invasion. Zoospores of this parasite did not encyst on powdered preparations of chitin or chitosan (deacetylated chitin). Upon dissolution of chitosan in dilute acid followed by drying these solutions to form thin, transparent films, zoospores readily encysted. The degree of reacetylation of these films and the spacing of acetylated and deacetylated residues had no significant effect on zoospore encystment. Zoospores of a strain of Lagenidiurn myophilurn isolated from marine shrimp, that also infects mosquito larvae, encysted on chitosan films. No encystment of spores of the plant parasite Phytophthora capsici was observed on chitin or chitosan films. Simulation of cuticle sclerotization by incubating chitosan films with different catecholamines and tyrosinase significantly reduced zoospore encystment. Zoospores that encysted on chitosan films did not germinate in distilled water. Germination could be induced by adding microgram quantities of bovine serum albumin or proteins secreted by motile zoospores into the water, and to a lesser degree by some amino acids, but not by various cations. Zoospores encysted and germinated on the pupal stage of some mos-*Correspondence and reprints: Botany Department, Box 351330, University of Washington, Seattle, WA 98195, U.S.A. quito species. Appressoria were occasionally fort-ned, but most subsequently sent out another mycelial branch, apparently without attempting to pierce the pupal cuticle. Methylation of pupal exuviae with ethereal diazomethane or methanol/HC1 significantly increased zoospore encystment. Modification of chitin by catecholamines, lipids and protein on the epicuticular larval surface all affected host invasion.

Carbohydrate regulation of attachment, encystment, and appressorium formation by Pythium porphyrae (Oomycota) zoospores on Porphyra yezoensis (Rhodophyta)

“…The early stages of the typical invasion process involve chemotaxis of the zoospores to the host followed by attachment and encystment on the host surface (Hickman and Ho 1966, Hardham 1989). Carbohydrates have been implicated as a factor in species‐specific parasitism by bacteria (Ofek et al 1977), algae (Talmadge and Burger 1975), and Oomycetes (Royle and Hickman 1964, Hinch and Clarke 1980, Kerwin et al 1992, Petersen et al 1997).…”

Screening of Oomycete Fungi for Their Potential Role in Reducing the Biting Midge (Diptera: Ceratopogonidae) Larval Populations in Hervey Bay, Queensland, Australia

“…Many of the Oomycetes including Pythium , Phytophthora , Saprolegina , Aphanomyces , and Lagenidium are parasites of plants or animals using zoospores to infect their hosts [ 27 ]. Dominant marine Oomycetes are species of Halophytophthora [ 28 ], which are probable decomposers of fallen leaves [ 28 ].…”

“…Investigations indicate that species of Halophytophthora are widespread in mangrove communities in southern coastal Queensland [ 29 ]. The Oomycete Lagenidium giganteum is a facultative fungal pathogen of mosquito larvae [ 30 ] and has been registered for use as a mosquito biological control agent with the United States Environmental Protection Authorities [ 27 , 31 ], and successfully controls mosquitoes from the genera Anopheles , Aedes , Culex , Culiseta and Psorophora [ 31 ].…”