REGULATION OF ATTACHMENT, GERMINATION, AND APPRESSORIUM FORMATION BY ZOOSPORES OF LAGENIDIUM-GIGANTEUM AND RELATED OOMYCETES BY CHITIN, CHITOSAN, AND CATECHOLAMINES

Lagenidium giganteum (Oomycetes: Lagenidiales), a facultative parasite of mosquito larvae. infects the larval stage of mast species of mosqu itoes and a very limited number of alternate hosts. West 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 eith er secreted by or on the surface of their hosts. Surface topography ha d no detectable effect on L. giganteum encystment or appressorium form ation. Scanning electron microscopy documented the detachment of flage lla during zoospore encystment Bulbous knobs at the basal end of the d etached flagellum were interpreted as encysting zoospores dropping the axoneme and/or the basal body and associated structures to which flag ella are attached. Multiple signals appear to be involved in the initi al steps of L. giganteum host invasion. Zoospores of this parasite did not encyst on powdered preparations of chitin or chitosan (deacetylat ed chitin). Upon dissolution of chitosan in dilute acid followed by dr ying these solutions to form thin, transparent films, zoospores readil y 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 Lagenidium myophilum iso lated from marine shrimp, that also infects mosquito larvae, encysted on chitosan films. No encystment of spores of the plant parasite Phyto phtora capsici was observed on chitin or chitosan films. Simulation of cuticle sclerotization by incubating chitosan films with different ca techolamines and tyrosinase significantly reduced zoospore encystment. Zoospores that encysted on chitosan films did not germinate in distil led 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 var ious cations. Zoospores encysted and germinated on the pupal stage of some mosquite species. Appressoria were occasionally formed, but most subsequently sent out another mycelial branch, apparently without atte mpting to pierce the pupal cuticle. Methylation of pupal exuviae with ethereal diazomethane or methanol/HCl significantly increased zoospore encystment. Modification of chitin by catecholamines, lipids and prot ein on the epicuticular larval surface all affected host invasion.