MORPHOLOGY AND BEHAVIOR OF THE SUCCESSIVE GENERATIONS OF SECONDARY ZOOSPORES OF THE DAMPINGOFF PATHOGEN APHANOMYCES COCHLIOIDES
M. Tofazzal Islam
Laterally biflagellated first generation secondary zoospores of the parasitic peronosporomycete, Aphanomyces cochlioides locate host plants guided by a hostspecific compound, cochliophilin A (5-hydroxy-6,7- methylenedioxyflavone), and then change morphologically to initiate infection. However, when zoospores fail to find a host during their motile life, they become cysts which generate daughter secondary zoospores up to third generation. This study aimed at comparing the morphology and chemotactic properties of the first, second and third generation of secondary zoospores of A. cochlioides. Scanning electron microscopic observation revealed that all three successive generations of A. cochlioides zoospores were structurally similar but the later generations were smaller in size than those of first generation secondary zoospores. However, all three generations of secondary zoospores responded equally to the host-specific attractants (cochliophilin A and Ntrans- feruloyl-4-O-methyldopamine) and other bioactive secondary metabolites but at varying threshold concentrations. These results confirm the hypothesis of repeated zoospore emergence in parasitic Aphanomyces as a possible adaptation to parasitism. A detailed life cycle of A. cochlioides is illustrated on the basis of microscopic studies and current knowledge.