HETEROKARYON FORMATION BY PROTOPLAST FUSION IN PHYTOPHTHORA MEGASPERMA FOR GENETIC STUDIES
Abstract
Phytophthora megasperma, fungal division Oomycetes, contains important pathogens of trees and legumes. Phytophthora megasperma Drechs. f.sp. glycinea Kuan and Erwin (Pmg) causes Phytophthora root rot of soybean (Glycine max (L.) Merr.). Breeding for disease resistance in the soybean does not provide long lasting control of Pmg because Pmg exhibits a large amount of pathogenic variability. How pathogenic variability is inherited in Pmg is unknown. Sexual genetic studies in Pmg are difficult because Pmg is multinucleate and self-fertilizing. Therefore interracial heterokaryons were produced to study the inheritance of pathogenicity. Race 1 and Race 3 metalaxyl and fluorotryptophan resistant mutants were obtained after zoospore mutagenesis. Protoplasts from fluorotryptophan and metalaxyl resistant isolates were fused and heterokaryons were selected by growth in the presence of both drugs. Because zoospores are uninucleate, the drug resistance phenotypes of the zoospore progeny may reflect the nuclear composition of the heterokaryon. Some of the heterokaryons produced zoospores resistant to both drugs, suggesting that karyogamy occurred. In the heterokaryons nonpathogenicity (race 1 phenotype) was dominant to pathogenicity (race 3 phenotype) as predicted by Flor's gene-for-gene hypothesis. Heterokaryon formation was also attempted between a fluorotryptophan resistant isolate of Pmg and a metalaxyl resistant isolate of the alfalfa pathogen (P. megasperma Drechs. f.sp. medicaginis Kuan and Erwin (Pmm)). Putative heterokaryons were characterized for drug resistance in the zoospore progeny, pathogenicity, and isozyme patterns. Only the Pmm phenotypes were recovered, suggesting that the putative heterokaryons were spontaneous mutations of the Pmm parent to fluorotryptophan resistance. Because Pmm-Pmg heterokaryons were not obtained Pmm and Pmg may not be as closely related as their subspecies designation suggests. Protoplast fusion is a useful tool for Phytophthora genetics. They can be used to study dominance relationships, make outcrosses in self-fertilizing species, and be used as the first step in parasexual genetics.
Degree
Ph.D.
Subject Area
Plant pathology
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.