OCCURRENCE OF A PARASEXUAL CYCLE FOLLOWING THE TRANSFER OF ISOLATED-NUCLEI INTO PROTOPLASTS OF PHYTOPHTHORA-PARASITICA

Nuclei isolated from metalaxyl-resistant (M-r) protoplasts of Phytopht hora parasitica were transferred into chloroneb-resistant (Cn(r)) prot oplasts of the same species, with an average successful rate of 1x10(- 4), using a selective medium containing both metalaxyl and chloroneb, No colonies appeared when self-fusion products of donor nuclei or reci pient protoplasts were exposed to the selective medium. Similar result s were obtained when the reciprocal nuclear transformation was perform ed. All the zoospore cultures produced by nuclear transformants contai ned both M-r and Cn(r) genes, demonstrating the occurrence of karyogam y since most zoospores are uninucleate. An average of 38% of zoospores produced by nuclear transformants were about twice the size of normal zoospores. Each large zoospore contained a single nucleus, also about twice the size of that in normal zoospores, indicating the occurrence of tetraploidy in these zoospores. The other 42% of zoospores were of normal size but contained both M-r and Cn(r) genes, suggesting the oc currence of mitotic crossing-over following the production of tetraplo idy and the subsequent occurrence of diploidization during zoosporogen esis. For the first time, this study demonstrated the completion of ev ents leading to a parasexual cycle following nuclear transfer.