Occurrence of parasexual cycle in Phytophthora parasitica following protoplast fusion.

Abstract

Metalaxyl-resistant (M(superscript r)) and chloroneb-resistant (Cn(superscript r)) protoplasts of Phytophthora parasitica were fused, with heterokaryon formation rates ranging from 2.6×l0((superscript -3) to 8.2×l0(superscript -3). The heterokaryosis was confirmed in six of seven putative heterokaryons by the production of both M(superscript r) zoospores and Cn(superscript r) zoospores. and/or M(superscript r)+Cn(superscript r) zoospores. The 7th putative heterokaryon produced only Cn(superscript r) zoospores, probably because of the loss of the M(superscript r) chromosome during subculture. The heterokaryons produced normal-sized zoospores as well as large zoospores that were almost twice the size of normal-sized zoospores. Fifty percent of the normal-sized zoospores contained M(superscript r) and Cn(superscript r) genes. The results suggested an occurrence of diploidization after karyogamy of the heterokaryon and thus demonstrated the completion of events leading to a parasexual cycle following protoplast fusion.