Variability Resulting from Selfing and Outcrossing in Phytophthora cactorum

Self and hybrid oospore progeny and zoospore progeny of two pairs of Phytophthora cactorum mutants carrying a metalaxyl-resistance (M^r) nuclear gene or a chloroneb resistance (Cn^r) mitochondrial gene were compared for variation in growth rate, production of sporangia and oospores, and pathogenicity. Zoospore progeny were relatively uniform, whereas oospore progeny resulting from both selfing and outcrossing displayed great dissimilarity in all the traits tested. For pathogenicity, both pairs of tested mutants had greater variation in hybrid progeny than self progeny. For growth rate and oospore production, the same is true only for the first pair, but not the second pair. The variation in sporangium production displayed by one parental self progeny was greater than that by hybrid progeny in both pairs tested. These results show that isolates and traits selected for study have considerable effect on the variation in hybrid and self progeny and suggest that different mechanisms may be operating in creating variation in these two types of progeny. Received 23 October 2000/ Accepted in revised form 11 January 2001