Interspecific hybridization and apomixis between Phytophthora capsici and Phytophthora tropicalis

Phytophthora capsici and the closely related Phytophthora tropicalis infect different hosts that have documented overlapping geographical distributions. Phytophthora capsici attacks annual vegetable hosts whereas P. tropicalis has been recovered from woody perennial hosts. Our objective was to test whether interspecific hybridization is possible and to characterize the resulting progeny. Crosses were made between P. capsici (LT263) from pumpkin to P. tropicalis from rhododendron (LT232) and to P. tropicalis from Theobroma cacao (LT12). The wild type isolates were analyzed for mitochondrial and nuclear DNA sequence diversity and progeny were tested for mating type (MT), AFLP marker profiles and mitochondrial DNA haplotype (mtDNA type). All oospore progeny from LT263 x LT12 were identical to LT263 whereas progeny from LT263 x LT232 were parental as well as hybrid. Hybrid progeny had either one or the other parent mtDNA type and there was no correlation between MT and mtDNA type. Attempts to generate an F2 population from the hybrids proved unsuccessful while a backcross to the P. capsici parent produced hybrid progeny. These results demonstrate that apomixis might play a significant role in species separation and that hybridization between P. capsici and P. tropicalis is possible beyond the F1 generation.