Natural hybrids of resident and introduced Phytophthora species proliferating on multiple new hosts
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Several atypical Phytophthora strains, isolated from a range of horticultural hosts, were tentatively identified as P. cactorum. Numerous abortive oospores were observed in these strains and isozyme analysis showed all were heterozygous for the dimeric malic enzyme (MDHP). More detailed comparisons indicated that their MDHP alleles matched those of both P. cactorum and P. hedraiandra. Cloning and sequencing of the nuclear ribosomal internal transcribed spacer (ITS) regions of the atypical P. cactorum strains demonstrated the presence of sequences characteristic for both P. cactorum and P. hedraiandra. It was concluded that the atypical strains represented hybrids between the resident P. cactorum and the apparently recently introduced P. hedraiandra. Most strains had the mitochondrially inherited cytochrome oxidase I (Cox I) gene typical of one putative parent P. hedraiandra, while one single strain had that of the other putative parent, consistent with the hybrid hypothesis. Our data also suggest that the hybrids are evolving. The hybrids have proliferated on multiple new hosts in the Netherlands.
KeywordsP. cactorum P. hedraiandra hybridization evolution
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The authors pay tribute to Brigitta Wessels-Berk, who discovered Phytophthora hedraiandra in the Netherlands. Unfortunately her mycological expertise is forever lost since she decided to devote her time to beetles. We also highly appreciate the skilful technical assistance of Karin Rosendahl-Peters, Ilse Heurneman-van Brouwershaven, Ferry Hagen and Jessica Cavaletto.
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