Abstract:
Surveys of Australian and South African rivers revealed numerous Phytophthora isolates residing
in clade 6 of the genus, with internal transcribed spacer (ITS) gene regions that were
either highly polymorphic or unsequenceable. These isolates were suspected to be hybrids.
Three nuclear loci, the ITS region, two single copy loci (antisilencing factor (ASF) and G protein
alpha subunit (GPA)), and one mitochondrial locus (cytochrome oxidase c subunit I
(coxI)) were amplified and sequenced to test this hypothesis. Abundant recombination
within the ITS region was observed. This, combined with phylogenetic comparisons of
the other three loci, confirmed the presence of four different hybrid types involving the
three described parent species Phytophthora amnicola, Phytophthora thermophila, and Phytophthora
taxon PgChlamydo. In all cases, only a single coxI allele was detected, suggesting that
hybrids arose from sexual recombination. All the hybrid isolates were sterile in culture and
all their physiological traits tended to resemble those of the maternal parents. Nothing is
known regarding their host range or pathogenicity. Nonetheless, as several isolates from
Western Australia were obtained from the rhizosphere soil of dying plants, they should
be regarded as potential threats to plant health. The frequent occurrence of the hybrids
and their parent species in Australia strongly suggests an Australian origin and a subsequent
introduction into South Africa.
