Isozyme analysis and sequence analysis of the internal transcribed spacer regions (ITS-1 and ITS-2) and the 5.8S subunit of the ribosomal DNA gene repeat were used to examine whether isolates of
Phytophthora porri from
Allium and
Brassica represent a single homogeneous species. Twenty-six strains of
P. porri, 16 strains isolated from the genus
Allium, and 10 strains isolated from the genus
Brassica, were analyzed using malate dehydrogenase (MDH), isocitrate dehydrogenase (IDH) and lactate dehydrogenase (LDH), represented altogether by four putative loci (
Mdh-2,
Idh-1,
Idh-2, and
Ldh-2). Isozyme analysis revealed that strains isolated from
Allium contained five private alleles at three isozyme loci (
Ldh-2
83,
Ldh-2
104,
Idh-1
108,
Idh-1
112, and
Idh-2
98), whereas six different alleles were observed at four isozyme loci (
Ldh-2
85,
Ldh-2
100,
Ldh-2
114,
Idh-1
100,
Idh-2
100, and
Mdh-2
111) in strains obtained from
Brassica. The heterozygosity at the
Ldh-2 locus, differing in allele composition, however, between strains from
Allium and
Brassica, was present in all strains, indicating that it is probably fixed. Sequence analysis of the ITS regions and the 5.8S subunit showed consistent differences between isolates from
Allium and isolates from
Brassica. Based on isozyme data, ITS sequence analysis and formerly published differences in restriction enzyme patterns of mitochondrial DNA, morphology and pathogenicity, it was concluded that the isolates of
P. porri Foister did not represent a homogeneous species. Isolates from
Brassica constitute a distinct species which is described here as
P. brassicae sp. nov. It was inferred from isozyme patterns, which were in no case intermediate between the two species, that
P. porri and
P. brassicae do not hybridize and are reproductively isolated by barriers to gene flow.
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