Publication Type:Journal Article
Source:Mycological Research, Volume 95, Issue 9, p.1025 - 1046 (1991)
Isoenzyme analysis was used to examine intraspecific diversity and interspecific relatedness of P. cactorum (group I) and 12 papillate species of Phytophthora within group II of Waterhouse's (1963) taxonomic scheme. Isolates of 6 species, specifically P. botryosa, P. heveae, P. katsurae, P. meadii, P. palmivora and P. parasitica (P. nicotianae var. parasitica), contained low levels of genetic diversity and were judged to be valid species. Based upon isoenzyme analysis, all isolates of P. arecae were found to be identical with some isolates of P. palmivora. Thus, it is proposed that these 2 species are synonymous. For P. nicotianae var. parasitica, differences in oospore size or host plant were contrasted with the isoenzyme data using single locus coefficients and no evidence was found to support the existence of distinct varieties. In contrast, several other species, particularly P. capsici, P. citrophthora and P. megakarya, demonstrated much higher levels of variation in isoenzyme pattern and subgroups were readily identified. Isolates of P. capsici, including those previously identified as P. palmivora 'MF4' separated into 3 subgroups CAP1, CAP2 and CAP3. Those in CAP1 were from widely distributed geographical locations on a range of hosts including Capsicum sp., tomato, cucurbits, as well as cocoa and black pepper and it contained the greatest amount of intraspecific diversity. A single representative of P. mexicana also clustered with this subgroup and as such is considered conspecific to P. capsici. CAP2 isolates were found primarily on black pepper and from Hawaii on Macadamia integrifolia as well as other hosts. In some instances, specifically with isolates from India and Indonesia on black pepper both CAP1 and CAP2 were found. However, the evidence from isoenzymes suggests that the 2 groups are genetically isolated from one another. All representatives of CAP3 were derived from diseased cocoa in Brazil and this subgroup was monomorphic for all loci examined. P. citrophthora was divided into 2 subgroups. Isolates of CTR1 were derived from a large range of hosts including citrus and kiwi fruit. CTR2 was composed of only Brazilian isolates from cocoa. P. megakarya, which has only been reported on cocoa from West Africa, was also divided into 2 subgroups which were separated geographically. Isolates of MGK1 originated from either Nigeria or Guinea and isolates of MGK2 were from Cameroon. P. boehmeriae was the most genetically diverse species examined with 4 isoenzymatically diverse ETs representing only 11 isolates. Interspecific comparisons revealed relationships which would not have been predicted based on morphological comparisons alone. For example, P. meadii and P. botryosa clustered together indicating a very close genetic relatedness. P. katsurae and P. heveae also formed a single cluster. However, comparison of oospores of these 2 species showed that the oogonial stalk length of P. katsurae was consistently much longer. Unexpectedly, P. capsici and P. citrophthora also formed a cluster indicating that they were genetically related. Finally, the results of this isoenzyme analysis suggest that the papillate species of Group II of Waterhouse's (1963) scheme may form at least 6 distinct evolutionary lines.