The taxonomic structure of Phytophthora megasperma: Evidence for emerging biological species groups

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Abstract:

Nomenclatural uncertainty surrounds P. megasperma as various authors, working with limited groups of isolates, offer their interpretations of this species based on pathology, morphology, or cytology. We compared 93 isolates, including many described by others, for classical morphological features, growth behaviour and appearance, electrophoretic pattern of total proteins, chromosome number and nuclear DNA content. Nine distinct sub-groups were distinguished. While most groups could be distinguished by each of the criteria, protein electrophoresis was the most sensitive. The groups included: ALF, pathogenic to alfalfa, n = 12–15; SOY, pathogenic to soybean, n = 12–15; CLO, pathogenic to clover, n = 11–15; DF, pathogenic to Douglas fir, n = 17–24; AC, isolated from rosaceous fruit trees; and BHR, a major group obtained from a broad range of hosts. The last two groups, distinguished primarily by protein pattern, comprised at least four karyotypes: KI, n = 12–17; KII, n = 15–23; KIII, n = 22–28; and KIV, n= 26–34. All four karyotypes occur within the BHR protein group, suggesting a polyploid series within a closely related genotype.

Two broad lines of evolution are hypothesized, a legume line comprising ALF, SOY, CLO, and perhaps DF isolates, and a Broad Host Range line of AC and BHR isolates. Sub-groups within each line may represent emerging biological species, isolated by host specificity or karyotype. Taxonomic designation for the various groups must await confirmation of the hypothesis by demonstration of the extent of barriers to gene flow between the groups.

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