Second Science Symposium
January 18 - 21, 2005
Adaptive differences between Phytophthora ramorum isolates from Europe and North America: Evidence for separate subspecies?
Clive Brasier, Susan Kirk and Joan Rose, Forest Research, Farnham, Surrey GI10 4LH UK; email@example.com
Samples of European and North American isolates of P. ramorum were compared for their comparative plant health risk, their status as a single cohesive species and their potential for genetic recombination. Growth rates across different environments (g x e tests), developmental stability, host range, pathogenic aggressiveness and sexual behaviour were examined. In multiple tests on the susceptible Quercus rubra, samples of EU isolates were on average significantly more aggressive than US isolates (wound inoculation of bark), though the ranges of the two groups always overlapped. The potential host ranges of EU and US isolates (wound inoculation of mature stems of ca 30 European and North American trees) were very similar.
EU isolates grew significantly faster on average than US
isolates on carrot agar (CA) at 20oC (35-39 isolates/sample). Often, total
separation of EU and US isolates occurred. An EU sample grew faster even
when the US sample was fresh from the field. The EU/US growth rate difference
was also maintained at 12.5, 15 and 27o, but mean growth curves for the
EU and US groups across all the different temperatures were similar. EU
isolates were consistently of a uniform ‘wild type’ colony
morphology. US isolates were either wild type or were morphologically
variable, often slow growing ‘non wild type’ colonies. Single
hyphal tip subcultures from selected EU and US isolates showed US isolates
were more intrinsically developmentally unstable, often changing from
wild type to unstable non wild type. No viral dsRNA was detected in selected
‘non wild type’ isolates.
It is concluded that EU and US isolates are conspecific.
However, their differences for continuous variables such as growth rate,
aggressiveness and developmental stability are likely to reflect differences
in fitness i.e. be adaptive differences. It is suggested that for the
present EU and US types be considered as distinct phenotypic populations.
Possible causes and possible evolutionary consequences of their differences
will be discussed; including the possibility that EU and US population
types might at some point need to be viewed as (partially reproductively
isolated?) subspecies of P. ramorum: eg. as P. ramorum subspecies
europaea and subspecies americana.
| Coordinated by:
USDA Forest Service Pacific Southwest Research Station
University of California Integrated Hardwood Range Management Program,
Center for Forestry, Division of Agriculture and Natural Resources, and
California Oak Mortality Task Force
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