Abstract: Phytophthora sojae race evolution

Brett Tyler bmtyler at UCDAVIS.EDU
Mon Aug 29 14:37:40 EST 1994

Phytophthora sojae races have arisen by clonal evolution and by rare outcrosses

Helga Forster(1), Brett M. Tyler(2) and Michael D. Coffey(1)
1 Department of Plant Pathology, University of California, Riverside, CA
2 Department of Plant Pathology,University of California, Davis CA 95616
Molec. Plant Microbe Ints. in press.

        An extensive set of nuclear and mitochondrial restriction fragment
length polymorphisms (RFLPs) was used to examine the genetic relationships
among 48 isolates of Phytophthora sojae, an oomycete pathogen of soybean. 
This organism is diploid and homothallic.  The isolates examined
encompassed 25 physiological races of the pathogen, including races 36 and
37 which we describe here for the first time.  The results reveal a
moderate degree of diversity within the species; about 18% of all detected
nuclear restriction fragments were polymorphic in at least one isolate. 
One group of isolates, representing seven physiological races, had RFLPs
nearly identical to isolates of the first described race, race 1, and
probably arose from race 1 isolates clonally by mutation.  There was much
more genetic variation among the remaining isolates.  The distribution of
RFLP alleles among these isolates suggests most of the genetic variation in
the species is found in four genotypes ("progenitor" lines) represented by
isolates P1658 (race 1), P7064 (race 7), P 7074 (race 17) and P7076 (race
19) respectively.  All the other isolates appear to have been produced by
rare outcrosses between representatives of these four genotypes. The
distribution of avirulence phenotypes against Rps genes (soybean resistance
genes against P.sojae)  is consistent with the reassortment of single
avirulence genes as a result of the same outcrosses.  Therefore it is
proposed that occasional outcrosses have been a major contributor to the
origin of new physiological races of P. sojae, in addition to clonal
evolution.  In concordance with these mechanisms, no correlation was
observed between particular RFLPs and race types.  Thus it will be very
difficult to use RFLP or RAPD markers to directly identify race types of
new field isolates unless the markers are derived directly from avirulence

Brett Tyler
bmtyler at ucdavis.edu

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