Book of Abstracts (PDF) - International Mycological Association

IMC7 Wednesday August 14th Lectures
206 - Influence of selection pressure on the outcome of
current interspecific gene flow between Ophiostoma
species
C.M. Brasier 1* , M. Paoletti 2 & K.W. Buck 2
1 Forest Research Agency, Farnham, Surrey GU10 4LH,
U.K. - 2 Biology Department, Imperial College, London
SW7 2AZ, U.K. - E-mail: clive.brasier@forestry.gsi.gov.uk
The Dutch elm disease fungus Ophiostoma novo-ulmi has
recently invaded areas of the northern hemisphere
previously occupied by another DED pathogen, O. ulmi.
This has enabled interactions between the two species,
resulting in a series of unusual evolutionary events. O.
novo-ulmi has physically replaced O. ulmi, forcing the
latter into extinction. While doing so, O. novo-ulmi has
spread as a series of single mating type single vegetative
compatibility type (vic) clones. Locally, these clones have
then become rapidly genetically diverse (c. 5-10 years),
with many new vic and other phenotypes appearing. The
mechanism for this rapid change from clonality to
heterogeneity was unknown. Evidence based on cosegregation
of AFLP markers and cloning and sequencing
of mating type loci indicates that O. novo-ulmi has
acquired both the new vic genes and the A-mating type
locus from O. ulmi via horizontal gene flow. The O. ulmi
vic and mat A genes appear to have been acquired by O.
novo-ulmi under selection pressure imposed by deleterious
viruses. Other 'less useful' O. ulmi genes acquired by O.
novo-ulmi, such as genes for pathogenicity or for ceratoulmin
(toxin) production, appear not to be selected,
probably because they render O. novo-ulmi less fit.
Furthermore, only c2% of 'foreign' DNA appears to be
acceptable if an introgressed O. novo-ulmi genotype is to
survive.
207 - Evolutionary processes and genome organisation
among emerging interspecific hybrids in Phytophthora
species
D.E.L. Cooke 1* , J.M. Duncan 1 , W.A. Man in't Veld 2 &
C.M. Brasier 3
1 Host Parasite Co-evolution, Scottish Crop Research
Institute, Invergowrie, Dundee, DD2 5DA, U.K. - 2 Plant
Protection Service, Department of Mycology, PO Box
9102, 6700 HC Wageningen, The Netherlands. -
3 Pathology Branch, Forest Research, Alice Holt Lodge,
Farnham, Surrey, GU10 4LH, U.K. - E-mail:
dcooke@scri.sari.ac.uk
Whilst common in plants, reports of interspecific
hybridisation amongst fungi are scarce. The risks, however,
should not be underestimated. Hybridisation may
accelerate the evolution of plant pathogenic fungi resulting
in rapid adaptation to new plant species or environments.
Increasing global trade in plants and, inadvertently, their
pathogens, creates opportunities for such events. The genus
Phytophthora comprises over sixty plant pathogenic
species, many with broad host ranges and some responsible
for severe crop epidemics or for destabilising terrestrial
ecosystems. This, alongside their global distribution,
cryptic nature and flexible diploid genetic system serves to
increase the risks and threats of hybridisation within the
genus. Cytological behaviour, additive nucleotide bases in
the ITS region and AFLP and isozyme profiles of a new
aggressive Phytophthora pathogen of alder trees spreading
in Europe have shown that it comprises a swarm of
heteroploid interspecific hybrids between a P. cambivoralike
species and an unknown P. fragariae-like taxon.
Developmental and cytological instability and marked
phenotypic and genotypic variation all indicate a recent
hybridisation. Moreover, mtDNA typing suggests that this
occurred more than once. Evidence for the continuing
evolution (or stabilisation) of the hybrid genomes will be
presented and the challenge of delimiting such hybrid taxa
in a way that is practicable for quarantine legislation and
diagnosis will be considered.
208 - Evolution of interspecific Melampsora hybrids
exhibiting new host specificities
G. Newcombe 1* & P. Frey 2
1 University of Idaho, Dept. Forest Resources, Moscow ID
83844-1133, U.S.A. - 2 INRA, INRA Forest Pathology,
54280 Champenoux, France. - E-mail:
georgen@uidaho.edu
Interspecific hybridization has been an important tool in
plant domestication. Domestication in Populus, and other
forest trees, is just in its infancy, and hybridization has
been the primary means by which poplar breeders have
produced desirable plantation phenotypes. Are the
pathogens of Populus undergoing parallel domestication?
The answer appears to be yes for Melampsora, and no for
Venturia. For Melampsora, interspecific hybridization of
M. medusae and M. occidentalis now matches that of the
respective hosts (i.e., Populus deltoides and P.
trichocarpa). Multiple, exapted genes for rust resistance
from both host species have been suddenly overcome in P.
trichocarpa x P. deltoides hybrids yielding new and
complex pathogenic variation. Venturia, and other
ascomycetous pathogens of Populus are similar to a point.
Multiple, exapted genes for resistance to Venturia (and
Taphrina, and Linospora as well) are also known (from a
common-garden study of P. trichocarpa x P. deltoides and
P. trichocarpa x P. maximowiczii pedigrees). However,
although European and native species of Venturia do cooccur
in the Pacific Northwest of the U.S., they have not
hybridized. Instead, they remain confined to their
respective hosts (i.e., the European Populus nigra and the
native P. trichocarpa, respectively).
Book of Abstracts 67