Affiliation:
1. Molecular Plant Pathology, Swammerdam Institute for Life Sciences, University of Amsterdam, Amsterdam, The Netherlands
Abstract
ABSTRACT
Race 1 isolates of
Fusarium oxysporum
f. sp.
lycopersici
(FOL) are characterized by the presence of
AVR1
in their genomes. The product of this gene, Avr1, triggers resistance in tomato cultivars carrying resistance gene
I
. In FOL race 2 and race 3 isolates,
AVR1
is absent, and hence they are virulent on tomato cultivars carrying
I
. In this study, we analyzed an approximately 100-kb genomic fragment containing the
AVR1
locus of FOL race 1 isolate 004 (FOL004) and compared it to the sequenced genome of FOL race 2 isolate 4287 (FOL4287). A genomic fragment of 31 kb containing
AVR1
was found to be missing in FOL4287. Further analysis suggests that race 2 evolved from race 1 by deletion of this 31-kb fragment due to a recombination event between two transposable elements bordering the fragment. A worldwide collection of 71 FOL isolates representing races 1, 2, and 3, all known vegetative compatibility groups (VCGs), and five continents was subjected to PCR analysis of the
AVR1
locus, including the two bordering transposable elements. Based on phylogenetic analysis using the EF1-α gene, five evolutionary lineages for FOL that correlate well with VCGs were identified. More importantly, we show that FOL races evolved in a stepwise manner within each VCG by the loss of function of avirulence genes in a number of alternative ways.
IMPORTANCE
Plant-pathogenic microorganisms frequently mutate to overcome disease resistance genes that have been introduced in crops. For the fungus
Fusarium oxysporum
f. sp.
lycopersici
, the causal agent of
Fusarium
wilt in tomato, we have identified the nature of the mutations that have led to the overcoming of the
I
and
I-2
resistance genes in all five known clonal lineages, which include a newly discovered lineage. Five different deletion events, at least several of which are caused by recombination between transposable elements, have led to loss of
AVR1
and overcoming of
I
. Two new events affecting
AVR2
that led to overcoming of
I-2
have been identified. We propose a reconstruction of the evolution of races in FOL, in which the same mutations in
AVR2
and
AVR3
have occurred in different lineages and the FOL pathogenicity chromosome has been transferred to new lineages several times.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology
Cited by
33 articles.
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