Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

Session 4: Population Dynamics
Population Genetics of Mycosphaerella graminicola and
Phaeosphaeria nodorum
B.A. McDonald, 1 C.C. Mundt, 2 and J. Zhan2 1 Institute of Plant Sciences, Phytopathology Group, Zürich, Switzerland
2 Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR, USA
Abstract
Restriction fragment length polymorphisms (RFLPs) in the nuclear (nu) and mitochondrial (mt) genomes were used to
determine the genetic structure of populations of Mycosphaerella graminicola and Phaeosphaeria nodorum from
around the world. Both fungi have genetic structures consistent with a regular sexual cycle and a high degree of gene flow
occurring on a global scale. Gene as well as genotype diversity in the nuDNA are high for both fungi. There was no
evidence for widespread clones within field populations of either fungus. While both fungi had less diversity in the mtDNA,
M. graminicola exhibited significantly less diversity for the mtDNA compared to P. nodorum. Mycosphaerella
graminicola populations from Patzcuaro, Mexico, and Australia exhibited significantly lower gene diversity, suggesting
that these populations originated from a limited number of founders. Collections of M. graminicola taken from the same
field between 1990 and 1995 showed that genetic drift is negligible, suggesting that effective population sizes are very large.
A replicated field experiment showed that selection can cause significant changes in genotype frequencies during the course
of a growing season, and that the contributions of immigration and recombination to genetic diversity in field populations
can change over the growing season.
Ten years ago, we began
developing DNA-based markers as
tools to learn about the population
genetics of the wheat leaf blotch
pathogen Mycosphaerella
graminicola. One year later, we
began parallel studies using the
same genetic tools for the wheat
glume blotch pathogen
Phaeosphaeria nodorum. We began
with two elementary questions
regarding the population genetics
of both fungi. How much genetic
diversity is present within
populations? How is genetic
diversity distributed within and
among populations? As our
knowledge of the genetic structure
of both pathogens deepened, we
addressed more complex questions.
What are the relative contributions
of sexual and asexual reproduction
to the genetic structure of
populations? How stable are
populations over time? Does
selection for specific pathogen
genotypes occur on particular host
genotypes? Is there evidence for
host specialization in these
pathosystems?
To address the latter questions,
we utilized increasingly
sophisticated field experiments to
differentiate among the various
evolutionary forces acting on
populations of these fungi. In this
manuscript, I will briefly review
our understanding of the
population genetics of both fungi at
this point in time. The majority of
this manuscript was distilled from a
review chapter written for the Long
Ashton Symposium on Septoria in
Cereals held in 1997. Detailed data
to support our interpretations are
presented in that chapter
(McDonald et al., 1999).
Materials and Methods
77
DNA markers
The RFLP markers utilized for
these studies were developed in the
same way for both fungi utilizing
the methods described in
McDonald and Martinez (1990b).
Single-locus probes were used to
measure gene diversity for
individual RFLP loci and to
measure population subdivision
and genetic similarity among
populations (McDonald and
Martinez, 1990a; Boeger et al., 1993;
McDonald et al., 1994; Keller et al.,
1997a,b). Probes that hybridized to
repetitive elements were used for