Handbook Part 2 - International Mycological Association

PS8-468-0434
Evolution of microsatellites in the mitochondrial genome of Rhynchosporium secalis
S Torriani 1, S Banke 3, C Linde 2, BA MacDonald 1
1 Institute of Integrative Biology, Zurich, Zurich, Switzerland, 2 School of Botany and Zoology, Canberra, Australia, 3
Biological Inst., Copenhagen, Denmark
The aim of this project was to investigate the evolution of microsatellites in a mitochondrial genome, more specific to
test hypothesis that the microsatellites evolve faster than the variable nucleotide regions and therefore could add to
the diversity of the genome.
Two variable microsatellite regions in the genome of Rhynchosporium secalis were identified. The microsatellite
repeats consisted of 2 and 7 base-pair repeat respectively. For the two loci, four and three alleles were identified
among 60 worldwide isolates of R. secalis. Variable flanking regions of the microsatellite and other variable regions of
the mitochondrial genome were sequenced in order to infer a phylogeny representing the mitochondrial genome.
No phylogenetic conflict was observed among the regions sequenced. All the sequence loci were therefore treated
as one DNA sequence locus. A haplotype network was generated from the combined sequence data and a total of
11 haplotypes was found. Only one missing haplotype was identified in the network, suggesting a good representation
of the diversity found in the mitochondrial genome. After mapping the microsatellites onto the haplotype network, we
found that one microsatellite increased the diversity in the haplotype network. The other microsatellite was responsible
for a conflict by introducing homoplasy in the network, suggesting that the same microsatellite repeat number might
have arisen twice. We conclude that the microsatellites found in the mitochondrial genome of R. secalis will be useful
to analyse populations of R. secalis. It also suggests that the mutation rates for the microsatellites are higher than the
nucleotide substitution rates in the mitochondrial genome.
PS8-469-0439
Population expansion-migration scenarios explain the demographic history of the fungal pathogen
Mycosphaerella graminicola
S Banke
Biological Inst., Copenhagen, Denmark
The following hypothesis was tested in this project: a population that evolve with a new host is likely to show high levels
of population expansion over time, where as population recently founded by gene flow show no population
expansion and a more disruptive phylogenetic network pattern.
Five global populations of Mycosphaerella graminicola were chosen for this study and information from 6 DNA
sequence loci and four microsatellite loci was used to estimate the different population genetic parameters and infer
haplotype networks. Haplotype networks from recently founded populations showed more disruptive networks, where
as the populations that potentially had evolved with the new host showed star like networks. Two different models
were used to estimate mainly population expansion and migration, the first model allowed for recombination and
estimated directional gene flow as well as possible growth for each population. The second model did not allow for
any recombination, but estimated divergence time, migration and population expansion between two populations
at the time. Both models supported the same scenarios where significant population expansion where seen in the
populations evolving with the host and no expansion in the recently founded populations. The migration estimates
helped to explain this pattern as most migration where coming from the populations evolving with the host to the
recently founded populations. We conclude that population expansion might play an important role in shaping the
population structure of a pathogen evolving on a new host, given that the new host is successful
PS8-470-0444
Origin And Expansion Of Rhynchosporium Secalis, A Fungal Leaf Pathogen Of Barley
CC Linde 1, BA McDonald 2, PL Zaffarano 1
1 School of Botany and Zoology, The Australian National University, Canberra, ACT, Australia, 2 Plant Pathology,
Institute of Integrative Biology, ETH, Zurich, Switzerland
Rhynchosporium secalis is a Deuteromycetous fungus with a proposed teleomorph in the Discomycetes. However, the
teleomorph has never been found and its existence is solely based on observed moderate levels of neutral genetic
diversity and frequency-dependent-selection on mating types as measured in some populations. It is a pathogen that
infects cultivated barley and other Hordeum relatives as well as eg couch grass (Agropyron repens), a common grass
species native to Eurasia. Because of its common association with cultivated barley, a coevolutionary relationship
between Hordeum species and R. secalis is assumed, which suggest a Middle Eastern origin for R. secalis. However,
neutral genetic variation in R. secalis, as obtained with RFLPs and 15 microsatellite loci, consistently reveal a significant
lower number of alleles and lower levels of gene and genotypic diversity in R. secalis populations originating from
cultivated barley in the Middle East, as well as from the non-cultivated Hordeum spontaneum populations from the
Middle East, the progenitor of cultivated barley. This suggests that R. secalis did not co-evolve with barley, but coevolved
with another host and only encountered barley when it was introduced as an agricultural crop.
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