The role of integrative conjugative elements in evolution of the kiwifruit pathogen Pseudomonas syringae pv. actinidiae : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Genetics at Massey University, Albany Campus, New Zealand
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Date
2017
DOI
Open Access Location
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Massey University
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Abstract
Horizontal
gene
transfer
(HGT)
is
a
major
force
driving
evolution
in
prokaryotes.
Among
the
different
contributions
to
bacterial
fitness,
HGT
underpins
the
evolution
of
pathogenicity
and
virulence
in
several
bacterial
pathogens.
Pseudomonas
syringae
pv.
actinidiae
(Psa)
first
emerged
as
a
pathogen
of
kiwifruit
in
Asia
during
the
1980’s.
In
2008
an
outbreak
occurred
in
Italy
that
rapidly
spread
to
major
kiwifruit
growing
areas
of
the
world.
During
its
global
journey
the
outbreak
lineage
independently
acquired
divergent
Integrative
and
Conjugative
Elements
(ICEs),
harbouring
an
identical
set
of
cargo
genes
that
are
hypothesized
to
be
involved
in
the
plant-‐pathogen
interaction.
Here
I
show
that
the
three
ICEs
acquired
by
Psa
belong
to
a
diverse
family
of
ICEs
present
only
in
plant-‐associated
Pseudomonas
ssp.
(the
PsICEs).
The
evolution of PsICEs is characterized by extensive inter-‐ICE recombination
events that are frequent enough to mask evolutionary history, producing
chimeras with variable patterns of similarity to each other, yet maintaining a
syntenic backbone where cargo genes are integrated in conserved positions.
Although there are different classes of PsICE cargo genes, one set was
frequently recovered: those contained on a Tn6212 element. Tn6212 confers a
selective benefit when Psa is grown on succinate, fumarate, or malate as the
only carbon source, but no phenotype was detected in planta. Members of the
PsICE family also confer copper resistance to Psa strains isolated in New
Zealand. I analyzed a
number
of
these
and
showed
transfer
in
vitro
and
in
planta.
I
also
measured
the
fitness
consequences
of
ICE
carriage,
captured
the
de
novo
formation
of
novel
recombinant
ICEs,
and
explored
ICE
host-‐range.
Together
my
work,
which
began
with
observations
from
genome
sequences
before
moving
to
experimental
studies
in
the
laboratory,
has
provided
new
insights
into
the
role
that
horizontal
gene
transfer
plays
in
the
evolution
of
virulence.
Description
Keywords
Pseudomonas syringae, Genetics, Kiwifruit, Diseases and pests, Research Subject Categories::NATURAL SCIENCES::Biology::Cell and molecular biology::Genetics