Biology Reference
In-Depth Information
Here again, sulfation is likely an important cue in signal recognition because
agarose, a neutral agar fraction, did not enhance appressorium formation. However,
O. porphyrae
ultimately only infects
Porphyra yezoensis
and
Bangia atropurpurea
and protoplast fusion of
P. yezoensis
with the resistant
Porphyra tenuipedalis
resulted in hybrids with reduced susceptibility. Therefore, probably other signals
are involved to increase specificity leading to selective infection.
Various oomycetes are able to infect other red algae, but the spectrum of
compatible hosts is usually narrow (West et al.
2006
). For example, a strain of
Olpidiopsis
sp. fromMadagascar that had been isolated from
Bostrychia moritziana
was also able to infect certain isolates of
B. tenella
,
B. radicans,
and
B. radicosa
,
but no other species (West et al.
2006
). Five other species of
Bostrychia
were
ultimately resistant, as were six different species of
Stictosiphonia
,
Lophosiphonia
,
Neosiphonia,
and
Polysiphonia
. Interestingly,
Bostrychia
isolates originating from
Madagascar were in most cases susceptible, and strains from other parts of the
world mostly resistant, which seemingly indicates the existence of pathovars
(strains of the same species with distinctive pathogenicity due to coevolution
with different hosts) among algal pathogens.
Surprisingly, basal oomycetes, such as
Eurychasma dicksonii
, which is the most
common and widespread eukaryotic pathogen of marine algae (Gachon et al.
2009
),
appear far less specific than
O. porphyrae
. It occurs in all cold and temperate seas
worldwide (Sekimoto et al.
2008
). In culture, it infects virtually all species of brown
algae tested (more than 40, M
uller et al.
1999
), including representatives of all major
orders of this phylum. qPCR and microscopic observations made on laboratory-
controlled cultures revealed that clonal brown algal strains exhibit different levels
of resistance against
Eurychasma
, ranging from high susceptibility to complete
absence of symptoms (Gachon et al.
2009
). In the coevolution race between parasites
and hosts, the parasites quickly overcome new resistance strategies by the host. This
process should lead to local adaptation, where a parasite population has a higher
mean performance on local (sympatric) versus foreign (allopatric) host populations.
Hence, this latter study opens the perspective of combining large-scale disease
monitoring in the field with laboratory-controlled experiments on the genome
model seaweed
Ectocarpus siliculosus
to improve our understanding of host speci-
ficity in brown algal diseases (Grenville-Briggs et al.
2011
).
€
“
Me
´
nages a
`
trois,
”
Quorum Sensing Mediated
Bacterial-Macroalgal Interactions Modulate Colonization
11.3.2
In marine environments, the space competitors include, in addition to macrophytes,
sedentary animals and the periphytic community of microalgae, cyanobacteria,
heterotrophic microbes, and protists (see Chap.
7
by Edwards and Connell). The
bacterial communities that are associated with macroalgae often provide further
settlement cues for macrofoulers, which has in particular been studied with
