Biology Reference
In-Depth Information
These interactions were extensively studied at a cell biology level by the group
of Lynda Goff and Annette Coleman since the 1980s (see Hancock et al.
2010
), but
little is known about how these algal parasites affect their hosts at an organismic
level and even less at the population level. Research has concentrated on assessing
changes in the host at the cellular level (Kugrens and West
1973
; Goff
1976
)orin
detecting translocation of substances between partners (e.g., Court
1980
; Kremer
1983
). Kremer (
1983
) showed that although there was carbon transfer in the form of
the photoassimilate digeneaside and of amino acids from the host
Rhodomela
confervoides
to its parasite
Harveyella mirabilis
, carbon fixed by the host was
enough to cover the needs of both the parasite and the host. There are, though,
indications that parasitized hosts are more susceptible to removal from the popula-
tion following local tissue destruction and a decrease in growing tips (Goff
1976
).
Few studies have specifically evaluated the effects of algal parasitic infections on
the host, and these demonstrated reduced growth rates and elongation of the host
thallus (Nonomura
1979
; Apt
1984
). It was shown that infections by the red alga
Hypneocolax stellaris
reduced the growth rate of its host
Hypnea musciformis
by
40% in the field and up to 70% in laboratory trials (Apt
1984
).
In contrast with the excellent knowledge on the life history strategies of red algal
parasites, the molecular and genomic consequences of becoming a parasite
remained unknown until the recent work by Hancock et al. (
2010
). It reports the
sequencing of the mitochondrial genomes of the free-living
Gracilariopsis
andersonii
and its closely related parasite
Gracilariophila oryzoides
. Whereas the
parasite genome is similar to the host in many ways, the genes encoding the
essential proteins
atp8
and
sdhc
are pseudogenes in the parasite. The mitochondrial
genome of a parasite from a different class of red algae,
Plocamiocolax pulvinata
,
has also lost the
atp
8 gene entirely, indicating that this gene is no longer critical
in red algal parasite mitochondria. The trend toward the loss of
atp8
is
significant considering that rapid rates of evolution are not occurring among the
majority of mitochondrial genes. This creates a bit of a paradox because genes
considered of vital importance are being lost while, at the same time, there are
clearly selective forces maintaining the genome sequence from further losses and
accelerated mutational rates. Further analysis of host and parasite nuclear DNAmay
elucidate some fundamental genomic shifts and molecular changes that have
resulted in a parasitic existence (Hancock et al.
2010
). In addition, it is interesting
to mention that the galactans from
Tikvahiella candida
, an adelphoparasite of
Solieria robusta
, closely resemble those of its host and furnish evidence in support
of a close phylogenetic relationship between the two species (Chiovitti et al.
1999
).
Similarly with the importance of cell wall galactans in the host perception by spores
of some green algal endophytes (Bouarab et al.
1999
) and oomycete spores
(Uppalapati et al.
2001
), it is tempting to speculate that part of the host specificity
in red alga parasitism has evolved through sophisticated surface signal perception.
Future work in this field will undoubtedly beneficiate of comparative transcriptome
analysis using RNAseq that will reveal the expression and the regulation of both host
and parasite genes in parasitized red algae as recently done in algal oomycete
interactions (see below).
