Biology Reference
In-Depth Information
only are there two different phases adapted to different seasonal conditions, but
there are two different hosts (a crustose and a bladed host) for the endophytic
sporophyte phase (Sussmann and DeWreede
2002
).
11.2.3 Eukaryotic Parasites and Pathogens
In addition with the case of hemiparasitism caused by
V. lanosa
in
A. nodosum
(see
above), strict parasitic relationships have been well studied in red algae, but
remained nearly unexplored from a physiological point of view in fungal or
oomycete colonizers of marine macroalgae.
11.2.3.1 Adelphoparasites and Alloparasites of Red Algae
Some of the algal endophytes in fronds of red algae are nonphotosynthetic and are
assumed to obtain carbon from their hosts (Goff
1982
). The evolution of these red algal
parasites is one of the most fascinating aspects of algal associations: It is common
among florideophyte red algae, involving about 8-10%of the species, and has a unique
type of development in which parasite nuclei are transferred to host cells and “control”
host cell development in a kind of hijacking. It was reinvestigated in the recent years
using new molecular tools to better decipher phylogenetic relationships between the
host and its parasite (Zuccarello et al.
2004
;Kuriharaetal.
2010
), as well as to generate
genomics data on the two partners. A recent work reported the sequencing of the
mitochondrial genomes of the free-living gracilarioid
Gracilariopsis andersonii
and
its closely related parasite
Gracilariophila oryzoides
to characterize the effect of a
parasitic lifestyle on the organellar genomes of red algal parasites (Hancock et al.
2010
). This latter paper provides a comprehensive review of the biology of aldelpho-
parasitism that is summarized below.
Red algal parasites are unusual because the vast majority (80%) of them parasit-
ize species with which they share a recent common ancestor. This strategy has
earned them the name “adelphoparasites,” from the Greek, adelpho, meaning “kin.”
Intracellular adelphoparasites are hardly observed in nature, morphologically
reduced to a miniature, often colorless pustule and, yet, have independently evolved
hundreds of times among the florideophyte red algae. Another part of red algal
parasites (20%) are known as alloparasites (“allo” meaning other), which also
diverged from free-living species, but then have radiated to exploit more distantly
related red algal hosts (Goff et al.
1997
; Kurihara et al.
2010
). Goff et al. (
1997
)
give two possible scenarios for the origin of parasitic red algae. In one, the parasites
are ancestrally epiphytic, later becoming endophytic and eventually parasitic. In the
second, the parasites derive directly from spores that lose the ability to survive
independently of the parent, following the evolution of a carposporophyte after
fertilization in the florideophyte. One possibility is that adelphoparasites arose by
the second process while alloparasites arose by the first.
