Biology Reference
In-Depth Information
many individual host thalli. To date, endophytic gametophytes have only been
observed either in situ or in vitro in cultures from the Pacific Ocean (Hubbard et al.
2003
). On the one hand, it is possible that the endophytic habit is an accident of
spore settlement and that it is of little or no adaptive significance. On the other hand,
endophytism may represent a facultative strategy of primary importance to game-
tophyte growth and reproduction, at least in some species or in some environments.
The irregular branching pattern and absence of heterotrichous development in the
kelp gametophytes are analogous to that found in many endophytic red algae (e.g.,
in Acrochaetiales, Garbary et al.
1982
) or endophytic green algae [e.g.,
Acrochaete
,
Endophyton
,
Phaeophila
(Nielsen and McLachlan
1986
)]. These common features
suggest a morphological convergence in red, brown, and green algae based on the
common adaptation for endophytism. The benefits and drawbacks of the endophytic
lifestyle for kelp gametophytes remain unresolved. The small morphology and low
density of endophytic gametophytes in culture suggests drawbacks. However, both
free-living and endophytic gametophytes can be found in nature (e.g., Kain
1979
;
Garbary et al.
1999a
,
b
), implying that both lifestyles represent adaptations. Sym-
biosis could provide protection from grazing and sedimentation, or improve access
to light. Fine sediments reduce kelp spore attachment and survival (Devinny and
Volse
1978
) and the understories of kelp stands experience higher levels of sedi-
mentation than adjacent cleared areas (Eckman et al.
1989
; Duggins et al.
1990
).
Anderson et al. (
1997
) found that juvenile
Ecklonia
were grazed less when they
grew on adult holdfasts. Similarly, red algal hosts may inhibit herbivores from
accessing the gametophytes, or the host surface may be more resistant than in
epilithic gametophytes (Padilla
1985
). It is also possible that endophytes have no
better chance of surviving than free-living gametophytes, but the increased diver-
sity of life history strategies may increase the overall likelihood of gametophytes
surviving. More recently, molecular and microscopy data revealed that
gametophytes of
Alaria
,
Macrocystis,
and
Nereocystis
epi/endophytically grow in
the older portions of the thallus of
Lessoniopsis littoralis
(Lane and Saunders
2005
).
However, developing sporophytes were hardly observed in the field attached to
L. littoralis
, suggesting that endophytic habit is an accident of spore settlement and
that it is of little or no adaptive significance.
Green Algal Endophytes as Alternate Life History Phases
of
Acrosiphonia
Species
Molecular studies have shown the relationship between the unicellular endophytes
(“
Codiolum
” and “
Chlorochytrium
”) and their alternative life history stages, the
gametophytes of
Acrosiphonia coalita
and
A. arcta
(Sussmann et al.
1999
). The
population dynamics of the endophyte and their common hosts (
Mazzaella
splendens
and “
Petrocelis”
) has been explored, showing a seasonal pattern of
abundance of the endophyte (Sussmann and DeWreede
2002
). A puzzling aspect
of the relationship is the presence of many endophytes at the time the host plant is
disintegrating. A bet-hedging strategy is proposed for this complex life history; not
