Biology Reference
In-Depth Information
T
RANSPORT
In bark beetleÏfungal symbioses, beneÝt through transport is gained strictly by the fungi. Bark
beetle-associated fungi are completely dependent, or nearly so, on their hosts for transport from
tree to tree. Sticky spores, or spore production within plant tissues, primarily in galleries, pupal
chambers, and the interface between the phloem and outer bark layers of the tree, preclude dispersal
by wind or rain splash. Spores of
Ophiostoma
may also sometimes be isolated from the bodies of
other insects that develop in bark beetleÏcolonized trees such as clerid beetles (D.L. Six, unpublished
data); however, incidence and reliability of dissemination by these potential alternate vectors is
unknown. In some cases, mites may be important vectors (Klepzig et al., 2001).
P
ROTECTION
As with nutrition, protection is a beneÝt that may be gained by both beetle and fungus. Beetle-
associated fungi likely gain direct physical protection from desiccation and UV light while in
transit within mycangia. Beetle brood, on the other hand, may in some instances receive protection
from detrimental contact with antagonistic fungi when developing in phloem colonized by
competitive beneÝcial fungi. Competition among fungal species has been demonstrated frequently
(Shearer, 1995), including among
Ophiostoma
species
(Klepzig, 1998; K. Bleiker and D.L. Six,
unpublished data),
Ophiostoma
,
Ceratocystiopsis
, and
Entomocorticium
(Klepzig and Wilkens,
1997; Klepzig, 1998), and
Ophiostoma
and yeasts (A.S. Adams and D.L. Six, unpublished data).
The outcome of competitive interactions may determine which fungi the larvae will contact and
ingest and consequently affect their development and survival. In the
D. frontalis
system, the
two mycangial fungi,
Entomocorticium
sp.
A and
C. ranaculosus
, and the nonmycangial antag-
onistic associate
O. minus
compete for uncolonized phloem during the initial colonization of the
tree (Klepzig and Wilkens, 1997). In this case,
O. minus
is the superior competitor, colonizing
uninfected phloem more rapidly than either
Entomocorticum
sp. A or
C. ranaculosus.
However,
the two mycangial fungi differ greatly in their ability to maintain resources once they have been
captured.
C. ranaculosus
is quickly overgrown by
O. minus,
while
Entomocorticium
sp. A is not
overgrown and is able to maintain resources in the presence of
O. minus
(Klepzig and Wilkens,
1997). Thus, larvae growing in tissues colonized by
Entomocorticium
sp. A may receive protec-
tion from
O. minus
, while those growing in tissues colonized by
C. ranaculosus
are more
susceptible to contact with the detrimental species.
O
THER
E
FFECTS
Effects of association, both direct and indirect, may also beneÝt host beetles in ways other than
through nutrition, transport, and protection. For example, yeasts isolated from
I. typographus
(L.) are capable of oxidizing
cis
-verbenol, an aggregation pheromone of the host, to verbenone,
an antiaggregation pheromone (Leufven et al., 1984). The abundance of yeasts associated with
individual beetles during host colonization increases during later stages of attack, coinciding
with the period when release of antiaggregation pheromones is at its peak (Leufven and Nehls,
1986). Two yeasts,
P. pini
and
P. capsulata
, associated with
D. ponderosae
convert
trans
-verbenol,
an aggregation pheromone, to verbenone, an antiaggregation pheromone, and this conversion
may play a role in terminating mass attacks on trees by this beetle (Hunt and Borden, 1990).
FACTORS CONSTRAINING MUTUALISM
IN BARK BEETLE-FUNGUS ASSOCIATIONS
The factors and conditions allowing the development and maintenance of mutualism are often
complex but at a minimum require that there is (1) a low cost to resource exchange, (2) a gain
