Biology Reference
In-Depth Information
(reward) for each partner through exchange of specialized resources, and (3) a strong probability
of continual contact between the associates.
C
OSTS
Costs of association are critical considerations when attempting to describe interactions between
symbionts. However, costs are difÝcult to quantify in biological systems. Regardless, we can at
least speculate as to what some may be. Structurally complex glandular mycangia are likely to
incur costs to the beetle through energy expenditures in the production of the structure and
secretions by glands; however, such costs are probably low given the relatively small size of
mycangia and the small overall amount of secretions produced. Although less complex mycangia
may incur lower costs, the costs of structurally complex glandular mycangia may be sufÝciently
offset through nutritional gains and other potential beneÝts to the beetle derived from maintaining
speciÝc fungi.
Costs to the fungi consist primarily of energy expended in production of mycelium ingested
by larvae and of spores ingested by teneral adult beetles during maturation feeding. These losses
are likely offset by increased resource availability, increased efÝciency in transport, and lower
overall spore production compared with fungi that do not rely on arthropod vectors. Noninsect-
vectored fungi rely instead on wind or rain-splash dissemination that requires the production of
extremely high numbers of spores, most of which die in transit or land on unsuitable substrates.
Other potential costs to the fungi include production of insect-adapted spores with a sticky
adherent matrix and the production of nutrients useful to the host beetle. Because fewer spores
are made, costs of producing more complex insect-adapted spores are likely to be insigniÝcant.
Likewise, the production or concentration of essential nutrients useful to the host insect may be
minimal or come at no additional cost. In fact, production or concentration of nutrients by fungi
may have been an important preadaptation that allowed some of these associations to form in
the Ýrst place.
R
EWARDS
The concept of rewards in mutualism is tightly linked to the magnitude of costs incurred. Costs
must be minimal and rewards must exceed costs. CostÏbeneÝt analyses are often complicated to
conduct, especially in systems such as those involving bark beetles where rearing the insect and
conducting manipulative experiments can be very difÝcult. Under these conditions, determining
rewards and, consequently, assigning a mutualistic role to interacting organisms must be carefully
considered. For example, in the
D. frontalis
system, at least one mycangial fungus,
Entomocor-
ticium
sp. A, has strong positive effects on host-beetle Ýtness and is clearly mutualistic (Barras,
1973; Bridges, 1983; Goldhammer et al., 1990). For the other two common fungal associates,
assignment of type of effect on the host is not so straightforward. The mycangial fungus,
C.
ranaculosus
, does not greatly beneÝt host development and may actually function more as a
commensal with the beetle. However, by displacing the more beneÝcial mycangial fungus it
reduces potential beneÝts to the host and so may be considered to act antagonistically. A
nonmycangial associate,
O. minus
, is highly antagonistic to developing beetle brood, which
greatly lowers host Ýtness. Despite strong negative effects on beetle Ýtness, some workers have
speculated that
O. minus
, which exhibits pathogenicity to host trees under certain circumstances,
may still be mutualistic with the beetle by contributing to the decline and death of the tree during
the early stages of colonization (Barras, 1970;
Klepzig et al., 2001). However,
O. minus
is not
required for successful colonization of the tree, and the beetle has a higher reproductive potential
in its absence. Thus, this fungus should not be considered mutualistic with
D. frontalis
. Rather,
when overall costs and beneÝts are considered, the Ýtness of
D. frontalis
would likely greatly
increase in the complete absence of
O. minus
, and therefore
O. minus
should be considered an
antagonistic associate.
