Biology Reference
In-Depth Information
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ONTACT
Phytophagous bark beetles are in contact with associated mycelial fungi except on occasions when
larval tunneling extends ahead of the growing front of the fungi. When this disassociation occurs,
the fungi reestablish contact with the beetle in the pupal chamber (Whitney, 1971). Contact with
yeasts is apparently always maintained (Whitney, 1971).
Progeny beetles probably develop most often with, and later disseminate, the same fungi as
were inoculated into the tree by their parents. However, occasionally exceptions may occur. Gal-
leries excavated by parental beetles and their brood may occasionally be colonized by a fungus
originating in an adjacent gallery of a conspeciÝc or other species of beetle, or from tissues infected
by fungi carried by other arthropods. The parent-inoculated fungus may be joined or even replaced
by another fungus when this occurs, resulting in an addition to or loss of the original symbiont
and its potential replacement with another. If replacement occurs, it may be with another individual
of the same fungus or by an individual of another species. The frequency of such replacements is
unknown. However, fungi associated with one bark beetle species have been isolated from another
when two or more beetle species cohabit the same trees. For example,
O. ips
can occasionally be
isolated from the exoskeleton of
D. ponderosae
when it cohabits trees with
I. pini,
a common host
of
O. ips
(D.L. Six, unpublished).
POTENTIAL ORIGINS AND CONSEQUENCES OF OLIGOPHILY
The potential for hosts to contact symbionts not of parental origin may be an important determinant
of whether associations are monophilic or oligophilic. Replacement of, or addition to, established
fungal symbionts with other fungi may at least in part explain why the majority of bark beetle
species thus far investigated are oligophilic (Six and Paine, 1999). Because bark beetleÏassociated
fungi may sometimes become disassociated from the host beetle for a period while growing within
host-tree tissues, and because several species of fungi are often present in a bark beetleÏcolonized
tree, the potential exists that contact with an established fungus will be lost and that contact with
other fungi may occur. Simultaneous colonization of host trees by two or more scolytid species as
well as by a large number of other potential arthropod vectors increases the probability of inoculation
of a large community of fungi into the tree. While some partitioning of resources within the tree
by insects occurs, galleries often intermingle, increasing the potential for contact of beetle progeny
with nonparental fungi. Such exposures may have allowed host shifts, as well as exploitation of
established mutualisms by nonbeneÝcial fungi, to occur (Six and Paine, 1999).
Despite the relatively high potential for contact between beetles and nonassociated fungi, it is
apparent that a high degree of speciÝcity exists in many associations. The mechanisms behind
maintaining such speciÝcity are unclear. However, such mechanisms are likely imperfect and may
allow the invasion and parasitism of coevolved associations. For example, many mycangial
Den-
droctonus
possess two mycangial fungi. For most, one associate appears to be highly coevolved
with the host beetle, while the other appears to be the result of a more recent colonization event
(Six and Paine, 1999). For coevolved associates, many appear to be mutualistic, while the more
recent associates appear to be antagonistic or at least confer less beneÝt to the beetle than do the
highly coevolved partners. The newer associates may be cheaters in the system, parasitizing existing
associations to their beneÝt while conferring few or no beneÝts to the host.
Certain characteristics may increase the ability of particular groups of fungi to invade already
established associations between beetles and fungi. For example, the sticky spores produced by
ophiostomatoid fungi may account, at least in part, for their widespread association with
scolytids. Furthermore, fungi closely related to established associates, and thus possessing
similar attributes, may more easily invade established associations. For example, several beetles
with coevolved
Ophiostoma
associates also possess
Ophiostoma
associates of more recent origin
(Six and Paine, 1999).
