Biology Reference
In-Depth Information
far surveyed, and for two of three beetle species where yeast associations have been quantiÝed
they are also the most prevalent associates (Shifrine and Phaff, 1956; Bridges et al., 1984; Leufven
and Nehls, 1986).
The yeasts associated with bark beetles do not exhibit pathogenicity to host trees (Callaham
and Shifrine, 1960). Like the ophiostomatoid fungi, they are not highly competitive and are limited
to colonizing tree tissues relatively early in the colonization process (Bridges et al., 1984).
MAINTENANCE OF ASSOCIATIONS
B
A
EETLE
DAPTATIONS
Many bark beetles possess structures of the integument that function in the transport of fungi and
contribute to the maintenance of bark beetleÏfungal symbioses. Some of these structures have been
termed mycangia (Batra, 1963). Use of the term mycangium can vary from highly restrictive to
very broad. In the most restrictive sense, a mycangium is deÝned as an invagination of the
integument lined with glands or secretory cells that is specialized for the acquisition and transport
of fungi (Batra, 1963; Levieux et al., 1991). More loosely deÝned, the term mycangium has been
applied to any structure that consistently transports fungi regardless of form or presence of secretory
cells (Farris and Funk, 1965; Livingston and Berryman, 1972; Nakashima, 1975; Beaver, 1986;
Furniss et al., 1987). This broader deÝnition allows the inclusion of shallow pits and setae along
with deeper pockets that act as fungal repositories but are not known to be associated with glands.
At present, strict categorization of beetles as mycangial or nonmycangial is difÝcult due to the
great variation in structures involved and the current ambiguity in the literature of what constitutes
a ÑtrueÒ mycangium. Given that there is a wide variety of structures, including pits, punctures, setal
brushes, and highly developed sac-like structures, that function in a biologically similar manner,
it may be appropriate to consider any structure that consistently functions to transport speciÝc fungi
as mycangia. Further subdivisions of mycangia could then be delineated on the basis of coarse and
Ýne structure, depending on what is known about the repository. Categories of coarse structure
would include pit, sac, and setal-brush mycangia. Pit mycangia include all fungal repositories
formed by shallow depressions of the exoskeleton. These pits may or may not be associated with
one or several setae. Sac mycangia consist of more complex invaginations forming deep pockets,
tubes, or cavities in the exoskeleton, while setal-brush mycangia consist of dense brushes of setae
that may or may not arise from depressions in the exoskeleton. Upon investigation of Ýne structure,
further subdivision could be made depending on the presence or absence of glands. In the future,
as more is learned about gland types or secretory cells associated with various bark beetle mycangia,
further subdivision by gland type may also prove useful.
The proposed two-tier system of classiÝcation of scolytid mycangia is presented in
Figure 7.2.
This system avoids the confusing and potentially misleading use of the term nonmycangial to refer
to beetles with fungal repositories and close associations with fungi but whose structures lack
glands, or for which the presence of glands is uninvestigated. For example, under the previous strict
deÝnition of what constitutes a mycangium,
would be considered non-
mycangial because the sac-like structures on the maxillary cardines described by Whitney and
Farris (1970) have not been investigated for the presence of glands, despite the fact that they are
involved in maintaining a highly consistent association with two speciÝc fungi and function
biologically as true mycangia. Additionally, beetles with pits that consistently transport speciÝc
fungi have been traditionally considered nonmycangial, even though at least some of those pits are
associated with glands. The proposed system allows initial classiÝcation by biological function
followed by subsequent classiÝcation by structure, rather than the reverse.
The presence of glands is uninvestigated for many mycangia. For some sac and pit mycangia,
the structures have been observed to be associated with waxy or oily secretions, indicating the
presence of glands (Livingston and Berryman, 1972; Furniss et al., 1987, 1995). For others, glands
Dendroctonus ponderosae
