303 Pacific
email: gcarroll@oregon.uoregon.edu
phone: 346-4522
Research interests
Symbiotic associations, in which two or more organisms live in intimate contact,
are now recognized to be extremely common throughout the biological world. I
work with a group of microfungi, termed fungal endophytes, which live completely
asymptomatically within leaves and stems of healthy plants. At the moment the
following general questions about these associations interest me.
1. Fungal endophytes are highly diverse, with many species occurring on a single
host and as many as 6-7 (and many more in the tropics) in a single leaf. How
can sampling schemes be devised to accurately describe this situation with the
least amount of work. Using hierarchical sampling schemes, how is the variance
in fungal infection frequencies partitioned among leaves within plants, among
plants at a given site and among sites in a given habitat? These are not trivial
questions since pharmaceutical companies who culture fungi for screening of
pharmacological activities would like to know how to sample fungal endophytes
most efficiently.
2. Do fungal endophytes play a role in deterring herbivory and thus function
as mutualists? This is a question which has bedeviled mycologists since endophytes
were first discovered in conifers of the Pacific Northwest. This hypothesis
has been proven in only several instances, usually with fungal endophytes which
attack the larvae of gall-forming insects. I would like to show that herbivore
deterrence provides the basis for endophyte mutualism in a much broader range
of situations, and am currently investigating the possibility that endophytes
in understory plants in old-growth Douglas fir forests serve to protect infected
leaves from grazing by slugs.
3. Have fungal endophytes evolved in parallel with their hosts? Did they “grow
up together” in an evolutionary sense. Little is known about the evolutionary
relationships of non-pathogenic fungi and their hosts. Some endophytes appear
to be absolutely host specific; however nobody has examined situations in which
apparently the same endophyte occurs on disjunct but closely related host species.
This problem bears on the question of how many species of fungi are extant and
how the distribution of hosts bears on the population biology of fungi. The
availability of phylogenetically informative DNA sequences now allows this question
to be addressed, and I have begun to begin work on this general problem using
host-specific endophytes on endemic, very locally distributed species of
Encephalartos, a genus of South African cycads
4. How can we explain the nearly ubiquitous presence of wood-decomposing Ascomycetes
as endophytes in leaves of plants where the active phases of the fungi are never
seen? How can we interpret the ubiquitous presence of Hypoxylon serpens,
a decomposer of maple wood, in needles of Douglas fir and leaves of twinflower?
I propose that such endophytic infections are foraging stages of these fungi,
dormant infections which allow the fungus to persist for long periods of time
in an environment where the host is absent or very patchily distributed. If
such is the case, these fungal endophytes must reproduce exclusively by asexual
means, and over time the effects of mutation accumulation should become evident.
Selected Publications
Carroll, G.C. 1992. Fungal Mutualism. In The Fungal Community. Eds. G.C.
Carroll and D.T. Wicklow, 327-354. New York: Marcel Denker, Inc.
Carroll, G.C. 1995. Forest endophytes: pattern and process. Canadian Journal
of Botany 73 (Suppl. 1): S1316-S1324.
Stone, J.K., Sherwood, M.A., and G.C. Carroll. 1996. Canopy microfungi: function
and diversity. Northwest Science .70:37-45.
Carroll, G.C. 1997. Fungus/Plant Interactions - An Overview. In The Mycota,
Vol 5A. Eds. G.C. Carroll and P. Tudzynski. Springer Verlag, Berlin, pp. 1-8.
Wilson, D. and G.C. Carroll. 1997 Avoidance of high-endophyte space by gall-forming
insects. Ecology 78: 2153-2163.