262 A. Elizabeth Arnold
Box 15.4 Are endophytes a distinct
group?
Endophytic fungi of tropical trees are distinct
from arbuscular mycorrhizal (AM) fungi on the
basis of habitat (aerial tissues versus rhizo-
sphere), taxonomy (primarily Ascomycota versus
Glomeromycota), and diversity (greater in endo-
phytes than in AM fungi). However, the occurrence
of entomopathogens as endophytes raises the
question: How distinct are endophytes relative to
other guilds of fungi? This has yet to be resolved,
refl ecting the lack of knowledge regarding patho-
genic and saprophytic fungi in tropical forests
(see Lodge 1997). Understanding where endo-
phytes fall on the pathogen-to-mutualist con-
tinuum, and whether they exploit non-living tis-
sues of plants, is key to interpreting endophyte
ecology.
Several researchers have suggested that
endophytes are saprotrophs waiting to happen:
endophytes undergo a latent phase as a prelude
to rapid growth following leaf death. In this way,
endophytes might resemble tropical pioneer spe-
cies – but with persistence in leaves instead of
a soil seed bank. Four observations support the
latent-saprophyte hypothesis: (1) the recovery
of putatively saprotrophic taxa such as Xylaria,
Colletotrichum, Alternaria, and Aureobasidium as
endophytes (Bills and Polishook 1994, Fröhlich
and Hyde 1999); (2) the ability of many endo-
phytes to grow on plant-derived media; (3) the
close phylogenetic relationship of many endophytes
to saprophytic species; and (4) the developmentof reproductive structures on surface-sterilized
leaves. Interestingly, fungi that are closely related
to endophytes of tropical plants, including mem-
bers of Cercospora and Fusarium, can produce
gibberellins and abscisic acid (Phelan and Stinner
1992). Could sapro-endophytes hasten leaf death
or leaf drop, and/or accelerate the transition of
leaves from carbon sources to sinks as a means
to further their reproductive success? The role
of endophytes in leaf hormone status and leaf
lifetimes should be explored.
Similarly, it has been suggested that a large
proportion of endophytes are latent pathogens.
Their close relationship to known pathogens
(Carroll 1986) is echoed by the observation that
many pathogenic species have long latent peri-
ods: endophytes may simply represent the long
end of the latent-period spectrum. Under this
scenario, endophytes await environmental cues
that allow them to manifest virulence, and then
reproduce via the formation of necrotic lesions
or other symptoms. Such cues could be abiotic
(e.g., water stress due to drought), intrinsic to the
host (e.g., tissue age), or could refl ect an interac-
tion with a property of the endophyte (e.g., accu-
mulation of suffi cient biomass to induce symptom
development). One intriguing possibility is that
plants serve as alternate hosts of one another’s
pathogens. The potential for a tree to harbor, at
little cost (and perhaps at benefi t) to itself, micro-
bial agents detrimental to its neighbors raises
a series of interesting questions regarding the
cryptic roles of endophytes in tropical forest
dynamics.species in tropical forests remains an open ques-
tion, and one that is more than academic as
we attempt to understand the ecological impor-
tance and potential applications of these cryptic
symbionts.
BEYOND ALPHA-DIVERSITY: HOST
AFFINITY AND SPATIAL
STRUCTURE
The host affinity and spatial heterogeneity of trop-
ical endophytes are much debated. One challenge
lies in the prevalence of singleton species: even
large-scale surveys, such as those at BCI, typi-
cally recover 50–65% of species only once (see
Arnoldet al.2000, Arnold and Lutzoni 2007).
Similar values have been observed for endophytes
in other forest types in Panama (e.g., mangroves:
62.4% of morphospecies were singletons; Gilbert
et al. 2002a) and for macroscopic fungi: at
BCI, Gilbert et al. (2002b) recovered 58.1%
of polypore species (shelf- and bracket-fungi)
only once. Because singletons must be excluded
from analyses of spatial structure or host affin-
ity, analyses frequently consider less than half
of the observed species. Consequently, conclu-
sions regarding spatial and host specificity are