TEPE, ERIC J.*, MICHAEL A. VINCENT, and LINDA E. WATSON. Department of Botany, 316 Pearson Hall, Miami University, Oxford, OH 45056, USA. - Evolution of ant mutualisms in Piper subg. Macrostachys (Piperaceae): variation in ant-associated characters.
Several Central American species of Piper have developed
obligate mutualisms with ants, in which plant partners provide nesting
sites and food for ant partners. In turn, the plants receive some
protection from herbivores and fungal infection. In addition to these
obligate ant-plants, some species of Piper are found that have
resident ants only sometimes (facultative), and still other
Piper species are never found with ants. This continuum is
found in Piper subg. Macrostachys, and the degree of ant
association is closely correlated with plant morphology. This
variation in morphology and ant presence has contributed to
confounding evolutionary relationships. In obligate ant-plants, the
sheathing petioles that are typical of some Piper become
tightly rolled into a tube. The ants, Pheidole bicornis in most
cases, move into this cavity, and in general, the stems become hollow
as the plant increases in size with the colony eventually occupying
the entire plant. The food bodies that the ants subsist on are
produced on the adaxial surface of the petioles (i.e. inside the
tube), thus available exclusively to the ants; furthermore they are
only produced in the presence of the ants. While food bodies and
hollow stems are only found in the obligate ant plants, variously
closed petiole domatia are found in other Piper species; an
increased degree of closure corresponds with increased density of ant
residents. In these facultative ant plants, the association between
ant and plant is less specific, and a number of ant species inhabit
these plants. This continuum of ant mutualisms and plant adaptations
is being examined in the context of an independently derived molecular
phylogeny of Piper subg. Macrostachys, with the ultimate
goal of understanding the evolutionary sequence of events and the key
adaptations that resulted in obligate associations between ants and
plants.
Key words: ant-plant mutualisms, evolution, phylogeny, Piper, Piperaceae, plant morphology