Evolution of the Attine Ant-Fungus Symbiosis

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9707209 Mueller Among the more fascinating cases of symbiosis in the American tropics is the interaction between leafcutter ants of the tribe Attini (Family Formicidae) and fungi of the mushroom family Lepiotaceae (Basidiomycetes) in which the ants maintain "gardens" of freshly cut leaves on which the fungi are tended and harvested for food. There are some 200 or so described species of leafcutter Attini ants, and advanced members of the group are known to maintain extensive gardens in which propagation of the fungi is obligately linked to strict vertical transmission of inocula from parent to offspring nests. This obligate transmission of food inocula raises the intriguing question whether mutational changes and evolutionary diversification in the ants is mirrored by change and diversification in their cultivated fungi, and leads to conjoint patterns of speciation and coevolution. The interaction is known to be even more complicated because other groups of putatively primitive Attini ants are opportunistic in their acquisition of fungi for their gardens, and may regularly harvest fresh inocula from wild sources. Which ant groups do this regularly, which have switched over to the obligate mode of growth and transmission, and which act in an intermediate fashion are all unanswered questions. Correspondingly, which species and genera of Lepiotaceae fungi are associated with the various ant cohorts in "open" or "closed" systems of inocula uptake and transmission are also unknown. Preliminary molecular DNA analyses by Dr. Ulrich Mueller at the University of Maryland and his colleagues Ted Schultz at the Smithsonian and Stephen Rehner at the University of Puerto Rico have opened a major technological door to the study of this conceptually challenging case of symbiosis. Molecular methods are particularly necessary in study of the fungi because the cultivated forms rarely if ever sporulate, and in the absence of spore-bearing structures, taxonomic identification is difficult if not impossible. Diagn ostic DNA markers, from nuclear or mitochondrial genes, hold hope for solving this problem, while providing data that can be used to infer phylogenetic or genealogical associations among the fungi. This collaborative team of entomologists and mycologists continues field collecting in Central and South America, to augment the nearly 140 ant species thus far collected, and in parallel to collect fungal material from ant nests. Field work with Brazilian and Argentinian colleagues will facilitate collecting in rich areas in South America. Molecular DNA sequencing and morphological analyses of the ants and of those fungi that can be induced to sporulate in culture will provide additional taxonomic and phylogenetic information. Study of the phylogenetic patterns and ecological habitats of associated ants and fungi will create an accessible model of symbiont evolution in tropical terrestrial ecosystems, comparable in its richness of interactions with the coral reef symbioses involving corals and photosynthetic dinoflagellates.