The origins of tropical diversity: phylogeography of leafcutter ants (Atta spp.)
Scott E. Solomon, Section of Integrative Biology, The University of Texas at Austin, 1 University Station, C0930, Austin, TX 78712 and Ulrich G. Mueller, Section of Integrative Biology, University of Texas at Austin, 2401 Speedway, Austin, TX 78705.
The mechanisms responsible for generating high species diversity in tropical organisms have remained elusive despite decades of research. Recently, molecular phylogeographic methods have provided a new approach to test predictions made by the leading hypotheses of tropical diversification (the Forest Refugia, Riverine Barrier, Habitat Gradient, and Marine Incursion models). To date, molecular approaches to test these hypotheses have focused exclusively on vertebrates, despite the fact that invertebrates, especially insects, comprise some of the most diverse tropical taxa. Using leafcutter ants of the genus Atta (Formicidae: Attini), we tested these four hypotheses by sequencing approximately 500 base-pairs of mitochondrial DNA, including part of the cytochrome oxidase I and cytochrome oxidase II genes, the entire tRNA-leucine gene, and an intergenic spacer region. We used analysis of molecular variance, partial Mantel tests, and parametric bootstrap analyses to determine whether refugia, rivers, ecotones, or areas of marine incursion have acted as barriers to gene flow in Atta spp. In addition, mismatch analyses and Tajima's D statistic were used to test for population expansion, as predicted by two of the four hypotheses. The results suggest that no single hypothesis can fully explain the patterns of population structure observed in extant Atta spp. We suggest that a combination of factors operating at different spatial and temporal scales is most likely responsible for generating population structure, and possibly speciation patterns, in leafcutter ants.