Breaking News: The Atta Phylogeny


Atta cephalotes carrying leaves, Ecuador

Molecular Phylogenetics and Evolution this morning has the first detailed molecular phylogeny of the leafcutting ant genus Atta.  MaurÃcio Bacci et al sequenced several mitochondrial genes and the nuclear gene EF-1a from 13 of the 15 described species-level taxa, using them to infer the evolutionary history of the genus.

This is an important paper. 

Atta is the classic leafcutter ant of the new world tropics and a model system for co-evolution among the ants, the fungi they cultivate, and a suite of microbes that either parasitize or protect the ant-fungus mutualism.  These ants are also one of the most important agricultural pests in the Neotropics, capable of defoliating entire trees overnight. An understanding of the evolutionary history of this group will be very helpful in uncovering details about the nature of the ant-fungus relationship and the evolutionary factors that have shaped it.

attaphylo1I've reproduced a simplified version of the Bacci et al tree in the figure at left, stripping out the species-level detail to show only the subgenera, each of which comprises several species.  The geography of the genus is represented by the color of the branches.

Two patterns jump out at me.  First is the geography.  The older lineages are northern and the younger ones southern, suggesting that Atta arose in Central America and speciated as it dispersed southward.  Oddly, the highest diversity of Atta occurs at the southern part of its range, near Paraguay, as if the farther afield the ants travelled the more species they generated.

Second, the molecules offer unambiguous support to classical taxonomy.  Each of the subgenera named by various morphologists- Archeatta, Atta s. str., Epiatta, and Neoatta- emerge with strong support, with Archeatta as the oldest.

I can't leave any paper without a bit of criticism, and mine is this: for all the different genes representing this phylogeny, the total number of base pairs is just a hair above 1,000.  That's not very many, and the results could be prone to statistical artifacts stemming from a small sample size.  Nonetheless, support for the classical arrangement is high, suggesting that this tree is a good one and should hold up as more data are added.

source: Bacci, M. et al. 2008. Phylogeny of leafcutter ants in the genus Atta Fabricius (Formicidae: Attini) based on mitochondrial and nuclear DNA sequences.  Molecular Phylogenetics and Evolution, online early, doi: 10.1016/j.ympev.2008.11.005.

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Does the paper address the biogeographic result? More speciose younger taxa is counter to what would be expected, right? Are the southern ants spread out over a more diverse landscape?

Hi Jim. The paper does mention the biogeography, concluding that "the historical separation of South America from Central and North America has played a role in speciation within this genus."

As to the expectedness of more speciose younger taxa, I'd tend to agree, at least if one assumes a predictably even speciation rate across a tree. But we see violations of this all the time. The African rift lake cichlids, for instance.

The thing is, I don't think the southern habitats really are more diverse. The northern species Atta mexicana is found in Mexican lowland rainforest, as well as in dry Sonoran Desert (the only U.S. population is at Organ Pipe Nat'l Monument).