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The Rise of Army Ants and Their Relatives: Diversification of Specialized Predatory Doryline Ants


The Rise of Army Ants and Their Relatives: Diversification of Specialized Predatory Doryline Ants

Seán G Brady et al. BMC Evol Biol.


Background: Army ants are dominant invertebrate predators in tropical and subtropical terrestrial ecosystems. Their close relatives within the dorylomorph group of ants are also highly specialized predators, although much less is known about their biology. We analyzed molecular data generated from 11 nuclear genes to infer a phylogeny for the major dorylomorph lineages, and incorporated fossil evidence to infer divergence times under a relaxed molecular clock.

Results: Because our results indicate that one subfamily and several genera of dorylomorphs are non-monophyletic, we propose to subsume the six previous dorylomorph subfamilies into a single subfamily, Dorylinae. We find the monophyly of Dorylinae to be strongly supported and estimate the crown age of the group at 87 (74-101) million years. Our phylogenetic analyses provide only weak support for army ant monophyly and also call into question a previous hypothesis that army ants underwent a fundamental split into New World and Old World lineages. Outside the army ants, our phylogeny reveals for the first time many old, distinct lineages in the Dorylinae. The genus Cerapachys is shown to be non-monophyletic and comprised of multiple lineages scattered across the Dorylinae tree. We recover, with strong support, novel relationships among these Cerapachys-like clades and other doryline genera, but divergences in the deepest parts of the tree are not well resolved. We find the genus Sphinctomyrmex, characterized by distinctive abdominal constrictions, to consist of two separate lineages with convergent morphologies, one inhabiting the Old World and the other the New World tropics.

Conclusions: While we obtain good resolution in many parts of the Dorylinae phylogeny, relationships deep in the tree remain unresolved, with major lineages joining each other in various ways depending upon the analytical method employed, but always with short internodes. This may be indicative of rapid radiation in the early history of the Dorylinae, but additional molecular data and more complete species sampling are needed for confirmation. Our phylogeny now provides a basic framework for comparative biological analyses, but much additional study on the behavior and morphology of doryline species is needed, especially investigations directed at the non-army ant taxa.


Figure 1
Figure 1
Summary of phylogenetic results for the dorylines based on analyses of the 83 taxon data set. The majority rule consensus of all post burnin trees from a Bayesian analysis of the full data set under standard nucleotide coding. The colored boxes above branches denote support values. The top row of each box indicates Bayesian posterior support values, expressed as percentages; the bottom row indicates maximum likelihood (ML) bootstrap values. The columns, from left to right, indicate (i) standard nucleotide coding; (ii) RY-coding of all third codon positions; (iii) exclusion of all third codon positions; (iv) BEAST posterior values under standard coding (top cell), or ML bootstrap values under a codon model (bottom cell). For each cell, white = 0–49, grey = 50–74, blue = 75–94, red = 95–100 (see key).
Figure 2
Figure 2
Morphological diversity within Dorylinae. 1–8, Lateral views of select Cerapachys species representing variation in a generalized morphology that probably reflects the doryline ancestral condition; 9–10, Dorsal head views of New World (Eciton) and Old World (Dorylus) army ant species illustrating some morphological characteristics such as falcate mandibles that are linked to their highly specialized foraging behavior; 11–12, Lateral views of New World and Old World Sphinctomyrmex species showing convergent evolution of distinctive abdominal constrictions.
Figure 3
Figure 3
Chronogram of major lineages within Dorylinae. Fossil-calibrated chronogram of the Dorylinae inferred under an uncorrelated lognormal relaxed clock model. Branch lengths are proportional to time (in units of millions of years) and horizontal blue bars indicate the 95% highest posterior density of estimated node ages. Support values for this topology are summarized in Figure 1 and Additional file 1.

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