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, 105 (39), 14913-7

Newly Discovered Sister Lineage Sheds Light on Early Ant Evolution

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Newly Discovered Sister Lineage Sheds Light on Early Ant Evolution

Christian Rabeling et al. Proc Natl Acad Sci U S A.

Abstract

Ants are the world's most conspicuous and important eusocial insects and their diversity, abundance, and extreme behavioral specializations make them a model system for several disciplines within the biological sciences. Here, we report the discovery of a new ant that appears to represent the sister lineage to all extant ants (Hymenoptera: Formicidae). The phylogenetic position of this cryptic predator from the soils of the Amazon rainforest was inferred from several nuclear genes, sequenced from a single leg. Martialis heureka (gen. et sp. nov.) also constitutes the sole representative of a new, morphologically distinct subfamily of ants, the Martialinae (subfam. nov.). Our analyses have reduced the likelihood of long-branch attraction artifacts that have troubled previous phylogenetic studies of early-diverging ants and therefore solidify the emerging view that the most basal extant ant lineages are cryptic, hypogaeic foragers. On the basis of morphological and phylogenetic evidence we suggest that these specialized subterranean predators are the sole surviving representatives of a highly divergent lineage that arose near the dawn of ant diversification and have persisted in ecologically stable environments like tropical soils over great spans of time.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Holotype worker of Martialis heureka gen. et sp. nov. The single specimen has been collected in the leaf litter of a terra firme rainforest near Manaus, Amazonas, Brazil. Martialis heureka is inferred to be the sister lineage to all extant ants. (A) Lateral and (B) dorsal view of the worker. (Scale bar: 1 mm.) Photographs courtesy of C. Rabeling and M. Verhaagh.
Fig. 2.
Fig. 2.
Worker of Martialis heureka foraging in a subterranean tunnel. This drawing depicts M. heureka in its assumed natural habitat as inferred from its external morphology. Please see the Inferred Biology section for a more detailed discussion of the species' biology. (Scale bar: 1 mm.) Color pencil drawing by Barrett A. Klein.
Fig. 3.
Fig. 3.
Maximum likelihood (ML) tree with Bayesian posterior probabilities (BPP) and ML bootstrap proportions (ML BP) support values. Tree is rooted on the branch leading to the outgroup, Pristocera. The formicoid clade has been collapsed to increase resolution of relationships among basal ant groups (Martialis, Leptanillinae, and poneroids). Bipartitions with strong Bayesian support are indicated by blue triangles (BPP = 1.0), green circles (0.95 < BPP < 1.0) or orange rectangles (0.9 < BPP < 0.95).
Fig. 4.
Fig. 4.
Alternative rootings evaluated with Bayes factors (BF) and the Shimodaira–Hasegawa (SH) test. Constraint trees used to evaluate alternative rootings are depicted on the left. Rootings 1–3 correspond to the first rooting considered by Brady et al. (20), but with alternative arrangements of Martialis and Leptanillinae. Rootings 4–7 correspond to Brady et al.'s rootings 2–5. For each alternative rooting, the corresponding value of 2ln(BF) is given for the comparison of that rooting against the maximum posterior probability (MPP) rooting 1. Values lower than −10 can be interpreted as strong evidence against the alternative. P values resulting from the comparison of alternative rootings to the maximum likelihood (ML) rooting 1 using the SH test are also given. Rootings significantly worse than the ML rooting would have P values < 0.05.

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