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Phylogenetic Tree Resolves Ant Placement Among Stinging Insects, Gives Insight into Evolution of Eusociality

NEW YORK (GenomeWeb News) – Social insects of the order Hymenoptera all belong to two groups that are marked by transporting food and building nests, according to a new phylogenetic study of ants, bees, and stinging wasps.

As they reported in the online early edition of Current Biology today, researchers from the University of California, Davis, constructed a phylogenetic tree of stinging insects based on new sequencing data from 11 key species, as well as previously published insect genomes. How the various stinging bugs are related to one another is a point of contention, the researchers noted.

"Multiple lines of evidence provide strong support for the monophyly of the Aculeata [stinging insects], but relationships among the major lineages within this group have been a matter of continued uncertainty," Philip Ward, a professor of entomology at UC-Davis, and his colleagues wrote. "The position of ants, the most species-rich and ecologically dominant of all eusocial insects, has been particularly problematic."

Social behavior has developed in a few groups of animals, and the clade aculeate Hymenoptera contains both insects that have developed eusocial behavior and those that have not. Having a resolved phylogenic tree, the researchers said, could help in understanding how traits like eusociality have evolved.

To draw their tree, the researchers first collected samples of 10 species from the field and one rare species from a preserved specimen for sequencing on the Illumina HiSeq 2000. They assembled those data into transcriptomes using the Trinity software package, yielding assembly sizes ranging between 46.7 million base pairs for the sweat bee and 219 million base pairs for the scoliid wasp.

They further drew upon published transcriptome data from one bee species and genome assemblies from three other bees, as well as three species of ant.

Different approaches — including maximum likelihood, Bayesian analyses, and species tree analyses — yielded similar phylogenetic tree topology and branch lengths, the researchers pointed out, noting that most nodes had robust support.

Overall, the phylogenetic tree constructed by Ward and his colleagues suggested some changes to how stinging insects are currently grouped. For example, they found that Apoidea, which includes spheciform wasps and bees, and ants are sister groups, despite earlier placements that had ants being more closely related to ectoparasitic wasps like scoliid wasps.

This placement "emphasizes a greater affinity of ants to the predatory wasps that characterize the earliest branching lineages of Apoidea than to scoliids, bradynobaenids, tiphiids, and other ectoparasitoid wasps with which they have been associated previously," the study authors said.

Eusociality, the researchers posited, arose once in common ancestor of those sister groups. Entomologists have long argued that building nests and amassing provisions like prey and pollen are a prerequisite for social traits, and the new positioning of ants near apoids, which also share those behavioral traits, indicated that those traits may have only developed once in that line and "that the preconditions for eusociality are rare and contingent," the researchers added.

The new phylogenetic tree also suggested a certain chain of behavior changes in stinging insects. Ward and his colleagues reported that, based on their results, the ancestral stinging wasp was an ectoparasite that attacked and paralyzed its host as food for its offspring. Two lineages — the ant and Apoidea line and the Vespidae line — then modified this behavior to become more predatory while also building nests and developing a greater emphasis on parental care. Within these two lineages, eusociality behavior then developed, while other remaining three stinging insect clades mainly kept to the ancestral ectoparasitoid behaviors.

"The new tree provides a robust framework for investigating the evolution of nesting, feeding, and social behavior within the stinging Hymenoptera, and for exploring genomic signatures of changes in these characteristics," the researchers added.

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