New study rewrites evolutionary history of vespid wasps

Scientists at the University of Illinois have conducted a genetic analysis of vespid wasps that revises the vespid family tree and challenges long-held views about how the wasps' social behaviours evolved. In the study, published in the Proceedings of the National Academy of Sciences, the researchers found genetic evidence that eusociality (the reproductive specialisation seen in some insects and other animals) evolved independently in two groups of vespid wasps. These findings contradict an earlier model of vespid wasp evolution, which placed the groups together in a single lineage with a common ancestor.

Eusocial behaviour is quite rare, and generally involves the breeding of different reproductive classes within a colony. The sterile members of the group perform tasks that support their fertile counterparts. Eusociality occurs in only a few species of insects, rodents, crustaceans and other arthropods. The evolution of eusociality in wasps has long been a source of debate, said Heather Hines and entomology professor Sydney Cameron, who is the principal investigator of the study. A prior model of vespid wasp evolution placed three subfamilies of wasps - the Polistinae, Vespinae and Stenogastrinae - together in a single evolutionary group with a common ancestor. This model did not rely on a genetic analysis of the wasps, but instead classified them according to several physical and behavioural traits. Cameron's team included University of Missouri biology professor James H. Hunt, an expert on the evolution of social behaviour in the vespid wasps. Hunt observed that many behavioural characteristics of the vespid wasps contradicted this model of the vespid family tree. Hunt's observations, along with those of other behavioural experts in the field, prompted the new analysis. Instead of affirming a linear, step-wise evolution of social behaviour from solitary to highly social, Cameron said, her team's analysis shows that the Polistinae and Vespinae wasp subfamilies evolved their eusocial characteristics separately from the eusocial Stenogastrinae subfamily of vespid wasps.

Experts on vespid wasp behaviour have long noted the significant behavioural differences between the Stenogastrinae subfamily and the group that includes Polistinae and Vespinae. And others have tried, unsuccessfully, to challenge the earlier non-genetic model of vespid wasp evolution. In 1998, German researchers J. Schmitz and R. Moritz also used a genetic analysis to propose that the subfamily Stenogastrinae was evolutionarily distinct from the Polistinae and Vespinae subfamilies. Proponents of the non-genetic model criticised their work, however, because it relied on an analysis of less than 600 base pairs from two genes (one ribosomal RNA, the other mitochondrial DNA) and included very few representative species, some of which were unsuitable for the analysis.

The new study examined variations in fragments of four genes across 30 species of vespid wasps. Four independent statistical analyses tested the reliability of the pattern of relationships that emerged from the data. This work confirms the ideas of Schmitz and Moritz, said Cameron, by adding to the weight of evidence that their hypothesis was accurate. The fact that eusociality evolved independently in two groups of vespid wasps also sheds light on the complexity of evolutionary processes, Cameron said. Scientists attempt to make generalisations and simplify the world. But the world isn't always simple and evolution isn't simple. This finding points to the complexity of life.