Monday, March 05, 2007

 

Evolutionary history of vespid wasps rewritten by New study

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 behaviors evolved. In the study [1], published in the February 21, 2007 Proceedings of the National Academy of Sciences journal, the researchers found genetic evidence that eusociality (the reproductive specialization 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 behavior 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 U. of I. entomology graduate student 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 behavioral traits.

Cameron's team included University of Missouri biology professor James H. Hunt, an expert on the evolution of social behavior in the vespid wasps. Hunt observed that many behavioral characteristics of the vespid wasps contradicted this model of the vespid family tree.

Hunt's observations, along with those of other behavioral experts in the field, prompted the new analysis.

Instead of affirming a linear, step-wise evolution of social behavior 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 behavior have long noted the significant behavioral 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 criticized 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 generalizations and simplify the world. But the world isn't always simple and evolution isn't simple. This finding points to the complexity of life."

Source (adapted): University of Illinois at Urbana-Champaign PR "New study rewrites evolutionary history of vespid wasps" March 1 2007

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[1] Based on the PNAS paper:

Multigene phylogeny reveals eusociality evolved twice in vespid wasps

Heather M. Hines, James H. Hunt, Timothy K. O'Connor, Joseph J. Gillespie, and Sydney A. Cameron

Published online before print February 21, 2007, 10.1073/pnas.0610140104
PNAS | February 27, 2007 | vol. 104 | no. 9 | 3295-3299

Eusocial wasps of the family Vespidae are thought to have derived their social behavior from a common ancestor that had a rudimentary caste-containing social system. In support of this behavioral scenario, the leading phylogenetic hypothesis of Vespidae places the eusocial wasps (subfamilies Stenogastrinae, Polistinae, and Vespinae) as a derived monophyletic clade, thus implying a single origin of eusocial behavior. This perspective has shaped the investigation and interpretation of vespid social evolution for more than two decades. Here we report a phylogeny of Vespidae based on data from four nuclear gene fragments (18S and 28S ribosomal DNA, abdominal-A and RNA polymerase II) and representatives from all six extant subfamilies. In contrast to the current phylogenetic perspective, our results indicate two independent origins of vespid eusociality, once in the clade Polistinae+Vespinae and once in the Stenogastrinae. The stenogastrines appear as an early diverging clade distantly related to the vespines and polistines and thus evolved their distinctive form of social behavior from a different ancestor than that of Polistinae+Vespinae. These results support earlier views based on life history and behavior and have important implications for interpreting transitional stages in vespid social evolution.

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Of related interest:

Eusociality: Origin and consequences

Edward O. Wilson, and Bert Holldobler

Published online before print September 12, 2005, 10.1073/pnas.0505858102
PNAS | September 20, 2005 | vol. 102 | no. 38 | 13367-13371

In this new assessment of the empirical evidence, an alternative to the standard model is proposed: group selection is the strong binding force in eusocial evolution; individual selection, the strong dissolutive force; and kin selection (narrowly defined), either a weak binding or weak dissolutive force, according to circumstance. Close kinship may be more a consequence of eusociality than a factor promoting its origin. A point of no return to the solitary state exists, as a rule when workers become anatomically differentiated. Eusociality has been rare in evolution, evidently due to the scarcity of environmental pressures adequate to tip the balance among countervailing forces in favor of group selection. Eusociality in ants and termites in the irreversible stage is the key to their ecological dominance and has (at least in ants) shaped some features of internal phylogeny. Their colonies are consistently superior to solitary and preeusocial competitors, due to the altruistic behavior among nestmates and their ability to organize coordinated action by pheromonal communication.

In eusociality, an evolutionarily advanced level of colonial existence, adult colonial members belong to two or more overlapping generations, care cooperatively for the young, and are divided into reproductive and nonreproductive (or at least less-reproductive) castes. The phenomenon is well marked and nearly confined to insects, especially ants, bees, wasps, and termites, where it has been subject to a large body of mostly specialized research scattered across disciplines from genetics to paleontology. It has long been the conviction of researchers on social insects that common principles exist at the organismic and superorganismic levels, thus between individual insects and the tightly integrated colonies they compose. Parallels have been persuasively drawn between the self-construction of organisms from molecules and tissues and that of superorganisms from interacting entire organisms. The principles can be further parsed into two segments of the time scale: the developmental decision rules that assemble organisms and colonies in each generation and the origin of these rules through evolutionary time.

Focusing here on the second principle, evolutionary process, we suggest how three seemingly disparate evolutionary phenomena can be causally linked: the selection forces that generate and shape eusociality, the rareness of the origin of eusociality, and the ecological hegemony of eusocial insects.

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