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MBE Advance Access originally published online on April 22, 2009
Molecular Biology and Evolution 2009 26(8):1791-1801; doi:10.1093/molbev/msp086
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© The Author 2009. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Research Articles

Rapid Evolution of Immune Proteins in Social Insects

Lumi Viljakainen*,{dagger}, Jay D. Evans{ddagger}, Martin Hasselmann§, Olav Rueppell||, Salim Tingek and Pekka Pamilo#

* Department of Biology
{dagger} Biocenter Oulu, University of Oulu, Finland
{ddagger} US Department of Agriculture— Agricultural Research Service Bee Research Lab, Beltsville, MD
§ Department of Genetics, Heinrich Heine University, Duesseldorf, Germany
|| Department of Biology, University of North Carolina
Agricultural Research Station, Tenom, Borneo, Malaysia
# Department of Biological and Environmental Sciences, University of Helsinki, Finland

E-mail: lumi.viljakainen{at}oulu.fi.

Accepted for publication April 18, 2009.

The existence of behavioral traits connected to defense against pathogens manifests the importance of pathogens in the evolution of social insects. However, very little is known about how pathogen pressure has affected the molecular evolution of genes involved in their innate immune system. We have studied the sequence evolution of several immune genes in ants and honeybees. The results show high rates of evolution in both ants and honeybees as measured by the ratio of amino acid changes to silent nucleotide changes, the ratio being clearly higher than in Drosophila immune genes or in nonimmunity genes of bees. This conforms to our expectations based on high pathogen pressure in social insects. The codon-based likelihood method found clear evidence of positive selection only in one ant gene, even though positive selection has earlier been found in both ant and termite immune genes. There is now indication that selection on the amino acid composition of the immune-related genes has been an important part in the fight against pathogens by social insects. However, we cannot distinguish in all the cases whether the high observed dN/dS ratio results from positive selection within a restricted part of the studied genes or from relaxation of purifying selection associated with effective measures of behaviorally based colony-level defenses.

Key Words: ants • honeybees • molecular evolution • positive selection • immune system

Adriana Briscoe, Associate Editor


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