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MBE Advance Access published online on August 18, 2004
Molecular Biology and Evolution, doi:10.1093/molbev/msh236
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2004; all rights reserved
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Accepted August 12, 2004

Original Article

Duplication and Diversifying Selection among Termite Antifungal Peptides

Mark S. Bulmer 1* Ross H. Crozier 1

1 School of Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia

* To whom correspondence should be addressed. E-mail: mark.bulmer{at}jcu.edu.au.


  Abstract

We have identified and analysed the mRNA sequence of 20 new defensin-like peptides from eleven Australian termite species of Nasutitermes and from an outgroup, Drepanotermes rubriceps. The sequence was amplified by reverse transcriptase PCR with a degenerate primer designed from termicin, an antifungal peptide previously characterized from the termite Pseudocanthotermes spiniger. All 20 genes show high sequence identity with P. spiniger termicin and have duplicated repeatedly during the radiation of Nasutitermes. Comparison of the relative fixation rates of synonymous (silent) and nonsynonymous (amino acid altering) mutations indicates that the Nasutitermes termicins are positively selected. This positive selection appears to drive a decrease in termicin charge. In termites with two genes, the decrease in charge is predominantly restricted to one termicin. Furthermore, the spread of charge is significantly greater within species than across species among amino acid sites that appear to be under strong positive selection and this spread is attributable to only three sites. Our results suggest that following termicin duplication, certain critical sites have maintained a positive charge in one duplicate and evolved towards neutrality in the other and that positive selection has directed these changes repeatedly and independently. This diversification among duplicated genes may be a counter-response to the evolution of fungal resistance in social insects that are particularly vulnerable to fungal epidemics.

Keywords: Immune genes; termites; diversifying selection; gene duplication; termicins; defensins; protein evolution.
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