MBE Advance Access published online on May 25, 2005
Molecular Biology and Evolution, doi:10.1093/molbev/msi171
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1 Umeå Plant Science Centre, Department of Ecology and Environmental Science, University of Umeå, SE-891 87 Umeå, Sweden
* To whom correspondence should be addressed. Plants defend themselves against the attack of natural enemies by using an array of both constitutively expressed and induced defenses. Long-lived woody perennials are over-represented among plant species that show strong induced defense responses, whereas annual plants and crop species are under-represented. However, most studies of plant defense genes have been performed on annual or short-lived perennial weeds or crop species. Here I use molecular population genetic methods to survey six wound inducible protease inhibitors (PI) in a long-lived woody, perennial plant species, the European aspen (Populus tremula), to evaluate the likelihood of either recurrent selective sweeps or balancing selection maintaining amino acid polymorphisms in these genes. Results show that none of the six PI genes have reduced diversities at synonymous sites, as would be expected in the presence of recurrent selective sweeps. Several genes show some evidence of non-neutral evolution, however, such as enhanced linkage disequilibrium and a large number of high frequency derived mutations. A group of at least four Kunitz trypsin inhibitor genes appear to have experienced elevated levels of non-synonymous substitutions, indicating allelic turnover on an evolutionary timescale. One gene, TI1, has enhanced levels of intraspecific polymorphism at non-synonymous sites and also has an unusual haplotype structure characterized by two divergent haplotypes occurring at roughly equal frequencies in the sample. One haplotype has very low levels of intra-allelic nucleotide diversity, whereas the other haplotype has levels of diversity comparable to other genes in P. tremula. Patterns of sequence diversity at TI1 do not fit a simple model of either balancing selection or recurrent selective sweeps. This suggests that selection at TI1 is more complex, possibly involving allelic cycling.
Accepted May 17, 2005
Research Article
Molecular Population Genetics of Herbivore-Induced Protease Inhibitor Genes in European Aspen (Populus tremula L., Salicaceae)
Pär K. Ingvarsson, E-mail: pelle{at}eg.umu.se
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