Contrasting patterns of evolution between allelic groups at a single locus in Arabidopsis.

Heterogeneities in evolutionary pattern among different loci are commonly observed. To see whether the heterogeneity can also be observed among allelic groups in a single locus, we investigated the coding sequence and the flanking regions of Rpp13, a disease resistance gene in up to 60 accession lines from worldwide populations in Arabidopsis thaliana. An extraordinarily high level of polymorphism (pi=0.098) and four distinct clades were found in the leucine-rich repeat (LRR) region in this gene. No obvious geographic relationship with the clades was observed, and such clades were not observed in the other regions in and around this gene. The average genetic diversity among the clades ranged from 10 to 14.6% in the LRR. The levels of polymorphism within each clade varied largely, and significant heterogeneity in evolutionary rates among clades was detected. A statistically significant departure from neutrality was also detected by Fu & Li's tests. These results suggest that both directional and diversifying selection are working on this locus, and that natural selection can cause heterogeneity in evolutionary rate, even among allele groups in a locus.