The PISTILLATA (PI) homologue, BpMADS2, was isolated from silver birch (Betula pendula Roth) and used to study nucleotide polymorphism. Two regions (together about 2450 bp) comprising mainly untranslated sequences were sequenced from 10 individuals from each of two populations in Finland. The nucleotide polymorphism was low in the BpMADS2 locus, especially in the coding region. The synonymous site overall nucleotide diversity (πs) was 0.0043 and the nonsynonymous nucleotide diversity (πa) was only 0.000052. For the whole region, the π values for the two populations were 0.0039 and 0.0045, and for the coding regions, the π values were only 0 and 0.00066 (for the corresponding coding regions of Arabidopsis thaliana PI world-wide π was 0.0021). Estimates of π or θ did not differ significantly between the two populations, and the two populations were not diverged from each other. Two classes of BpMADS2 alleles were present in both populations, suggesting that this gene exhibits allelic dimorphism. In addition to the nucleotide site variation, two microsatellites were also associated within the haplotypes. This allelic dimorphism might be the result of postglacial re-colonization partly from northwestern, partly from southeastern/eastern refugia. The sequence comparison detected five recombination events in the regions studied. The large number of microsatellites in all of the three introns studied suggests that BpMADS2 is a hotspot for microsatellite formation.
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