Multilocus nested haplotype networks extended with DNA fingerprints show common origin and fine-scale, ongoing genetic divergence in a wild microbial metapopulation.

Nested haplotype networks for three loci in a haploid, fungal plant pathogen, Sclerotinia sclerotiorum, in two natural, Norwegian populations of the woodland buttercup, Ranunculus ficaria, were extended with DNA fingerprints to determine fine-scale population divergence. To preserve the cladistic structure in the network for both nonrecombinant and postrecombinant haplotypes in highly recombinant clades, recombinant events were not removed ('peeled off'), but instead were examined in alternative (marginal) networks. Fungi from both sampling locations share a common origin with subsequent genetic divergence, consistent with expectations for metapopulation structure. Evidence for divergence includes (i) lack of shared fingerprints between the two locations, (ii) evolution of new fingerprints, via transposition and recombination, within 2 years on a fine spatial scale within one sampling location, and (iii) increase in the size of the intergenic spacer (IGS) in both sampling locations. Sites of microsatellite repeat expansion and of an insertion were consistent with the boundaries of two recombination blocks in the IGS. Both alternative networks based on the recombination blocks were essential to finding all associations of DNA fingerprints with IGS size, sampling site, sampling year and mycelial compatibility group. Variation in the elongation factor 1alpha and calmodulin loci supported the topologies and the recurrent, ongoing polarity of change in fingerprints and IGS size inferred from the IGS.