Strong differences in genetic structure across disjunct, edge, and core populations of the distylous forest herb Pulmonaria officinalis (Boraginaceae).

Premise Of The Study: Populations at the edge of a species' distribution area are often small and have low levels of gene flow resulting in lower genetic variation and higher differentiation compared to core populations. This study examined genetic variation among populations of the distylous temperate forest herb Pulmonaria officinalis located in the core, the edge, and outside the species' main distribution range.

Methods: We compared patterns of genetic variation for eight microsatellite loci between disjunct (Belgium), edge (western Germany), and core (eastern Germany) populations of P. officinalis.

Key Results: Disjunct populations contained only a subset of alleles found in edge and core populations and had significantly lower within-population genetic variation. No significant differences, however, in within-population genetic variation were found between edge and core populations, except for allelic and genotypic richness. Genetic differentiation was highest among disjunct (F(ST) = 0.11) and lowest among core populations (F(ST) = 0.03). Significant (P < 0.01) isolation by distance was found for disjunct and edge populations (r(M) = 0.29 and 0.50, respectively), but not for core populations (r(M) = 0.18).

Conclusions: The results are best interpreted from a "dynamic range" point of view in which the observed low levels of genetic diversity and high genetic differentiation in disjunct populations are best explained through historical processes, most likely the introduction of the species in medieval times. Lower levels of gene flow caused by the pronounced fragmentation of forests in Belgium may further have contributed to the genetic structure of P. officinalis in these disjunct populations.