Genetic and morphological differentiation across a contact zone between two postglacial immigration lineages of Helianthemum nummularium (Cistaceae) in southern Scandinavia

Helianthemum nummularium (L.) Mill. has two subspecies in southern Scandinavia, which differ in an indumentum character that is inherited as a Mendelian gene with two alleles. The dominant allele is the genetic basis for the key character of subsp. obscurum (absence of a dense felt of stellate hairs on the abaxial surface of the leaf), whereas the recessive allele is the genetic basis for the key character of subsp. nummularium (presence of a dense felt of stellate hairs). Subsp. obscurum occurs in southwest Scandinavia, whereas subsp. nummularium occurs in southeast Scandinavia. The distributions of the two subspecies overlap in a wide zone in south Sweden, where natural populations consist of a mixture of the two subspecies in various proportions. Eighty-five percent of the investigated populations in this zone are polymorphic. A significant positive correlation exists between latitude/longitude across the zone and the frequency of the recessive indumentum allele. There is no significant difference in the leaf shape between the two subspecies, but there is a geographical trend across the zone. Twenty-seven populations displayed six plastid DNA haplotypes in Scandinavia. The geographic distribution of four of these haplotypes supports an eastern postglacial immigration lineage, whereas the distribution of two others haplotypes supports a postglacial immigration lineage from southwest.