Primary hybrid zone formation in Tephroseris helenitis (Asteraceae), following postglacial range expansion along the central Northern Alps

Secondary vs. primary hybrid zone formation remains a challenging task as the time window in which these historical (vicariant) vs. contemporary (environmental-selective) processes are distinguishable may be relatively narrow.

To examine the origin and structure of a transition zone between two subspecies of Tephroseris helenitis along the central Northern Alps, molecular (AFLP) and morphological (achene type) data as well as ecological niche models (ENMs) were used. Samples were collected over a c. 350 km-long transect, largely covered by ice during the Last Glacial Maximum (LGM).

Genetically non-admixed individuals of subspp. helenitis vs. salisburgensis dominated the westernmost vs. eastern transect areas, with admixed individuals occurring in between. Cline for achene morphology was steep, largely non-coincidental, and displaced to the east of the cline centre for neutral AFLPs. During the LGM, ssp. helenitis should have been able to persist in a refugium southwest of the transect, while suitable habitat for ssp. salisburgensis was apparently absent at this time.

Patterns of genetic and clinal variation in combination with our ENM data are suggestive of a primary hybrid zone that originated after the species’ postglacial, eastward expansion.