Climate, colonisation and celibacy: Population structure in central European Trichomanes speciosum (Pteridophyta)

The Killarney fern Trichomanes speciosum Willd. (Hymenophyllaceae) is uniqu e in possessing both extensive sexual (sporophyte and gametophyte generatio ns present) and asexual (gametophyte only) ranges. It was first discovered in central Europe in 1993 and is represented in this area only by its peren nial, vegetatively propagating gametophyte generation. Genetic variation ha s been investigated at 35 sites. Allozyme diversity is partitioned primaril y between, not within, sites. Although genetic variation exists at a fine s cale (<5 m) within some populations, the results suggest that clones were n ot intimately associated in these cases. The majority of sites support uniq ue multilocus phenotypes. Where phenotypes were present at more than one si te they tended to recur at the next closest site. However, similar phenotyp es link eastern and western Pfalzerwald sites up to c. 70 km apart. This pa ttern of diversity suggests that colonisation was not solely of a "stepping stone" or "leading edge" type. We suggest that during a climatically favou rable period, probably the Atlantic hypsithermal, there may have been an ex plosive colonisation by long-distance dispersal from refugial areas. This w as followed by a short period during which sporophyte production, sexual re production and local spread were possible. With climatic change, reduction in the available habitat and the loss of the sporophyte generation, differe nt individual genets became fixed within small, favourable, but scattered, sites. The possibility that some central European sites north of the Alps a cted as periglacial refugia cannot be discounted, but would appear less lik ely than (re-)colonisation from the Atlantic fringe.