Population structure and conservation biology of the endangered fern Trichomanes speciosum Willd. (Hymenophyllaceae) at its northern distributional limit

Genetic diversity in the Killarney fern, Trichomanes speciosum Willd. has b een investigated in south-western Scotland, the northern-most limit of the distribution of the sporophyte. T. speciosum is unique amongst European pte ridophytes in that both phases of the life cycle are perennial and capable of vegetative propagation. Within sites no variation was revealed by allozy me electrophoresis, even where both generations were growing together. In c ontrast, diversity was observed among sites, with seven different multilocu s phenotypes (MLPs) present in the aroa. Two of these MLPs covered large ar eas while the others were restricted to one, or few localities. Asexual rep roduction of the gametophyte via gemmae is assumed to be the main means of dispersal in recent times, allowing single clones to become widespread, whi le the overall genetic variability may be attributed to sexual reproduction and spore dispersal in historic times under more favourable climatic condi tions. We suggest that it is not inbreeding, nor lack of genetic variation that limits sporophyte production, but rather the prevailing climatic condi tions. The sporophyte is extremely rare and vulnerable. However, when the g ametophyte is considered, the species is neither threatened with extinction ; nor does it appear to face the danger of marked genetic erosion, because the long-lived gametophyte stage contains all of the genetic variability pr esent in the area and can be regarded as a valuable 'seed-bank'. (C) 1999 T he Linnean Society of London.