ON HYBRID FORMATION IN THE ROCK FERN ASPLENIUM X ALTERNIFOLIUM (ASPLENIACEAE, PTERIDOPHYTA)

Length polymorphism in a non-coding spacer (trnL(UAA)-trnF(GAA)) in th e chloroplast DNA was used in the investigation of the origin of the m ost common and conspicuous European fern hybrid, Asplenium x alternifo lium (Aspleniaceae, Pteridophyta). The origins of A. x alternifolium, the hybrid between A. trichomanes s.l. and A. septentrionale s.l. was studied at three ploidy levels, diploid, triploid and tetraploid. The cpDNA technique allowed us to investigate the mode of hybrid formation between sexual species for the first time over a wide geographic rang e and with a large sample rite. Morphological variation in this hybrid has previously been attributed to different reciprocal parental combi nations, and to the influence of chloroplast genes on morphogenesis. O ur results demonstrate that one parent, A. septentrionale s.l., acts p redominantly as the female parent in these hybrids, with only one popu lation of A. x alternifolium showing reciprocal hybridity. The discove ry of predominantly unidirectional hybrid formation in this hybrid may be explained by the different breeding systems of the parental taxa. The role of gametophyte ecology is also assessed.