Genetic diversity of the epiphytic bryophyte Leucodon sciuroides in formerly glaciated versus nonglaciated parts of Europe.

Twelve populations of the epiphytic bryophyte Leucodon sciuroides from three major regions representing formerly glaciated and nonglaciated regions of Europe were screened for polymorphisms at 15 putative isozyme loci. The populations clustered into three distinct groups consisting of: (i) a single population from Crete, representing a cryptic unknown taxon; (ii) four Scandinavian populations and two populations from northern Greece; and (iii) the remaining populations from mainland Greece and Crete. The Scandinavian populations were genetically depleted compared with most Greek populations, thus fitting the expectation of generally lower levels of variation in formerly glaciated areas. The transition zone between genetically diverse and depleted populations appears to be located through northern Greece, coinciding with the northern limit of the Mediterranean region. This indicates that genetic variation was lost in populations at the northern limit of glacial refugia. The two groups of populations fit a progenitor-derivative model. They also have contrasting reproductive strategies: the Mediterranean populations reproduce sexually, whereas the other populations propagate vegetatively. Epiphytic species, growing on substrates that are limited in space and time, appear to be especially vulnerable to loss of genetic variation. Lack of genetic variation and therefore low adaptability to increased levels of atmospheric pollution may explain why many epiphytic lichen and bryophytes, including L. sciuroides, are declining over much of Europe.