Genetic structure in three haploid peat mosses (Sphagnum)

Over the past 20 years, studies have revealed levels of genetic variation in bryophytes that are similar to those found in vascular plants. This has led many to question the traditional view of bryophyte evolution, which holds that these organisms have a low evolutionary rate. RAPD and isozyme analyses were used to measure genetic variation in 18 populations of several Sphagnum taxa, with special emphasis on the bisexual S. lindbergii and the unisexual S. angustifolium, S. fallax and S. isoviitae. Both types of markers were found to be selectively neutral. A test of population differentiation showed no significant divergence between S. fallax and S. isoviitae growing in sympatry; these taxa were therefore treated as conspecific. Only S. angustifolium had polymorphic isozyme loci. The highest genetic variation in RAPD loci was found in S. angustifolium; the lowest in S. lindbergii. There seemed to be a high turnover rate of individuals in S. angustifolium populations. Populations of S. fallax coll. were strongly differentiated for RAPD markers, whereas S. angustifolium populations were only weakly differentiated for any marker, even for populations from different continents. Populations of S. lindbergii were not differentiated at all. Most studied populations did not fit the 'Conocephalum - Plagiomnium' model of bryophyte population structure. The observed patterns could best be explained by assuming a low evolutionary rate, at least in S. angustifolium, meaning that high levels of molecular variability seem not to be incompatible with slow evolution.