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 dierentiation showed no signi®cant divergence between S. fallax and S. isoviitae growing in sympatry; these taxa were therefore treated as conspeci®c. 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 dierentiated for RAPD markers, whereas S. angustifolium populations were only weakly dierentiated for any marker, even for populations from dierent continents. Populations of S. lindbergii were not dierentiated at all. Most studied populations did not ®t 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.Keywords: gene¯ow, genetic drift, isozymes, mutation, RAPD. IntroductionThe so-called`traditional view' of bryophyte population biology suggests that genetic variability is severely restricted in mosses and liverworts by the dominant haploid part of their life cycle, their widespread asexuality and the assumed predominant inbreeding in bisexual taxa (e.g. Anderson, 1963; Crum, 1972). Crum (1972) states that bryophytes are a genetically depleted group with limited evolutionary potential. This view is supported by palaeobotanical studies, which show that bryophytes evolved early and remained morphologically unchanged through geological time. Moreover, the existence of highly disjunct conspeci®c populations with little to no morphological divergence has been used to support the traditional view.In the late 1970s and early 1980s isozyme studies performed on bryophytes revealed unexpectedly high amounts of genetic variation (e.g. Cummins & Wyatt, 1981; Yamazaki, 1981). This led to a re-evaluation of bryophyte population biology and the evolutionary rate of these organisms. The morphological similarities through time and space are partly explained by the presence of physiological and biochemical, rather than morphological, evolution (creating sibling species) and convergent evolution (Wyatt, 1985). The so-called Conocephalum ± Plagiomnium' model (Wyatt, 1985;Wyatt et al., 1989) has been proposed to describe the various population structures in bryophytes. Here, two species are thoug...

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Secondary hybrid zones are not uncommon in Dactylorhiza, but knowledge of ecological and evolutionary consequences of hybridization are scarce. Here, we assess interploidal gene flow and introgression in a hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta (2n ¼ 2x ¼ 40) and its putative allotetraploid derivative D. lapponica (2n ¼ 4x ¼ 80). Photometric quantification of DNA content and morphology confirmed that triploids are abundant in sympatric populations in our study area. Allozyme segregation patterns in D. lapponica supported an allopolyploid origin, although unbalanced genotypes suggested rare pairings between homoeologous chromosomes. Photometric data and chromosome counts suggest backcrossing between the triploid hybrid and D. lapponica, and hence some hybrid fertility. Triploids are morphologically more similar to the tetraploids than the diploids, maybe owing to the hybrid origin of both triploids and tetraploids. The diploids and tetraploids were not more similar in the parapatric populations compared to when they occur in allopatry. This indicates that backcrossing rarely leads to introgression, or alternatively that allopatric populations are not isolated enough to prevent influx of pollen from the other species. Despite some evidence of backcrossing, our study gives few indications that widespread hybridization entails local breakdown of species boundaries. Rather, the hybrid zone may be a transient phenomenon due to intensive mowing, resulting in the opening of habitats and hence bringing the parental species into close contact. Heredity (2005) 94, 488-496.

Random amplified polymorphic DNA (RAPD) markers are sensitive to changes in reaction conditions and may express polymorphisms of nongenetic origin. Taxa with variable chromosome numbers are particularly challenging cases, as differences in DNA content may also influence marker reproducibility. We addressed these problems by comparing RAPD and amplified fragment length polymorphism (AFLP) analyses of clonal identity and relationships in a chromosomally variable arctic plant, the polyploid Saxifraga cernua, which has been thought to be monoclonal over large geographical distances. Fifty-seven plants from four Greenland populations were analysed using a conservative scoring approach. In total, 26 AFLP and 32 RAPD multilocus phenotypes (putative clones) were identified, of which 21 were identical and each of the remaining five AFLP clones was split into two to three very similar RAPD clones. This minor difference can be explained by sampling error and stochastic variation. The pattern observed in Greenland corroborates our previous results from Svalbard, suggesting that rare sexual events in S. cernua are sufficient to maintain high levels of clonal diversity even at small spatial scales. We conclude that although AFLP analysis is superior in terms of efficiency, RAPDs may still be used as reliable markers in small low-tech laboratories.

Genetic variation in the expanding moss species Pogonatum dentatum was studied using intersimple sequence repeat (ISSR) markers. The genetic consequences of range expansion were studied by comparing source populations in a mountain area with populations from a recently colonized lowland area in Sweden. Indices of genetic variation show slightly lower number of alleles per locus in the lowlands and a similar gene diversity in both areas. Three of four lowland populations had evidence of a recently passed bottleneck. Considerably higher haplotype diversity was found in the recently colonized lowlands compared to source populations in the mountains. Patterns of allelic diversity suggest that P. dentatum experiences loss of genetic variation through founder effects and genetic drift when expanding its distribution range. Higher haplotypic diversity, less linkage disequilibrium, and fewer compatible loci indicate that sexual recombination is relatively more important in the lowlands compared to the mountains. A likely explanation is higher success of establishment from spores in the lowlands, while clonal propagation predominates in the mountains. Less genetic differentiation among lowland populations indicates more gene flow in the lowland area, involving more spores and/or fragments moving among populations.