Genetic structure in three haploid peat mosses (Sphagnum)

Abstract

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...

The narrow endemic Norwegian peat moss Sphagnum troendelagicum originated before the last glacial maximum

“…However, a scenario involving multiple origins, perhaps one origin at each local site, should yield high genetic differentiation among populations if there is limited gene flow. A peat moss without sexual reproduction and spore production should have an extremely Natural history of Sphagnum troendelagicum HK Stenien et al limited gene flow, even though limited dispersal by vegetative fragments could in theory be possible over short distances (Stenien and Såstad, 1999). The results reported by Stenien and Flatberg (2000) could therefore perhaps equally likely fit a scenario of a single origin of S. troendelagicum, given sufficient time since origin for the accumulation of genetic variation by mutations, some sexual reproduction and some gene flow between populations.…”

“…On the basis of the combined ABC and IM results, although, it seems unlikely that S. troendelagicum is younger than the last glacial maximum (c. 11 600 years) or older than 80 000 years. Stenien and Flatberg (2000) reported relatively high genetic variability within S. troendelagicum compared with other studied peat mosses (for example, gene diversity comparable with levels found in the common circumboreal S. fallax; Stenien and Såstad, 1999). They also found very low levels of LD (average P d equal to 4.8%) and low differentiation between S. troendelagicum populations (average F ST equal to 0.07).…”

“…Loci with missing alleles were excluded from LD analyses to obtain maximally conservative estimates. The percentage of linked loci in relation to the maximum number of linked loci (P d ) was estimated for each population as described in Stenien and Såstad (1999). We estimated fixation indices, F ST , among S. troendelagicum populations, and the corrected numbers of pairwise differences between S. troendelagicum populations, and between S. troendelagicum and parental species populations (Weir and Cockerham, 1984;Michalakis and Excoffier, 1996).…”

Adaptive basis of codon usage in the haploid moss Physcomitrella patens

“…Still, weak selection for codon usage seem to operate at the genomic level, implying either that the overall effective population size is larger than expected in this species or that genetic drift do not significantly influence levels of weak molecular selection on codon usage, as it seems to be the case in A. thaliana. If this latter is the case, then this seeming paradox is analogous to the problem of explaining the high within population genetic variability in molecular markers (eg Wyatt et al, 1989;Stenøien and Såstad, 1999;Stenøien and Flatberg, 2000;Wilson and Provan, 2003), as well as quantitative traits (Stenøien et al, 1997) found in many primitive plants that are expected to experience substantial genetic drift.…”

Glacial refugia and post-glacial colonization patterns in European bryophytes

“…Homalothecium sericeum (Hedderson and Nowell 2006) Admixture in contact zone leading to unique haplotypes (Hassel et al 2005) Root of gene trees or haplotype network close to haplotypes found in an area e.g. Grimmia montana (Vanderpoorten et al 2008) Several refugial areas (Laenen et al 2011) Low mutation rate and/or large effective population size causing incomplete lineage sorting (Stenøien and Såstad 1999) probable origin before the LGM (Stenøien et al 2011b). This could indicate glacial survival in Scandinavia, but it could also mean that the species originated outside the ice sheet and colonized Norway after the ice retreated (Stenøien et al 2011b).…”