Sigurd M. Såstad

Colonization of old-forest lichens in a young and an old boreal Picea abies forest: an experimental approach

Abstract Understanding the factors limiting the distribution of a species is crucial for designing conservation strategies. We evaluated whether the scarcity of old-forest lichens in young forests was due to unfavourable environmental conditions for colonization in young stands or to dispersal limitations. Vegetative diaspores of Lobaria scrobiculata , Platismatia glauca and Platismatia norvegica were sown on 240 spruce twigs transplanted to a young and an old stand of spruce. Our results demonstrate that the old-forest species established and grew as rapidly in the young as in the old forest. Higher light levels in the young, compared with the old forest, did not reduce diaspore development. Moreover, greater numbers of juvenile thalli of Platismatia were found on control twigs (unsown) in the old forest, compared with the young forest, suggesting higher propagule density and more efficient dispersal in the old stand. Our results indicate that life-history characteristics, which include dispersal characters, are important for explaining the species scarcity in younger stands. Minimizing the distance between regeneration units and potential sources of propagules is probably important for maintaining lichen biodiversity in managed forest ecosystems.

Amplified fragment length polymorphism versus random amplified polymorphic DNA markers: clonal diversity in Saxifraga cernua.

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 and structure in the expanding moss Pogonatum dentatum (Polytrichaceae) in its area of origin and in a recently colonized area

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.

Origin, taxonomy and population structure of the allopolyploid peat mossSphagnum majus

The polyploid peat mossSphagnum majus shows considerable phenotypic plasticity along ecological gradients in mires. It is considered taxonomically heterogeneous, and two subspecies have been described. Isozyme analyses were carried out on populations ofS. majus from Central Norway and from eastern coast of North America in order to assess the origin, taxonomy and population structure of this species. High levels of fixed heterozygosity in the populations demonstrate thatS. majus is a genetic allopolyploid. At all ‘loci’ screened, extant populations ofS. cuspidatum shared enzyme bands withS. majus. The other most likely progenitor based on morphology,S. annulatum, was fixed for enzyme bands not found inS. majus. The progenitor genotype ofS. annulatum may have been missed because of inadequate sampling or extinction. Alternatively, another extinct or undetected taxon may constitute the second progenitor. The observed patterns of genetic variation and linkage disequilibria were uncorrelated with the previously proposed subspecific classification ofS. majus. Lack of genetic divergence between continents suggests that the origins ofS. majus in Europe and North America were not independent. Low mutation rates and large effective population sizes may be important causing populations to diverge slowly, and may explain the observed patterns without hypothesising frequent long-distance dispersal.

The Narrow Endemic Sphagnum troendelagicum is an Allopolyploid Derivative of the Widespread S. balticum and S. tenellum

Abstract Sphagnum troendelagicum is only known from five localities in central Norway. Three populations were analyzed to assess its genetic affinities with other Sphagnum species. Isozyme and cytological data indicate that the species is allopolyploid. Morphological, RAPD, and isozyme data further indicate that it has originated from hybridization between Sphagnum tenellum (sect. Mollusca) and S. balticum (sect. Cuspidata). Sphagnum troendelagicum is morphologically distinct, with unique combinations of character states found in sect. Cuspidata and sect. Mollusca. Sphagnum tenellum is genetically similar to S. balticum, although these species are morphologically divergent. Our data support the inclusion of S. tenellum within sect. Cuspidata. Sphagnum troendelagicum appears to have originated recurrently; thus its restricted distribution cannot be ascribed to a single origin. The progenitor species are widespread in the Northern Hemisphere and often grow sympatrically in boreal areas. Hypotheses that may account for the restricted distribution of S. troendelagicum, including ecological tolerance of the allopolyploid and opportunities for hybridization between the progenitors, are discussed. Communicating Editor: Kathleen A. Kron

Habitat-specific genetic effects on growth rate and morphology across pH and water-level gradients within a population of the moss Sphagnum angustifolium (Sphagnaceae).

To study genetic adaptations in bryophytes on small ecological and spatial scales and to assess the adaptive significance of morphological trait variation, genotypes of Sphagnum angustifolium originating from habitats characterized by different pH and height above water table were clonally propagated and grown along the same gradients that exist in the field. Clones from ombrotrophic habitats grew consistently better ombrotrophically than clones from minerotrophic habitats and vice versa, suggesting that the genotypes were adapted to different pH levels. Genetic variation was found in several morphological traits, but habitat-specific genetic effects were detected only in length of spreading branches. Covariation between morphology and growth was generally environmentally induced. Positive and negative cross-environment genetic correlations suggested the presence of constraints on adaptive reaction norm evolution. The indications of small-scale genetic adaptations suggest either selective establishment of genotypes adapted to specific habitats, strong selective forces operating at the later stages of the life cycle, restricted gene flow over short distances, or a combination of these. In contrast to prevailing views, these results indicate that bryophytes are likely to respond genetically to small-scale environmental gradients.

Genetic variability in bryophytes: does mating system really matter?

Abstract Mating patterns do not alter allelic frequencies in natural populations. Therefore, if evolution is defined as changes in allelic frequencies in a population through time, then mating system is not an evolutionary force. There is no direct causal link between mating patterns and the familiar measures of genetic variability applied to allele frequency data from haploid gametophyte populations. In this paper we focus on how mating patterns may affect intralocus variability when acting together with evolutionary forces, like selection and drift. Moreover, we address how levels of inbreeding can have a profound influence on haplotypic variation. Measures for estimating the extent of recombination from haplotypic variation are presented. It seems that high levels of selfing are not necessarily associated with bisexuality in bryophytes.

Interploidal gene flow and introgression in bryophytes: Sphagnum girgensohnii × S. russowii, a case of spontaneous neotriploidy

Abstract Hybrid zones between female sporophyte-bearing haploid Sphagnum girgensohnii and male allodiploid S. russowii were studied. The existence of hybrids was initially hypothesized based on incidence of conspicuously large sporophytes in S. girgensohnii in the presence of male S. russowii. Measurements of nuclear DNA content, morphology and isozymes provide the first evidence of spontaneous hybridization across ploidal levels in bryophytes. Viable spores from the hybrid capsules yielded triploid protonemata and juvenile gametophores in culture. Isozyme profiles of the triploid gametophores displayed unbalanced heterozygous patterns containing two alleles. As S. russowii is an allodiploid species with S. girgensohnii and S. rubellum as progenitors, isozyme banding patterns of the triploids are interpreted as consisting of a single allele copy from the S. rubellum genome in S. russowii, and two copies of another allele at the same locus inherited from S. girgensohnii. The triploid sporophytes can be considered as F2 hybrids formed by introgression. The allotriploid gametophores are formed through diplospory or syndiploidy in the hybrid capsules. Comparison of hybrid sporophyte morphology to S. girgensohnii and S. russowii confirmed that hybrid capsules are larger and with an abnormally swollen sporophyte foot. The spore sizes of the hybrid capsules were significantly larger than spores from normal, intraspecific S. girgensohnii capsules. The percentage of germinated spores was <5% in cultures from hybrid sporophytes, which is much lower than in intraspecific sporophytes. The new knowledge opens the way for re-evaluation of polyploid evolution in bryophytes with emphasis on gametic non-reduction, introgression and two-step models of polyploid formation.

Population Structure and Taxonomy of Sphagnum cuspidatum and S. viride

Abstract Based on morphology, Sphagnum cuspidatum and S. viride have been recognized as two different species. Morphs corresponding to the two taxa were investigated in nine populations situated in Norway, Denmark, Canada, and U.S.A. Eleven putative isozyme loci were used to map genetic variation and to assess the taxonomic relationships. Cluster analysis of allele frequency data did not produce groups corresponding either to the two species or to geographic regions. Low evolutionary rates because of low mutation rates and large effective population sizes may account for the lack of divergence between populations, even across continents. A test of population differentiation showed significant divergence between the two morphs in one population indicating that they might be reproductively isolated. However, in five other populations containing both morphs no such patterns were found suggesting that the speciation process is incomplete. A species level recognition of these taxa is not supported by isozyme data, rather taxonomic recognition at the subspecific level may be more appropriate. A population of S. trinitense expressed unique allele combinations, supporting the view that it is a distinct species different from S. cuspidatum.

Species Delimitation and Relationships of the Sphagnum recurvum Complex (Bryophyta)-as Revealed by Isozyme and RAPD Markers

Isozyme data from 26 populations of six species within the Sphagnum recurvum complex, as well as from the two outgroups S. balticum and S. lindbergii, were used to infer the relationships of these populations, and to assess to what degree these relationships are reflected in alternative classifications. A subset of eight populations, comprising four species, were analyzed for both isozyme and RAPD markers. Population aggregation analysis, cluster analysis of pairwise distances and parsimony analysis of allele fre- quency data grouped populations with brown spores in three distinct clusters corresponding to S. angusti- folium, S. flexuosum and S. recurvum s. str., whereas no clusters corresponding to any of the yellow-spored taxa (S. brevifolium, S. fallax and S. isoviitae) could be discerned. Genetic data corroborates morphological evidence suggesting that species level recognition is not warranted for S. fallax and S. isoviitae, whereas the status of S. brevifolium remains more uncertain. S. lindbergii deviated strongly from the remaining material in terms of both isozymes and RAPDs supporting its status as outgroup. S. balticum showed close genetic affinity to the yellow-spored taxa, suggesting that the S. recurvum complex is not monophyletic. Isozyme and RAPD markers yielded similar clustering patterns.