Nils Cronberg

GENOTYPIC DIVERSITY, MOLECULAR MARKERS AND SPATIAL DISTRIBUTION OF GENETS IN CLONAL PLANTS, A LITERATURE SURVEY

We present a literature survey of studies using molecular markers to investigate genet diversity and structure in clonal plants. The data from 40 studies comprised 45 species of which only two were studied by DNA methods, the rest by isozyme markers. Less than one third of the studies provided information about the spatial distribution of individual genets within populations, and only 12.5% of the studies used mapping of all ramets within plots or part of the population in combination with identification of multilocus genotypes. We also present two case studies. InGlechoma hederacea morphological criteria were used to select clones. Multi-samples of ramets within these “clones” turned out to be variable using isozymes indicating that these “clones” in most cases consisted of several genets. One individual multilocus genotype covered tens of square metres. InHylocomium splendens samples from 10×10 cm plots usually consisted of a mixture of multilocus genotypes, but occasionally the whole plot consisted of one genet.

Clonal diversity and allozyme variation in populations of the arctic sedge Carex bigelowii (Cyperaceae).

1 A study of allozyme variation in vegetatively propagating populations of the rhizomatous sedge Carex bigelowii, revealed high levels of clonal diversity (genet diversity) within populations. The structure of allelic variation within the populations suggests that sexual reproduction has played a significant role in these C. bigelowii populations, despite present lack of seedling recruitment. The study was carried out in moss-heath communities on Icelandic lava-fields. Two adjacent populations were studied at one site, while a third population was studied at a second site, 35 km away from the first two populations. 2 The number of genets detected in each population, among 85-88 analysed ramets, ranged from 41 to 55 (minimum estimate). Samples were taken every 4 m along transects in the populations. Ramets with the same allozyme genotype were often spatially aggregated. No seedlings have been observed in the populations during five years of demographic studies. 3 All the populations studied showed a diploid expression of allozymes and high levels of allelic variation, with on average 491.77 alleles per locus (A) and an allelic diversity (HS) of 0.167. Similar levels of within-population variability are found in many wind-pollinated and outcrossing plant species. 4 The difference between observed and expected heterozygosity was small in all populations, suggesting high levels of outbreeding. 5 Comparisons with other Carex taxa show that the levels of and structuring of allozyme diversity in C. bigelowii is similar to that in other outbreeding species (usually rhizomatous), and much higher than in inbreeding species (which are usually caespitose). 6 Only 5the total allelic diversity was explained by differences between the two study sites (G ST = 0.055), suggesting extensive recent or historic gene-flow. (Less)

Genetic diversity in populations of Sphagnum capillifolium from the mountains of Bulgaria, and their possible refugial role

Abstract Genetic diversity in eight populations of Sphagnum capillifolium from different Bulgarian mountains was investigated by means of isozyme electrophoresis. High levels of allelic diversity were found (HS = 0.119), comparable to earlier estimates for northern European populations (HS = 0.116). Strong differentiation among populations and a low number of widespread genotypes suggest a high degree of isolation and restricted gene flow between populations, which is consistent with generally small and scattered populations. The large proportion of distinguishable genotypes (mean 0.498) suggests high levels of out-crossing either currently or in the past. Introgression between S. capillifolium and S. rubellum, a species not found in Bulgaria, was suggested by the occurrence of rubellum-alleles in five populations from different mountains. This could be explained by an ancient hybridization event in a sympatric population. Based on (1) the high genetic diversity, (2) the fairly wide distribution of alien alleles, and (3) the isolated distribution of populations even within one mountain, a possible survival of S. capillifolium in the Balkan area during the Quaternary ice periods is hypothesized.

Genetic variation in the clonal bryophyte Hylocomium splendens at hierarchical geographical scales in Scandinavia

The widespread bryophyte Hylocomium splendens was sampled in a hierarchical fashion from populations representing four Scandinavian vegetation zones. Allozyme electrophoresis revealed variation at 11 out of 13 screened loci, allowing accurate identification of genotypes. From a total sample of 298 shoots 79 genotypes could be detected, giving the proportion of distinguishable genotypes (PD) of 0.265. The total allelic diversity (GST) based on polymorphic loci was 0.274. The relative differentiation among populations was low (GST = 0.073), indicating a high level of gene flow. Differences in population structuring occurred between a subarctic-alpine site vs. three lowland sites. The subarctic-alpine population had one widespread clone, which appeared to be propagated by dispersal of vegetative fragments. That population also comprised many rare genotypes, often occurring together within 10 × 10 cm patches. The lowland populations had genotypes that were locally common and often dominant within the patches. When identical genotypes were observed in multiple patches within these populations, it was usually statistically highly probable that they had arisen by independent sexual recombinations.

Isozyme evidence of relationships within Sphagnum sect. Acutifolia (Sphagnaceae, Bryophyta)

Ten enzyme systems coding for 13 putative loci were studied in 12 European species ofSphagnum sect.Acutifolia (peat mosses). On average seven plants from each of 73 collections made throughout N Europe were analysed. Despite complex morphological variation, all species expressed unique allele combinations. Principal components and cluster analyses based on allele frequencies showed that sect.Acutifolia can be divided into two major groups; one comprisingSphagnum fimbriatum, S. girgensohnii andS. molle; the second comprisingS. capillifolium, S. angermanicum, S. fuscum, S. quinquefarium, S. rubellum, S. subfulvum, S. subnitens andS. warnstorfii. These groups are linked byS. russowii, which is shown to be an allopolyploid, with fixed heterozygosity at 6 to 8 of the loci. The presumed progenitors ofS. russowii areS. girgensohnii andS. rubellum. Allelic data also indicate thatS. teres from sect.Squarrosa is related toS. girgensohnii andS. fimbriatum. The bisexual species tend to be less variable than the unisexual species.

Microarthropods Mediate Sperm Transfer in Mosses

Among flowering plants, animals commonly act as pollinators. We showed that fertile moss shoots attract springtails and mites, which in turn carry moss sperm, thereby enhancing the fertilization process. Previously, fertilization of mosses was thought to depend on the capacity of individual sperm to swim through a continuous water layer. The role of microarthropods in moss fertilization resembles the role of animals as pollinators of flowering plants but may be evolutionarily much older because of the antiquity of the organism groups involved.

Population structure and interspecific differentiation of peat moss sister species Sphagnum capillifolium and S. rubellum in northern Europe.

Isozyme electrophoresis was used to study the morphologically similar sister speciesSphagnum rubellum andS. capillifolium from a sample of 1313 plants representing 37 populations from Scandinavia, Great Britain and S Germany. The mean pairwise genetic identities (I) among conspecific populations were 0.976 forS. rubellum and 0.969 forS. capillifolium, versus 0.627 between populations of the two species. Interspecific gene flow was indicated by the observation of occasional plants in sympatric populations with alleles otherwise unique to the other species. Populations of bisexualS. capillifolium were significantly more variable than populations of unisexualS. rubellum. Alpine populations ofS. rubellum andS. capillifolium were dominated by few genotypes, and differentiation among populations was pronounced, indicating a low level of sexual recombination. InS. rubellum, maximum variability was found in western areas with high annual precipitation. Distribution of alleles inS. rubellum indicated restricted gene flow between Great Britain and Scandinavia. Postglacial migration from separate refugia may explain large-scale variation inS. rubellum.

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.

Maternal transmission of cytoplasmic DNA in interspecific hybrids of peat mosses, Sphagnum (Bryophyta)

The progeny of spontaneous interspecific hybrid sporophytes of Sphagnum were used to analyse the inheritance of cytoplasmic DNA. The analysis showed that only the female parent donated chloroplasts and mitochondria in Sphagnum hybrids. Thus, this is the first study demonstrating maternal cytoplasmic inheritance in a nonvascular land plant. This finding has important implications for phylogenetic reconstructions utilizing chloroplast and mitochondrial DNA sequences as well as for the evolution of cytoplasmic inheritance in relation to the life cycle of land plants.

Clonal distribution, fertility and sex ratios of the moss Plagiomnium affine (Bland.) T.Kop. in forests of contrasting age

Abstract Six populations of the clonal forest floor moss Plagiomnium affine from forests of different age were screened for genetic variation at 23 allozyme loci, of which nine were polymorphic. Samples consisting of two adjacent unconnected shoots were taken at regular intervals along one transect from each population. A total of 602 shoots was analysed. Almost 80% of the shoots were sterile (i.e. not expressing male or female gender). Sex remained unknown for only 10% of shoots after identification of genets based on electrophoretic data. We identified a mean number of 3.7 fertile clones per population. The mean length of clones along transects in each population ranged between 2 and 3 m. The size distribution within populations was bimodal, with a few dominant clones and a varying number of much smaller clones. The overall sex ratio was slightly female biased at the ramet level, but balanced at the genet level. Forest age was negatively correlated with percentage of sterile shoots and positively correlated with frequency of sporophytes. In both cases correlations were significant only if population 1, which was subject to extreme soil disturbance by badgers, was excluded. We conclude that the effective population size is larger, and the susceptibility to genetic drift is lower, in old forests.