Studying population genetic parameters of dominantly asexual bryophyte species is interesting due to the unique features of their life cycle, such as a relatively short distance dispersal capacity and a lack of the advantages of sexual reproduction. Studying asexual species becomes even more interesting when the study species are also habitat specialists, are rare in part of their global distribution, and have a dominant haploid life cycle. We chose a species that falls into all the above categories, Crossocalyx hellerianus (Nees ex Lindenb.) Meyl.; a minute dioicous epixylic liverwort with dominant asexual reproduction, which inhabits decaying logs and stumps of coniferous trees in boreal and subalpine forests with a circumboreal- subcontinental distribution. The objective of this study is to identify the spatial pattern of genetic variation and structure of C. hellerianus from population (site) to regional (100 km) and intercontinental scales (10 000 km) and to subsequently answer the question, how do the current distribution of the species and dispersal limitations shape the spatial genetic patterns of C. hellerianus?
Samples were collected from three provinces in Canada: Quebec (15 populations, 104 individuals), Alberta (six populations, 50 individuals), and New Brunswick (one population, nine individuals). Using already published data, we compared Canadian and European populations with six populations (99 individuals) from the Czech Republic and four populations (241 individuals) from Finland. Six polymorphic microsatellite markers were used to genotype the individuals. The sequenced data were analyzed for genetic diversity estimations, genetic variation, and population structure parameters using different software packages.
Despite the dominant asexual reproduction mode of this species, we observed a high level of genetic diversity even within colony and population levels. Also, we found some evidence of long-distance dispersal of asexual propagules of the species. In terms of genetic variation and structure, we observed two main clusters in North America and few barriers to gene flow, which is a pattern similar to that found in boreal tree species in North America that were influenced by post-glacial dispersal patterns. The populations from the Gulf of St-Lawrence region were differentiated from the other populations. At the intercontinental level, three significant clusters were observed as Canadian, Czech, and Finnish populations each formed a cluster, although a connection between Europe and North America is suggested via the Gulf of St-Lawrence region.
The results of this research indicate that the populations of asexually reproducing species can be as genetically diverse as sexually reproducing species. Furthermore, the genetic structure of the species in North America has been shaped by post-glacial dispersal patterns and biogeographic connections between North American and Europe.