Decoding cryptic diversity of moss populations in a forest-tundra ecotone

Cryptic speciation is widespread among bryophytes, and it appears common in arctic and subarctic mosses where cryptic lineages may occur sympatrically in the environment. However, cryptic lineages are rarely considered in genetic diversity assessments. This situation poses a challenge, as the complex population structure resulting from cryptic speciation, along with factors like the predominance of clonality, can lead to inaccurate estimates of biodiversity. In this sense, we studied two populations of the subarctic moss Racomitrium lanuginosum in the forest-tundra ecotone to test the impact of accounting for differentiated genetic groups on moss genetic diversity estimates, clonal structure and microbial covariation. We performed genotyping-by-sequencing to infer genetic diversity and structure in the forest tundra and the shrub tundra. Genetic groups were identified using haplotype-based coancestry matrices and phylogenetic analyses. The clonal structure was explored by determining multilocus genotypes at the population and a finer scale (225 cm2). The covariation between genetic groups and microbial communities (bacterial and diazotrophic) was explored. The recognition of cryptic lineages in genetic diversity estimations revealed differences between habitats that remained undetected when treating R. lanuginosum as a single species. Clonal growth seemed to predominate and affect the genetic structure at the local scale. Finally, genetic groups did not host specific microbiomes, suggesting that moss microbial associations in the forest-tundra ecotone did strongly respond to a genetic component. This study highlights the importance of accounting for cryptic lineages in genetic diversity estimations for a precise biodiversity assessment in subarctic and arctic ecosystems.