Conservation genomics of a rare fern and a moss on insular erratic boulders

Erratic boulders are habitat islands for rock-dwelling species and contribute to the biodiversity of landscapes. In the calcareous Swiss lowlands, siliceous erratic boulders form habitat islands for the regionally critically endangered fern Asplenium septentrionale, about 20 bryophyte species and numerous lichens. Focusing on island biogeographical processes, we analysed the conservation genomics of A. septentrionale and the moss Hedwigia ciliata on insular erratic boulders and adjacent mainlands in siliceous mountain areas with double digest restriction associated DNA sequencing (ddRAD). In the tetraploid A. septentrionale, frequent identical multilocus genotypes within populations suggested prevalent intragametophytic selfing, and six of eight boulder populations each consisting of a single multilocus genotype indicated single spore funder events. The mainland populations genetic structure coincided with Pleistocene refugia. In H. ciliata, four different genetic lineages were discriminated, and populations consisting of only one multilocus genotype were less frequent than in A. septentrionale. No distinct population structure was evident. In both taxa, multilocus genotype diversity on boulders was – as expected by island biogeographical theory – lower than in mainland populations. The absence of a common genetic group for boulder populations as well as the absence of isolation by distance patterns suggested that boulders were colonised by independent long-distance dispersal events. Successful boulder colonisation of A. septentrionale seemed to be rare, while colonisation by H. ciliata spores appeared more frequent. Our study suggests that pivotal principles of conservation biology such as connectivity and genetic diversity play a subordinate role for species on insular erratic boulders, as evidenced by A. septentrionale.