Population genomics of Tillandsia landbeckii reveals unbalanced genetic diversity and founder effects in the Atacama Desert

In hyper-arid habitats vegetation tends to be highly patchy with individual plant populations set widely apart from each other. In the Atacama Desert of northern Chile, rainfall is essentially absent, but fog occurring both at the coast and sometimes reaching inland areas supports patches of vegetation in an otherwise barren environment. Tillandsia landbeckii (Bromeliaceae), a epiarenic plant without functional roots, completely depends on fog as source for water, therefore it is found only in fog corridors. We investigated the genetic connectivity within and between populations of T. landbeckii, using genome-wide single-nucleotide-polymorphisms (SNP) obtained through genotyping-by-sequencing (GBS). The 21 sampled populations from the Chilean Atacama Desert are distributed in three geographically ordered south to north clusters, with the southern cluster containing only one population that is genetically very distant from the others. We obtained three genetic groups that corresponded to these three geographical clusters, with the exception of the two populations, where genetic affiliation lies at least in part with the central cluster. Our data indicates that several populations are strongly affected by clonal reproduction, while others are mostly reproducing sexually. We interpret these data as the result of genetic bottlenecks due to founder events involving few dispersing genotypes combined with strong geographical isolation for the northern and southern populations, following stepping stone dispersal of Tillandsia during more climatically favorable episodes.