Conservation genomics of an Australian cycad Cycas calcicola, and the Absence of Key Genotypes in Botanic Gardens

Understanding the genetic diversity of wild populations is fundamental to conserving species in-situ and ex-situ. To aid conservation plans and to inform ex-situ conservation, we examined the genetic diversity of the cycad Cycas calcicola (Cycadaceae). Samples were collected from wild populations in the Litchfield National Park and Katherine regions in the Northern Territory, Australia. Additional samples were obtained from botanic garden plants that were originally collected in the Katherine region, Daly River and Spirit Hills in the Northern Territory, Australia. Using RADseq we recovered 2271 informative genome-wide SNPs, revealing low to moderate levels of gene diversity (uHₑ = 0.037 to 0.135), very low levels of gene flow, and significant levels of inbreeding (mean FIS = 0.491). Population structure and multivariate analysis showed that populations fall into two genetic groups (Katherine vs Litchfield + Daly River + Spirit Hills). Genetic differentiation was twice as high between populations of the Katherine and Litchfield regions (FST ~ 0.1) compared to within these two regions (FST ~ 0.05). Increasing population fragmentation together with high levels of inbreeding and very little gene flow are concerning for the future adaptability of this species. The results indicated that the ex-situ collections (1) had significantly lower genetic diversity than the wild populations, and (2) only partly capture the genetic diversity present, particularly because the Litchfield National Park populations are not represented. We recommend that ex-situ collections be expanded to incorporate the genetic diversity found in Litchfield National Park and to increase the number of representatives from Daly River/Spirit Hills, and that in-situ populations from the Katherine and Greater Litchfield regions be conserved as separate management units.