Considering evolutionary processes in cycad conservation: identification of evolutionarily significant units within Dioon sonorense (Zamiaceae) in northwestern Mexico

Cycads are considered to be the most threatened plant group on Earth. Thus, the identification of important biological units for conservation is crucial for their management and protection. Taxonomic studies have enormously contributed to cycad conservation, since species descriptions formally recognize biodiversity components that can be considered in conservation. However, because multidisciplinary studies that evaluate the biodiversity within cycad species are almost non-existent intraspecific evolutionarily significant units (ESUs) have not been considered when making conservation plans. The cycad Dioon sonorense (Zamiaceae) has a relatively wide distribution range along a vegetation gradient in Northwest Mexico: the southern populations of D. sonorense occur in tropical forests, whereas the northern populations occur in more xeric environments in the Sonoran Desert. Although both habitats might have promoted important evolutionary trends within D. sonorense , only the southern populations are being conserved. Here, we evaluated whether intrinsic differentiation among populations of D. sonorense exists, and whether northern populations may merit attention in conservation programs. We analyzed the variation of genome-wide loci to evaluate the genetic structure among six populations. We found two clusters, northern and southern, that can be recognized using 361 neutral loci. Such genetic structure is consistent with the differentiation observed from the variation of 16 macromorphological and 9 epidermal traits. Southern and northern regions are clearly differentiated in genetic, morphological and anatomical analyses. These multidisciplinary results suggest that the northernmost populations of D. sonorense represent an ESU that should be included in conservation programs. This study highlights that the evaluation of intraspecific variation may contribute effective strategies for cycad conservation, and that evolutionary processes should be considered in the delimitation of conservation units.