Perspectives on the systematics and phylogenetics of Guaiacum (Zygophyllaceae): complexities in conservation of endangered hardwoods due to fragmentation, introgression, and intermittent gene flow

Abstract

The systematics of the economically important, endangered hardwoods in Guaiacum are unclear withregard to taxonomic ranks, and the relationships among taxa. This is partially due to a lack of diagnostic charactersand minimal geographic sampling in previous studies. Nevertheless, systematic relationships are important to informtrade regulations and management practices for these species. This is especially true for Mexico, which is both theprimary exporter and the center of diversity for Guaiacum. Systematic and biogeographic issues in Guaiacum wereinvestigated by analyses of nuclear and chloroplast DNA markers from specimens sampled throughout the range.Phylogenetic and statistical parsimony analyses showed well-supported divergences within the group, including adeep divergence between G. officinale and other taxa with additional biogeographically correlated subdivisions. Thereis also an indication that accessions from Chiapas, Mexico are genetically intermediate between G. sanctum and G.coulteri, while minor segregates of Guaiacum (G. palmeri, G. guatemalense) were not well distinguished by eithermorphological or molecular characters. The genetic structuring among the major groups of Guaiacum shows evidenceof isolation induced by fragmentation of the range, with the structure largely maintained with only occasional longdistance gene flow between remote populations.