Establishing the phylogenetic origin, history, and age of the narrow endemic Viola guadalupensis (Violaceae)

Abstract

Climate change and shifts in land use are two major threats to biodiversity and are likely to disproportionately impact narrow endemics. Understanding their origins and the extent of their genetic diversity will enable land managers to better conserve these unique, highly localized gene pools. Viola guadalupensis is a narrow endemic of the Guadalupe Mountains (west Texas, USA). Its affinities within Viola section Chamaemelanium have been the subject of some debate. Furthermore, the polyploid and presumably reticulate relationships within this section remain largely unknown. We counted chromosomes for V. guadalupensis. Phylogenies for the chloroplast trnL-F region and the low-copy nuclear gene GPI for 24 Viola taxa were generated and used to produce a polyploid phylogenetic network. Divergence dates were obtained by fossil calibration. Meiotic chromosome counts revealed that V. guadalupensis is tetraploid (n = 12), and the presence of two GPI homoeologs further suggested allotetraploidy. Phylogenetic reconstructions showed that it originated through hybridization between unidentified members of subsection Canadenses (paternal parent) and subsection Nuttallianae (maternal parent). A fossil-calibrated relaxed clock dating analysis of GPI estimated the maximum age of V. guadalupensis to be 8.6 (5.7-11.6) Myr, suggesting the species evolved after the Guadalupe Mountains formed 12-13 Ma. Viola guadalupensis originated by intersubsectional hybridization followed by polyploidization. Within section Chamaemelanium, this phenomenon has occurred repeatedly in the last 9 Myr (at least for V. bakeri, V. douglasii, V. glabella, and V. sempervirens). Consequences for the systematics of the section are discussed.