INVESTIGATING PUTATIVE HYBRIDS OF FREMONTODENDRON DECUMBENS AND F. CALIFORNICUM IN THE SIERRA FOOTHILLS

Fremontodendron (Flannelbush, Fremontia, Malvaceae) is a genus of shrubs native to California, Arizona, and Baja California Norte. There are three species in the genus: Fremontodendron californicum (Torr.) Coville is widespread throughout the range and is highly variable in its appearance, F. mexicanum Davidson is endemic to Otay Mountain in southern California, and F. decumbens R.M.Lloyd is endemic to gabbro soils in the Sierra Nevada foothills. Fremontodendron decumbens is currently known only from the Pine Hill Preserve and adjacent areas in El Dorado County and was listed as endangered in 1996. Fremontodendron decumbens is mainly distinguished from F. californicum by its orange or reddish flowers and decumbent habit. Many plants with a decumbent habit but lacking the floral characters to be placed in F. decumbens have been collected over the years, primarily in the Northern Sierra Nevada/Cascade foothills, and there has long been speculation as to whether these could be hybrids between F. californicum and F. decumbens. Here we investigated the potential hybrid origin of morphologically intermediate individuals using full genome resequencing data from 99 individuals of F. californicum, F. decumbens, and potential hybrids, mapped to a F. californicum reference genome. We investigated population structure and potential admixture using principal components analysis, model-based evolutionary clustering, and D-statistics, as well as additional analyses. We also systematically investigated the morphology of F. californicum, F. decumbens, and potentially intermediate individuals by measuring a suite of ten characters in the field on 68 individuals. We found no evidence that the morphologically intermediate forms are of hybrid origin; rather, they generally group genetically with F. californicum. We also found that specimens from populations noted to be morphologically intermediate showed considerable overlap with F. californicum, but not F. decumbens, based on a morphological principal components analysis. The results lead us to conclude that F. decumbens is indeed a genetically distinct taxon and that individuals of F. californicum with a decumbent habit, but lacking the distinct floral characters of F. decumbens, are not the result of hybridization.