Cooperative Fish and Wildlife Research Units Program: Arizona
Education, Research and Technical Assistance for Managing Our Natural Resources

A series of 900–1200 m desert peaks surrounded by arid lowlands occur throughout the southwestern U.S. and northwestern Mexico where temperate affiliated species occur at highest elevations. The presence of disjunct long-lived plant taxa on under-explored summits, especially Isla Tiburón at 29º latitude in the Gulf of California, suggests a more southerly extent of Ice Age woodlands than previously understood. The phylogeography of the desert edge species Canotia holacantha (Celastraceae) was investigated to test the hypothesis that insular desert peak populations represent remnants of Pleistocene woodlands rather than recent dispersal events. Sequences of four chloroplast DNA regions totaling 2,032 bp were amplified from 74 individuals of 14 populations across the entire range of C. holacantha as well as nine individuals that represented the other two species in its clade (C. wendtii and Acanthothamnus aphyllus) and two outgroups. Results suggest that a Canotia common ancestor occurred on the landscape, which underwent a population contraction ca. 15 kya. The Isla Tiburón C. holacantha population and the Chihuahuan Desert microendemic C. wendtiihave the greatest genetic differentiation, are sister to one another, and basal to all other Canotia populations. Three haplotypes within C. holacantha were recovered, which correspond to regional geography and thus identified as the Arizona, Sonora, and Tiburón haplotypes, within which Acanthothamnus aphyllus is nested rather than as a related genus. These results indicate a once broad distribution of Canotia / Acanthothamnus during the Pleistocene, now present in relict populations on the fringes of the southern desert, in the Chihuahuan Desert, with scattered populations on desert peaks and a common or abundant distribution at the norther ecotone of the Sonoran Desert. These results suggest Canotia has tracked the shift of the desert’s edge both in latitude and elevation since the end of the last Ice Age.