Assembly and origin of the flora of the Chihuahuan Desert: the case of sclerophyllous Rosaceae

Aim To test hypotheses on the origin and assembly of the flora of the Chihuahuan Desert using as a system the hard-leaved shrubby lineages of the Rosaceae distributed in North American Deserts.

Location North American Deserts, Chihuahuan Desert, Mexican Plateau, Tehuacán Valley.

Methods Phylogenetic analyses were conducted based on nine chloroplast and nuclear molecular markers of eleven newly sequenced species of Rosaceae distributed in the Chihuahuan Desert along with previous sequences of 41 species in 25 genera representative of the major clades in the family. Based on phylogeny, divergence times, migration, vicariance events and ancestral biogeographic areas were determined. Ecological niche-based modeling will predict changes in distribution during Pleistocene climate oscillations. Traitgrams were constructted to visualize departure among lineages in the most significant climate variables.

Results The lineages studied diverged during the Eocene-Oligocene (53 – 33Mya). The ancestral area for most of the taxa was in the Mojave and Chihuahuan Deserts. Most dispersal events occurred from the Mojave Desert to the central area of the Chihuahuan Desert on the Mexican Plateau. Ecological niche-based modeling suggests a contraction of suitable habitat during the Last Interglacial period (~120,000 - 140,000 years), followed by expansion during the Last Glacial Maximum (~ 22,000 years) and the Middle Holocene (~ 6000 years).

Main conclusions Our results confirm the Axelrod’s hypothesis of an origin of the flora of the Chihuahuan Desert in SW North America migrating to occupy North American arid lands. Vicariance and the expansion of arid lands allowed divergence, migration and permanence of the lineages studied on the Mexican Plateau. Divergence occurred prior to the onset of aridification. The Mexican Plateau acted as a refuge for hard-leaved lineages during the Miocene. Shifts in ecological niches of related species allowed them to colonize new areas with different temperatures and precipitation. Current distribution of lineages is the most limited.

Methods
Eleven species of Rosaceae distributed in the Chihuahuan Desert were newly sequenced. Samples were collected taking into account their entire range in this arid land  including the Tehuacán Valley. Samples were obtained either from the field or from herbarium specimens (HGOM, SLPM, MEXU, and XAL). In total, sequences from 9 molecular markers: one nuclear ribosomal (ITS) and eight plastid markers (atpB-rbcL, matK, psbA-trnH, rpL16, rps16, trnH-rpl2, trnL-trnF, trnS- trnG) were analized.