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.

研究目标：以分布于北美荒漠的蔷薇科（Rosaceae）硬叶灌木类群为研究系统，检验奇瓦瓦沙漠（Chihuahuan Desert）植物区系的起源与组装相关假说。研究区域：北美荒漠、奇瓦瓦沙漠、墨西哥高原与特瓦坎谷地。研究方法：基于新测序的11个分布于奇瓦瓦沙漠的蔷薇科物种的9个叶绿体与核分子标记，并结合蔷薇科主要演化分支代表类群的25个属内41个物种的已有序列，开展系统发育分析。基于系统发育树，估算类群分化时间、迁移与隔离分化事件，并重建祖先生物地理区系。采用生态位模型预测更新世气候振荡期间的物种分布变化；构建性状图以可视化不同类群在关键气候变量上的差异。研究结果：本研究涉及的类群分化发生于始新世-渐新世（5300万至3300万年前）。多数类群的祖先分布区位于莫哈韦沙漠（Mojave Desert）与奇瓦瓦沙漠。多数物种扩散事件发生于从莫哈韦沙漠向墨西哥高原上的奇瓦瓦沙漠中部区域。生态位模型结果显示，适宜生境在末次间冰期（约12万至14万年前）发生收缩，随后在末次盛冰期（约2.2万年前）与中全新世（约6000年前）出现扩张。主要结论：本研究结果证实了阿克塞尔罗德（Axelrod）提出的奇瓦瓦沙漠植物区系起源于北美西南部，并迁移至北美干旱区域的假说。隔离分化与干旱区域的扩张推动了研究类群在墨西哥高原上的分化、迁移与存续。类群分化发生于气候干旱化进程启动之前。中新世时期，墨西哥高原作为硬叶灌木类群的避难所。相关物种的生态位分化使其能够定植于温度与降水条件各异的新区域。当前类群的分布范围最为受限。