Museomics Clarifies the Classification of Aloidendron (Asphodelaceae), the Iconic African Tree Aloes

Arborescent succulent plants are regarded as keystone and indicator species in desert ecosystems due to their large stature and long lifespans. Tree aloes, the genus <i>Aloidendron</i>, are icons of the southern African deserts yet have proved elusive subjects due to the difficulty of obtaining material of known provenance for comparative study. Consequently, evolutionary relationships among representatives of the unusual arborescent life form have remained unclear until now. We used a museomics approach to overcome this challenge. Chloroplast genomes of six <i>Aloidendron</i> species and 12 other members of Asphodelaceae were sequenced from modern living collections and herbarium specimens, including the type specimens of all but two <i>Aloidendron</i> species, the earliest of which was collected 130 years ago. Maximum-likelihood trees estimated from full chloroplast genomes and the nuclear internal transcribed spacer (ITS) region show that <i>Aloidendron sabaeum</i>, from the Arabian Peninsula, is nested within <i>Aloe</i> while the Madagascar endemic <i>Aloestrela suzannae</i> is most closely related to the Somalian <i>Aloidendron eminens</i>. We observed phylogenetic conflicts between the plastid and nuclear topologies, which may be indicative of recurrent hybridisation or incomplete lineage sorting events in <i>Aloe</i> and in <i>Aloidendron</i>. Comparing species ecology in the context provided by our phylogeny suggests that habitat preference to either xeric deserts or humid forests/thickets evolved repeatedly in <i>Aloidendron</i>. Our findings demonstrate the value of botanical collections for the study and classification of taxonomically challenging succulent plants.