It’s spring break season across the United States, which means many undergraduates are shedding their winter layers and flocking to warm, tropical destinations. After a week of fun in the sun, I’m sure many of them will rely on Aloe vera to soothe their sunburns.
In a recent BMC Evolutionary Biology paper, Grace et al. (2015) examined the phylogenetic history of Aloe vera and its relatives, tracing the medicinal use of plants in the genus. The authors collected sequence data from nuclear and plastid loci for 239 taxa in the family Xanthorrhoeaceae, including 197 species in the Aloe, Aloidendron, Aloiampelos, Aristaloe, Gonialoe and Kumara genera. They constructed phylogenetic trees, estimated divergence dates, and obtained information from a dataset of 1400+ records from the literature to test for phylogenetic signal in the use of aloes.
Low genetic polymorphism, taxonomic complexity, and incomplete taxon sampling have hampered the study of aloe and here Grace et al. present the first phylogeny to include >10% of Aloe species.
Branching tree aloes (Aloidendron) are basal to the remainder of the alooids. A clade comprising the Cape endemic genus Kumara and Haworthia s.s. is sister to Aloiampelos, which is in turn sister to Aloe. Within the large Aloe clade (184 species), well-supported terminal branches highlight species-level relationships but the clades, which will ultimately underpin a taxonomic revision, are incompletely resolved.
Divergence dating and ancestral area reconstructions suggest Aloe originated about 19 million years ago in southern Africa. Diversification in the genus began about 16 million years ago with a range expansion of ancestral taxa into a region encompassing Zambia, Ethiopia, and Somalia about 10 million years ago (see figure below adapted from Figure 2 in Grace et al. 2015).
Leaf succulence in aloes has increased with the radiation out of southern Africa, although succulence has been almost completely lost in several members of the genus. In terms of medicinal use, succulent aloes are significantly more likely to have a history of medicinal use than non-succulent species.
Of the 81 Aloe species in our analysis that have documented medicinal use, 98% have succulent leaves. By contrast, in 87% of the 15 species in which succulent leaf mesophyll has been almost entirely lost, there is negligible documented tradition of medicinal use, even in regions with thoroughly documented ethnoflora, such as South Africa.
If you hit the beach this spring, hopefully you will remember to apply sunscreen early and often, but if not, extracts from a succulent Aloe vera plant will help take the sting out of your sunburn.
Reference
Grace, O. M., Buerki, S., Symonds, M. R., Forest, F., van Wyk, A. E., Smith, G., … & Rønsted, N. (2015). Evolutionary history and leaf succulence as explanations for medicinal use in aloes and the global popularity of Aloe vera. BMC Evolutionary Biology. DOI: 10.1186/s12862-015-0291-7