Thousands of species of succulent plants occur in the dry regions of the world. These plants are adapted to thrive in harsh environments where drought is common. Succulent plants display an extraordinary array of features to contend with these conditions: modified photosynthesis, efficient water use and ways to withstand the temperature fluctuations typical of desert habitats. Our understanding of these adaptations in the plant kingdom and the evolutionary processes underlying them is patchy: most studies have focused on the enormous cacti (Cactaceae), ice plant (Aizoaceae) and spurge (Euphorbiaceae) families. I aim to contribute to a more complete picture of evolution among communities of succulent plants, particularly in Africa.
Evolution of aloes
Aloes are botanical icons of Africa. They are easily recognised and highly appreciated wherever they occur, from the southernmost tip of Africa to the Horn of Africa, the Arabian Peninsula and the islands off the eastern coast of Africa, including Madagascar. There are over 500 species of aloes, and the ways in which many species are related to one another have long challenged botanists.
We now know that the tree aloes (members of the genus Aloidendron, Xanthorrhoeaceae subf. Asphodeloideae) are the oldest of the extant aloes. The rambling aloes (Aloiampelos) and fan aloes endemic to the Cape (Kumara) likely evolved before the lineage that gave rise to the aloes in the strict sense – the genus Aloe. New insights into the evolution of aloes have revealed multiple innovations in leaf succulence, pollination syndromes and habitat specialism. More detailed study of systematic relationships and ecological success are now possible. Why are closely related genera found only in southern Africa, whereas Aloe is widely distributed? What environmental conditions historically supported speciation in Aloe? Does the leaf chemistry for which aloes are valued reflect the diversity of the genus? I am pursuing studies on various aspects of the systematics and ecology of aloes with my collaborators.
Predictive phylogenetics in succulent plants
Aloe and Euphorbia (Euphorbiaceae) are both examples of large succulent genera in Africa of considerable medicinal value. In Aloe, the leaf exudate and the succulent tissue are used, whereas in Euphorbia, the toxic milky latex is used. I am interested in the emerging topic of predictive phylogenetics—the study of patterns in evolutionary data to prioritise and predict species of interest for a given attribute.
Predictive phylogenetics seeks to detect patterns in evolutionary data, and estimate the repetition of these patterns in species for which data are lacking. This exciting new approach is being tested in various ways to identify species of interest for their properties, conservation concern or uses. I work closely with Nina Rønsted‘s lab investigating the evolution of chemical traits and uses in valuable succulent lineages. Currently, Madeleine Ernst is researching chemical diversity and utility in the super-genus Euphorbia for her PhD in the MedPlant Initial Training Network funded by the European Commission, and Louise Ahl is investigating chemical diversity of Aloe for her PhD, in collaboration with William Willats, funded by the Villum Foundation.