Morphometrics and phylogenomics of coca (Erythroxylum spp.) illuminate its reticulate evolution, with implications for taxonomy

Abstract

South American coca (Erythroxylum coca and E. novogranatense) has been a key-stone crop for many Andean and Amazonian communities for at least 8,000 years. However, over the last half century, global demand for cocaine has placed this plant in the centre of armed conflict, deforestation, and explosive growth of illegal economies. While national and international agencies progress from a war on drugs policy model towards locally appropriate, data-informed strategies to tackle coca plantations, monitoring their expansion and composition remains essential. The principal means to identify coca plants is leaf morphology, yet the extent to which it is reflected in taxonomy is uncertain. Here, we analyse the consistency of the current naming system of coca and its four closest wild relatives (the coca clade), using morphometrics, phylogenomics, and population genomics. We include the name-bearing type specimens of cocas closest wild relatives E. gracilipes and E. cataractarum. Morphometrics of 342 digitized herbarium specimens show that leaf shape and size fail to reliably discriminate between species and varieties. However, the rounder and more obovate leaves of certain coca varieties could be associated with domestication syndrome of this crop. Our phylogenomic data indicate gene flow involving monophyletic clades of E. gracilipes and the E. coca clade. These results further clarify the evolution of coca and support a taxonomic framework wherein E. gracilipes is retained as a single species. Our findings have implications for the development of cost-effective genotyping methods to effectively discriminate varieties of cultural significance from high-yielding cultivars fuelling the lucrative cocaine market.