More than one sweet tabaiba: Disentangling the systematics of the succulent dendroid shrub Euphorbia balsamifera

Abstract

The sweet tabaiba (Euphorbia balsamifera Ait.) is one of the classical examples of the disjunct biogeographic pattern known as Rand Flora. This species is currently circumscribed to comprise two subspecies, Euphorbia balsamifera subsp. balsamifera and E. balsamifera subsp. adenensis (Deflers) P.R.O. Bally, with their respective areas of distribution separated by a gap of about 2000 km across Central‐East Africa. We use multiple sources of evidence including phylogenomics, morphometrics, lineage ages, and climatic niche analysis to disentangle the confusing taxonomy around this charismatic Euphorbia lineage. Based on our integrative approach, we reestablish the two current subspecies to the rank of species (E. balsamifera Ait., E. adenensis Deflers) and resurrect the long forgotten E. sepium N.E. Br., a commonly used species in the western Sub‐Saharan and Sahelian regions. The taxonomic treatment presented here includes an identification key, updated morphological descriptions, and lectotypification of several names.

Rare and widespread: integrating Bayesian MCMC approaches, Sanger sequencing and Hyb‐Seq phylogenomics to reconstruct the origin of the enigmatic Rand Flora genus Camptoloma

“…All this evidence supports a paleoendemic scenario with historically high extinction rates and population‐contraction events. The abovementioned studies were based on Sanger sequencing of a few chloroplast and nuclear loci, which sometimes translated in low levels of resolution; our study, and other recent studies by our team (Villaverde et al, 2018; Riina et al, 2020), show that the use of genomic data can be a complementary approach that can help us clarify patterns that were previously less specific.…”

“…Intracontinental biodiversity disjunctions, in which climatic barriers isolate disjunct sister taxa within a continent, provide ideal scenarios for testing past climatic change hypotheses (Crisp and Cook, 2007; Mairal et al, 2017). One of the best studied examples is the Rand Flora (RF) pattern, a floristic biogeographic pattern where unrelated plant lineages with subhumid or xeric affinities share a similar disjunct distribution on the edges of the African continent and adjacent islands (Christ, 1892; Bramwell, 1985; Sanmartín et al, 2010; Pokorny et al, 2015; Mairal et al, 2015a, 2017; Villaverde et al, 2018; Riina et al, 2020). Recent studies using phylogenetic, divergence time, paleoclimate, and niche‐modeling data indicate that the RF pattern was formed over the last 20 million years through cycles of range contraction (i.e., by ecological vicariance and population extinction) and range expansion via short and medium‐range dispersal.…”

“…Two biogeographic distributions are typically found within RF lineages: (1) an east/west northern disjunction between Macaronesia and northwestern Africa (Western Sahara) on one side, and the region including Horn of Africa, Southern Arabia, and the Socotra Archipelago on the other. Taxa like the genus Canarina (Campanulaceae; Mairal et al, 2015a, 2017) or the Euphorbia balsamifera complex (Euphorbiaceae; Villaverde et al, 2018; Riina et al, 2020) show this type of disjunction. (2) A south/east disjunction between southern Africa and the Ethiopian Highlands and Eastern African mountain ranges can be found in the Euphorbia aphyllis group (Pokorny et al, 2015).…”

Ecological and geological processes impacting speciation modes drive the formation of wide‐range disjunctions within tribe Putorieae (Rubiaceae)

“…The opposite pattern is found in "ecological speciation," in which population divergence is driven by the adaptation to different ecological settings within the species geographical range (Smith et al, 2001;Couvreur et al, 2011;Schnitzler et al, 2012). In the "ecological speciation" mode, intense ecological selection leads to rapid interruption of gene flow between the "budding" species, so a relatively short temporal gap is expected between stem and crown age estimates (Czekanski-Moir & Rundell, 2019;Riina et al, 2020), resulting in sister species with different ecological niches but sympatric geographical ranges (Table 1). An intermediate speciation mode would be "ecogeographical vicariance" (Gittenberger, 1991;Rundell & Price, 2009), which involves both allopatry and ecological adaptation.…”

“…Unlike other Rand Flora taxa, such as genera Canarina (Mairal et al, 2015) and Camptoloma (Culshaw et al, in press) or Euphorbia sect. Balsamis (Villaverde et al, 2018; Riina et al, 2020), the distribution of Plocama extends beyond the Rand Flora range, exhibiting two minor centers of diversity in the Middle East (Fig. 1, cea ) and, especially, in the mountains and plateaus of Iran–Afghanistan, where a large number of montane‐adapted species with nearly sympatric ranges occur (Fig.…”