Solanum L. (Solanaceae) is one of the largest genera of angiosperms and presents difficulties in identification due to lack of regional keys to all groups. Here we provide keys to all 135 species of Solanum native and naturalised in Africa (as defined by World Geographical Scheme for Recording Plant Distributions): continental Africa, Madagascar (incl. the Indian Ocean islands of Mauritius, La Réunion, the Comoros and the Seychelles), Macaronesia and the Cape Verde Islands. Some of these have previously been published in the context of monographic works, but here we include all taxa. The paper is designed to be used in conjunction with the web resource Solanaceae Source (www.solanaceaesource.org) and hyperlinks provide access to online descriptions, synonymy and images (where available) of each species. All taxa treated and specimens seen are included in searchable Suppl. material 1, 2.

Africa, Aldabra, Azores, Canary Islands, Cape Verde, Comoros, cultivated plants, identification, keys, Madagascar, Madeira, Mauritius, La Réunion, Seychelles, Solanum, weeds

Solanum L. (Solanaceae) is one of the largest of angiosperm genera (Frodin 2004) with ca. 1,200 species distributed worldwide with species on all continents except Antarctica. The greatest species diversity in the genus occurs in the Neotropics (see Bohs 2005), but significant diversity also occurs in the Old World, with Africa and Australia particularly important areas for diversification (see Vorontsova and Knapp 2016; Echeverría-Londoño et al. 2018). Due to its large number of species and the number of introductions and cultivated taxa, Solanum is often an identification challenge for non-specialists. Recent completion of several large monographic treatments of the Solanum of Africa (e.g. Vorontsova and Knapp 2016; Knapp and Vorontsova 2016; Särkinen et al. 2018) as part of the US National Science Foundation funded Planetary Biodiversity Inventory project “PBI Solanum” means we can now provide keys for the genus across the continent and for adjacent islands such as Madagascar and Macaronesia. Some of these have been published in the Open Access literature (e.g. Knapp 2013; Knapp and Vorontsova 2016; Särkinen et al. 2018), but the largest of these, treating the spiny solanums (Vorontsova and Knapp 2016) is not. Several species that are either naturalised (e.g. members of the Brevantherum Clade) or cultivated (tree tomatoes, pepinos, potatoes and tomatoes) in the region are also not treated as part of these monographic treatments, although complete species descriptions and photographs are provided on the web resource Solanaceae Source (www.solanaceaesource.org).

Here we provide dichotomous keys that include all groups and species (native, naturalised and widely cultivated; see Table 1 for species list) of Solanum occurring in continental Africa, Madagascar (incl. the Indian Ocean islands of Mauritius, La Réunion, the Comoros, and the Seychelles), Macaronesia and the Cape Verde Islands to facilitate identification across the region. Taxa occurring in each country in the region are shown in Table 2 and a map of Solanum diversity (all taxa) is presented in Figure 1. Keys to individual groups are also provided for the 135 Solanum species occurring in the region (see Table 1 and Appendix 1 for a species list). We hope that these keys will encourage collection and documentation of Solanum across Africa and uncover new distributions and perhaps new species for the region.

Figure 1. 

Heat map of Solanum diversity in Africa. Darker degree squares indicate greater species richness. The middle to high elevation regions of eastern Africa (Kenya/Tanzania) have the highest high species diversity, followed by secondary areas of species richness in the Ethiopian plateaus, dry areas of central Madagascar, South Africa and the area around Mount Cameroon. We have not analysed how collecting effort has influenced these patterns, but it is likely to be important. As the Leptostemonum Clade has the largest number of species in Africa, diversity in that clade drives species richness overall (see Vorontsova and Knapp 2016, figure 2). Map prepared by Sarah Ficinski.

We modified keys from published monographs for groups of Solanum from the botanical continent “Africa” as defined in the World Geographical Scheme for Recording Plant Distributions (WGSRPD; Brummitt 2001). This corresponds basically to the countries of the continent of Africa, but excludes the Sinai Peninsula (politically part of Egypt and in WGSRPD part of Western Asia). It also includes islands grouped as Macaronesia (Azores, Canary Islands, Madeira and the Cape Verde Islands ) and Madagascar and other Indian Ocean islands east to Rodrigues.

We assessed distribution using the published monographs, with additional data points added from subsequent herbarium visits. All specimens seen for these keys can be seen in the Supplemental File and in the dataset published on the NHM Data Portal (https://doi.org/10.5519/0042549). For descriptions of the taxa, users are referred to the original publications or the Solanaceae Source website (www.solanaceaesource.org), where all species treated here are described and synonymy listed.

To access descriptions on the Solanaceae Source website, begin by typing the species name in the search box in the upper right-hand part of the screen banner (tick the option “Taxonomy” below the box); when the correct name you are searching for appears, select it, then push the “Search” button to the right of the search box (if you do not push the “Search” button, nothing will happen). You will be taken to the species page, where images and synonyms appear on the opening page; to access descriptions, click on the “Description” tab where information can be obtained. Up-to-date specimen details are not currently available on the website but can be found as described above.

Solanum can be divided into 13 major clades or monophyletic groups (Bohs 2005; Weese and Bohs 2007; Särkinen et al. 2013; see Figures 2 and 3 for photographs illustrating representative morphology of these groups in Africa). The largest monophyletic clade is the Leptostemonum clade, or the “spiny solanums”, which comprises approximately half of the species diversity of the genus; divisions within that clade have been defined by Stern et al. (2011), Vorontsova et al. (2013) and Aubriot et al. (2016). This group is rapidly diversifying in the Old World (Echeverría-Londoño et al. 2018), with most taxa occurring in the Old World belonging to a single monophyletic group. Previous treatments (e.g. Whalen 1984; Jaeger 1985; Jaeger and Hepper 1986) had suggested the African taxa were members of, or closely related to New World groups. More information on the phylogenetic relationships of African and Asian members of the Leptostemonum Clade can be found in Vorontsova et al. (2013) and Aubriot et al. (2016). Other clades with significant species diversity in Africa (as defined here) are the African non-spiny (ANS) and Normania Clades (both endemic to the region; see Bohs and Olmstead 2001) and the Morelloid Clade (with a number of widespread weedy taxa, see Särkinen et al. 2018). Other clades such as the Geminata, Brevantherum and Potato Clades are represented only by introduced or cultivated species. The Dulcamaroid Clade has a single species native to Mediterranean northern Africa and Macaronesia and two cultivated taxa that can become naturalised (Knapp 2013). In order to facilitate identification and to assist with the discovery of novelties from the region, we provide a key to the major groups (clades) of Solanum following the most recent phylogeny of the genus (Särkinen et al. 2013) and additional dichotomous keys to the species within each group. Groups are ordered as they occur as branches in the phylogeny of Särkinen et al. (2013).

Figure 2. 

A, B Solanum wendlandii Hook.f. (Allophyllum-Wendlandii Clade) C Solanum tarderemotum Bitter (Morelloid Clade) D Solanum scabrum Mill. (Morelloid Clade) E Solanum pyracanthos Lam. (Leptostemonum Clade) F Solanum aculeastrum Dunal (Leptostemonum Clade) G Solanum nigriviolaceum Bitter (Leptostemonum Clade) H Solanum usambarense Bitter & Dammer (Leptostemonum Clade). Photos A, B, F, G H by M.S. Vorontosova C, D, E by S. Knapp.

Figure 3. 

A Solanum terminale Forssk. (African non-spiny Clade) B Solanum madagascariense Dunal (African non-spiny Clade) C Solanum mauritianum Scop. (Brevantherum Clade) D Solanum laxum Spreng. (Dulcamaroid Clade) E Solanum trisectum Dunal (Normania Clade) F Solanum diphyllum L. (Geminata Clade) G Solanum tuberosum L. (Potato Clade) H Solanum laciniatum Aiton (Archaeosolanum Clade). Photos A, C, D, E, F, G, H by S. Knapp B by M.S. Vorontsova.

Four species have been recorded from this area, for which we have only seen single specimens, all of which are cultivated and not naturalised. Solanum laciniatum Aiton (Archaesolanum Clade), the kangaroo apple from Australia and New Zealand, has been recorded from Morocco, S. agrarium Sendtn. (Leptostemonum Clade, section Acanthophora Dunal sensu Nee 1979) from Brazil has only recently been collected in the Cape Verde Islands and S. atropurpureum Schrank (Leptostemonum Clade, section Acanthophora Dunal sensu Nee 1979) from Brazil and S. pectinatum Dunal (Leptostemonum Clade, section Lasiocarpa Dunal sensu Whalen et al. 1981) from Mexico have been recorded from Tanzania in a botanical garden. These singletons have been included in Table 1, but not in the keys below; descriptions should be checked if identification is ambiguous.

Each species name is hyperlinked to its page on Solanaceae Source (www.solanaceaesource.org) where photographs (if available), descriptions and other information can be found. An expanded key to all of the thirteen major clades of Solanum worldwide is in preparation (R. Hilgenhof and T. Särkinen, pers. comm.). Instructions on how to use Solanaceae source are included in the Materials and Methods.