A new tetraploid species of Solanum L. sect. Solanum (Solanaceae) from Tanzania

Abstract Solanum umalilaense Manoko sp. nov. (Solanaceae) is described from the Umalila area, in the southern highlands of Tanzania. Its novelty is supported with both morphological and AFLP data. Phenetic and phylogenetic analyses place Solanum umalilaense as a unique and well-supported taxon among tetraploid species of Solanum sect. Solanum from Africa. It can be distinguished from other African species by its extremely developed branching, each branch producing many multi-flowered inflorescences, flowers with short calyx lobes and its persistent, small, light yellowish brown fruits.


Introduction
Solanum L. section Solanum is a pantropical group of about 55 species, most of which are found in the New World, and about 10 in Africa; the group forms part of the Morelloid clade of Bohs (2005). Solanum nigrum L. is the type species for the section, and of the genus. Species in the section Solanum are herbs, sometimes suf-frutescent and occasionally small shrubs. Th ey also have simple, unbranched, uniseriate, multicellular hairs with or without glandular heads. Th eir ploidy levels range from diploid to hexaploid. Species with branched or stellate hairs do not fall in this section as traditionally circumscribed (Gray 1968). In Africa, section Solanum species form one of the largest groups of leafy vegetables.
Th e most thorough account of the African species of the section Solanum, with good morphological descriptions and keys, was published by Edmonds and Chweya (1997). Th eir list of native and introduced members of the section found in Africa includes S. americanum Mill., S. chenopodioides Lam., S. nigrum L., S. physalifolium Rusby var. nitidibaccatum (Bitter) Edmonds, S. retrofl exum Dunal, S. sarrachoides Sendtn., S. scabrum Mill., S. villosum Mill. and other taxa such as S. fl orulentum Bitter, S. grossidentatum A. Rich., S. hirsutum (Vahl) Dunal and S. tarderemotum Bitter, which may represent good species and need taxonomic revision. Delimitation of species based on morphological data alone is complicated by phenotypic plasticity, polyploidy, natural hybridization and discordant variation in this group (Edmonds and Chweya 1997). Jacoby (2003) and Olet (2004) studied accessions of some species of the section Solanum found in Africa using also a molecular approach (Amplifi ed Fragment Length Polymorphism, AFLP). Jacoby found that 15 morphological characters refl ected the genetic variation observed in the AFLP analysis, and Olet defi ned species of the section Solanum found in Uganda by using morphological characterization and AFLP on living accessions. Whereas eight species were defi ned by morphological characters, only fi ve were recognized using AFLPs.
While studying accessions of Solanum section Solanum from Africa at the Radboud University Nijmegen, it became apparent that plants grown from seeds of accession A24750133 collected from the Umalila area in the southern highlands of Tanzania were strikingly diff erent from other African accessions. Chromosome counts revealed that they had 48 chromosomes, in common with some other species of the section in Africa (Manoko 2007). Th e plants were temporarily designated as "species A" in Schippers (2004).
During a herbarium visit in Dar es Salaam, Tanzania, we found a specimen of the same taxon (R.E. Gereau et al. 5084, DSM) collected on the 14 th of November 1992 from Umalila forest reserve. Duplicate specimens were present in NHT, MO and K. Annotation labels on these specimens indicated that they were identifi ed by W.G. D'Arcy as S. aff . americanum Mill. and by L.B. Mwasumbi (DSM) as S. nigrum L.
We also located an additional specimen collected as a weed in maize fi eld in Uyole (Mbeya) in the DSM herbarium (A.A. Mwambunga A.A.M.6). Fieldwork (July 2010) in Tanzania enabled us to collect a number of accessions of this species and here we formally describe it. Voucher specimens have been deposited at the Experimental Garden and Genebank herbarium of the Radboud University Nijmegen, DSM and WAG.
Chromosome counts on accession A24750133 using standard cytological methods and fl ow cytometry on root and leaf cells respectively, showed that S. umalilaense had 48 chromosomes (2n=4x=48). Th e same was found for related African species such as S. fl orulentum, S. tarderemotum, S. hirsutum, S. grossidentatum and S. retrofl exum, indicating that all these species are tetraploid.
In phenetic and cladistic analyses of an AFLP (Vos et al. 1995) dataset of the tetraploid species from Africa, including S. villosum, S. retrofl exum, S. fl orulentum , S. tarderemotum , S. grossidentatum and S. hirsutum , S. umalilaense had 100% jackknife support in the neighbour joining and maximum parsimony analyses (Manoko 2007). A NJ tree was constructed using 435 polymorphic AFLP loci generated from 102 tetraploid individuals. Here only results for 44 individuals are shown, representing about 50% of the tree (Fig. 2; Manoko 2007), with S. umalilaense indicated as "sp A". Cluster I was made up of individuals morphologically similar but not identical (Manoko 2007). Part of the material conformed to S.tarderemotum forms A and B described by Olet (2004), and two remaining accessions were S. fl orulentum. Individuals that conformed to S. tarderemotum forms A and B in this cluster shared morphological characteristics not only with S. fl orulentum, but also with S. tarderemotum form C. Cluster II was composed of S. retrofl exum and a taxon that we identifi ed as S.  hirsutum (Manoko 2007). Both taxa have ovate calyx lobes, covering only half of the fruit during development, but refl exed away from mature fruits. Th e globose fruits, turning from green to purple/black, remain with the pedicel on the plant and are not aromatic. Cluster III is composed only of individuals of accession A24750133, which we describe here as the new species S. umalilaense (Fig. 2).
Ecology. During our collecting trip in the Umalila area (July 7-10, 2010) we found many plants in cultivated fi elds (Fig. 3E,F) on the slope of mountains, or left in abandoned cultivated plots (Fig. 3H), which also contained maize and beans.
According to the information on Gereau et al. 5084 the species is frequent on the ash layer in charcoal-burning areas. Th e Umalila area is located in the Mbeya region, in the South West of Tanzania at the border with Malawi and Zambia (Figure 3). Because of its elevation, the region is also known as the Southern Highlands of Tanzania, with volcanic type of soil, temperatures ranging from 12 to 23°C, and annual rainfall levels from 1500 to 2700 mm. Th e vegetation is mountainous, with cool temperature grasslands and the region is good for cultivation of coff ee, maize, beans and vegetables (Anon. 1997).
Etymology. Th e name S. umalilaense refers to the area the species was found. Umalila (Malilaor Umalila) is the highland area of the Malila, a relatively unknown ethnic group in south-west Tanzania (Walsh 1998).
Conservation status. Based on the information on the type specimen, S.umalilaense grows in the Umalila forest reserve. We visited the villages of Maganjo, Isangati, Igala, and diff erent wards and found that the species was cultivated as an important and well known leafy vegetable. It has been cultivated for many generations and farmers collect fruits and keep dried seeds for the next growing season. Since this species is cultivated as a food crop and has become almost domesticated, its preliminary conservation status can be considered to be of Least Concern (LC, IUCN 2001) although its very narrow distribution may be cause for further analysis.
Discussion. Solanum umalilaense diff ers from all other species that have been described or studied so far from Africa (Bitter 1912;1913;Bukenya and Hall 1988; D' Arcy and Rakotozafy 1994; Edmonds and Chweya, 1997;Bosser et al. 2000;Jacoby 2003;Olet 2004;Gonçalves 2005;Edmonds 2006 a, b). Th e new species produces a large number of infl orescences such that at full anthesis the plant appears to be covered with white fl owers, strikingly diff erent from other species in the section. Seeds of "species A", as it was provisionally labeled, were received from the Southern Highlands of Tanzania where this taxon is a common leafy vegetable. Morphologically, it could not be assigned to any known species, therefore Schippers (2004) had given it the name species A. Herbaria surveys at NHT and DSM in Tanzania produced comparable material, all collected from Umalila Forest Reserve in Mbeya by R.E. Gereau et al. in 1992 (R.E. Gereau, D.K. Harder, C.J. & M.J. Kayombo 5084, the DSM specimen of which we designate as holotype). Th ey had found these plants 'frequent on ash, in charcoal-burning area'. During our collection-trip in Tanzania in the same region, however, we always found this species in cultivated plots, never in ash. Gereau's material had been fi rst determined by the late W.G. D' Arcy (MO) in 1993 as S. aff . americanum Mill. Later, L.B. Mwasumbi (DSM) changed the name to S.nigrum L. Th ese names refer to a diploid and to a hexaploid species, respectively, whereas S. umalilaense is tetraploid. Th is clearly demonstrates that the suggested affi nities with S. americanum and S. nigrum were not correct.
Edmonds's opinion (JM Edmonds pers. comm.) that it might be a hybrid between two tetraploid species was investigated. We found that for the three generations we grew the plants (between 2002-2004) none of the characters showed segregation.
All together, the distinct morphology, chromosome number diff erent from S. americanum and S. nigrum, and a clear separate clustering from other species of the section, make this African species unique in section Solanum in Africa.