Interspecific hybridization between closely related species is considered inevitable even between good biological species which by definition are supposed to be biologically isolated from other such species (Grant 1981; Paris et al. 1989). This occur because it takes a long period of time for species to be completely isolated after speciation process and before the period of completeness the boundaries between related species may be semipemeable or porous (Lorenz-Lemke et al. 2006; Harrison and Larson 2014). Semipemeable boundaries allow gene flow and introgression and have been reported in plants, animals and insects (Paris et la. 1989; Grzebelus et al. 200; Besansky 2003; Uthicke et al. 2005; Van droogenbroeck et al. 2006; Zhou et al. 2010, Nadeau et al. 2013; Harrison and Larson 2014; Lopez-Camal and Tovar-Sanchez 2014; Zhao et al. 2014; Rodionov et al. 2019; Turchetto et al. 2019; Valencia-Montoya et al. 2019). Interspecific hybrids are common in the wild, occurring in 40% of families and 16% of genera, with an overall frequency of 0.09 hybrids per nonhybrid species (Whitney et al. 2010). Mallet (2005) estimated that at least 25% of plant species, especially the younger ones in evolutionary terms, hybridize readily involving possibility of introgression. Hybrid is one of the major forces of speciation (Grant 1981, Riserberg 1997; Arnold 2006; Soltis and Soltis 2009; Rodionov et al. 2019).
Evolutionary speaking, as it is for incomplete lineage separation, interspecific hybridization makes delimitation of species complicated not only when morphological data are used but even when gene trees such as mitchondrial genes or barcodes are used (Hickerson et al. 2006; Naciri and Linder 2015; Nicholls et al. 2012; Dupont 2016; Johnston et al. 2017; McFadden et al. 2017). It is on this basis that hybridization is considered one of the seven process that acts as veils to obscure species boundaries and relationships (Naciri and Linder 2015).
Confused species delimitation is one of the major isues at species level taxonomy. Carmago and Sites (2013) indicates that 227 articles on species delimitation were published since 2000 of which 60% got published after 2008. Confused species delimitation is reported in different groups in the genus Solanum (Spooner and vanden berg 1992; Spooner 2016; Tu 2018). Interspecific hybridization has been mentioned and hypothesized repeatedly to be one of the major causes of taxonomic complexity in the section Solanum also known as Solanum nigrum complex (Edmonds 1972; Edmonds and Chweya, 1997; Jacoby 2003; Dehmer and Hammer 2004; Särkinen et al. 2018) but exist limited systematic investigation on the same. This study was designed to assess whether or not hybrids confused the taxonomy of two lesser known African species species; Solanum tarderemotum Bitter and S. florulentum Bitter.
Solanum terderemotum Bitt. and S. florulentum Bitt. confuusion
Solanum terderemotum Bitt. and S. florulentum Bitt. Are lesser known African species that fall in the section Solanum which in the most recent treatment lies within the Morelloid clade (Bohs 2005; Weese and Bohs 2007). The boundaries of the formerly two species have been changed significantly through history. Solanum tarderemotum Bitter and S. florulentum Bitter were all published in Feddes Repertorium by Bitter in 1912 each based on a type collected from Tanzania. These taxa has been in DR Congo, Rwanda, Burundi, Ethiopia, Kenya, Uganda and Tanzania (Mwai and Schippers 2004). Their Swahili name is Mnafu. Edmonds and Chweya (1997), recognized them as separate species placing them in the list of species that required revision. Olet (2004) recognizes three morphological forms of S. tarderemotum (forms A, B and C). Form A and B made up of sub-glabrous to glabrous individuals with either simple or forked inflorescences on same individual but differing only on the prominence of the wings on the stem. Form C was said to be moderately pilose only with simple inflorescences. Based on crossing experiment Olet (2004) concluded that S. florulentum and S. tarderemotum were separate species but her conclusion was not supported with AFLP markers. Solanum tarderemotum has been nicknamed S. eldorettii (sometime as S. eldoretium) in East Africa but this name was never published officially (Schippers 2004; Mwai and Schippers, 2004).
Using AFLP markers (Manoko 2007) proposed that S. tarderemotum and S. florulentum were two separate species yet S. florulentum individuals were found to be embedded within individuals that Olet (2004) named S. tarderemotum forms A and B. Basing on crossing experiments, Manoko (2007) proposed that individuals with simple and branched inflorescence on the same individual were most likely hybrids of the two Bitter species; S. tarderemotum and S. florulentum. This proposition was however limited with the number of crosses performed. The three forms of S. tarderemotum are not reported neither in Flora Zambesiaca (Edmonds 2005) nor in Flora Ethiopia and Eritrea (Edmonds 2006) where S. tarderemotum is described but not S. florulentum. In these two Flora the description of S. tarderemotum included only individuals with simple inflorescence.
In Flora of Tropical East Africa (FTEA) Edmonds (2012) recognized both S. tarderemotum and S. florulentum however the description of each of the two taxa covered individuals with branched and simple inflorescence on the same stem for each of the two species. Särkinen et al. (2018), reduced S. florulentum to a synonym of S. tarderemotum Bitter expanding the description not only to cover the two former Bitter species but also the three forms of S. tarderemotum recognized by Olet in 2004. This historical event suggest existence of taxonomic iissues that is yet to be resokved. Clear delimitation of species is not only important for safer utilization of a species but also for conservation of biodiversity planning (Cracraft 2002).
In the present study, a wider sampling was done and AFLP markers used because of their potent in identifying hybrids and hybridization in complex groups (Bensch et al. 2002; Gobert 2002, Pooler et al. 2002; Joly and Bruneau 2007; Gosh et al. 2011; Schnittler et al. 2019; Vela et al. 2011). AFLP also have been used to delimit other good species in the section Solanum without problems (Jacoby et al. 2003; Dehmer and Hammer 2004, Manoko et al. 2007, Manoko et al. 2012). AFLP markers show species specific bands, have successfully been used to study recent evolution and its amplicons are highly conserved (Santos and Simon 2002; Gili et al. 2004, Reck-Kortmann et al. 2017). AFLPs markers are arbitrary spread throughout the genome and in closely related groups, co-migrating bands homologous (Waugh et al. 1997; Radmaker et al. 2000). In some studies results obtained from AFLP markers were comaprable to those obtained from gene trees (Weber et al. 2017).
In the present study, clustering pattern of individuals from disfferent accessins based on AFLP markers were asses, Number of fragment shared beteeen clusters and their percentage and pattern of segregetaion between memebers of the same accession. At morphological level only character states (simple, branched and mixed i.e. simple and branched on same stem) on the inflorescence were recorded. After AFLP analtysis these character atates were mapped printed on the AFLP tree.