Phylogeny of the Carolinense Clade of Solanum (Solanaceae) Inferred from Nuclear and Plastid DNA Sequences

The large and economically important genus Solanum contains ca. 1,400 species distributed worldwide. One of the 12–14 major clades identified in the genus is the Leptostemonum clade, or the “spiny solanums.” Previous molecular phylogenetic studies have identified 14 major clades in the spiny solanums. Among these is the Carolinense clade, which contains Solanum carolinense, the type species of S. subsect. Lathyrocarpum. However, previous results have suggested that the species composition of the Carolinense clade is only partially congruent with taxonomic circumscriptions of S. subsect. Lathyrocarpum. The main goal of this study was to increase taxon sampling over previous molecular phylogenies in order to better understand the composition of the Carolinense clade. We newly sequenced 31 accessions of 17 taxa putatively belonging to the clade for two nuclear (ITS, waxy) and one plastid gene region (trnT–trnF) and combined these with previously generated molecular data. Phylogenetic analyses of separate and combined DNA matrices were done using maximum parsimony and Bayesian methods. Results from analyses of the combined nuclear and plastid data set showed 10 taxa to be resolved within a wellsupported Carolinense clade. However, three species, Solanum dimidiatum, S. perplexum, and S. hieronymi, showed conflicting placements in phylogenies based on analyses of nuclear vs. plastid data sets, suggesting a history of allopolyploidy or introgression. Within the Carolinense clade, the North and South American species were both recovered as well-supported clades, implying a single dispersal event from South to North America. Keywords—Allopolyploidy, granule-bound starch synthase (GBSSI), hybridization, phylogeny, Solanum subsect. Lathyrocarpum, Solanum carolinense. The genus Solanum L. (Solanaceae), with a worldwide distribution and ca. 1,400 species, is among the largest genera of flowering plants and contains several economically important species (Bohs 2005). Previous molecular phylogenetic studies in Solanum have identified 12–14 major clades in the genus (Bohs 2005; Weese and Bohs 2007), including the large Leptostemonum clade, which corresponds closely to Solanum subg. Leptostemonum (Dunal) Bitter. Solanum subg. Leptostemonum, often referred to as the “spiny solanums,” consists of 350–450 species distributed worldwide and includes the cultivated eggplant, S. melongena L. Species placed in the subgenus typically share the unique combination of stellate hairs, prickles, and tapered anthers (Whalen 1984). Whalen (1984) and Nee (1999) provided synopses of infrageneric groupings in Solanum subg. Leptostemonum based on morphological characters. Whalen (1984) proposed 33 informal species groups within the subgenus and left 36 species unplaced. He included seven South American species in the Solanum multispinum species group (Table 1), characterized by erect or prostrate rhizomatous plants with abundant needle-like prickles, usually densely spiny calyces, white to violet corollas, a weakly andromonoecious reproductive system, and glabrous, smallto medium-sized green to yellow fruits. Among the North American spiny solanums, he left S. carolinense (the type species of S. subsect. Lathyrocarpum G. Don) and S. dimidiatum unplaced, although they both exhibit most of the morphological characters used to define the S. multispinum species group. Solanum pumilum was not included in his classification, and two other taxa were treated as synonyms: S. carolinense var. floridanum under var. carolinense, and S. perplexum under S. dimidiatum (Whalen 1984). Nee (1999) treated the New World species of spiny solanums, placing them into 10 sections. In Solanum sect. Melongena (Mill.) Dunal subsect. Lathyrocarpum, Nee (1999) included 17 described and two undescribed species that he distributed into four series [see Table 1; all three species of Nee’s series 4 were found to belong to the Elaeagnifolium clade (Levin et al. 2006; Stern et al. 2011) and are not listed]. Series 1 through 3 of Nee’s S. subsect. Lathyrocarpum included all the taxa of Whalen’s S. multispinum group, three species not treated by Whalen (S. mortonii, S. pumilum, and S. moxosense), four species Whalen left unplaced (S. carolinense, S. chamaeacanthum, S. dimidiatum, S. euacanthum), and S. hasslerianum, which was placed in Whalen’s S. polytrichum species group. Recent phylogenetic studies have aimed to identify and to delimit monophyletic groups within Solanum subg. Leptostemonum and to compare them with the species groups and infrageneric taxa proposed by Whalen (1984) and Nee (1999), respectively. Levin et al. (2006) provided the first large-scale molecular phylogeny of the spiny solanums based on data from three molecular markers. They identified 10 major wellsupported clades within the spiny solanums, one of which was the Carolinense clade. In their sampling scheme, the clade consisted of the North American S. carolinense and two South American species, S. comptum and S. conditum (the latter now known as S. aridum). The Carolinense clade was also recovered in the expanded phylogeny of spiny solanums by Stern et al. (2011) using slightly different molecular markers and additional taxon sampling. In their study, the Carolinense clade was composed of S. carolinense and four South American species: S. aridum, S. comptum, S. juvenale, and S. moxosense. Levin et al. (2006) and Stern et al. (2011) also sampled single accessions of S. hieronymi and S. multispinum from Whalen’s S. multispinum group, but in both molecular studies these species fell outside the Carolinense clade. Both studies also recovered the three species of Nee’s series 4 (S. elaeagnifolium, S. hindsianum, S. tridynamum) in a strongly supported Elaeagnifolium clade. Throughout this paper, the infrageneric taxon Solanum subsect. Lathyrocarpum is used in reference to the most recent