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

Abstract

The large and economically important genus Solanum contains ca. 1,400 species distributed worldwide. One of the major clades identified in the genus is the Leptostemonum clade, or 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 well-supported 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.