A molecular phylogeny and new subgeneric classification of Sporobolus (Poaceae: Chloridoideae: Sporobolinae)

The grass stibtribe Sporobolinae contains six genera: Calamovilfa (5 spp. endemic to North America), Crypsis (10 spp. endemic to Asia and Africa), Psilolemma (1 sp. endemic to Africa), Spartina (17 spp. centered in North America), Sporobolus (186 spp. distributed worldwide), and Thellungia (1 sp. endemic to Australia). Most species in this subtribe have spikelets with a single floret, 1-veined (occasionally 3 or more) lemmas, a ciliate membrane or line of hairs for a ligule, and fruits with free pericarps (modified caryopses). Phylogenetic analyses were conducted on 177 species (281 samples), of which 145 species were in the Sporobolinae, using sequence data from four plastid regions (rp132-trnL spacer, ndhA intron, rps16-trnK spacer, rps16 intron) and the nuclear ribosomal internal transcribed spacer regions (ITS) to infer evolutionary relationships and provide an evolutionary framework on which to revise the classification. The phylogenetic analysis provides weak to moderate support for a paraphyletic Sporobolus that includes Calamovilfa, Crypsis, Spartina, and Thellungia. In the combined plastid tree, Psilolemma jaegeri is sister to a trichotomy that includes an unsupported Urochondra-Zoysia clade (subtr. Zoysiinae), a strongly supported Sporobolus somalensis lineage, and a weakly supported Sporobolus s.l. lineage. In the ITS tree the Zoysiinae is sister to a highly supported Sporobolinae in which a Psilolemmajaegeri Sporobolus somalensis clade is sister to the remaining species of Sporobolus s.l. Within Sporobolus s.l. the nuclear and plastid analyses identify the same 16 major clades of which 11 are strongly supported in the ITS tree and 12 are strongly supported in the combined plastid tree. The positions of three of these clades representing proposed sections Crypsis, Fimbriatae, and Triachyrum are discordant in the nuclear and plastid trees, indicating their origins may involve hybridization. Seven species fall outside the major clades in both trees, and the placement of ten species of Sporobolus are discordant in the nuclear and plastid trees. We propose incorporating Calamovilfa, Crypsis, Spartina, Thellungia, and Eragrostis megalosperma within Sporobolus, and make the requisite 35 new combinations or new names. The molecular results support the recognition of 11 sections and 11 subsections within Sporobolus s.1.; four sections are new: Airoides, Clandestini, Cryptandri, and Pyramidati; three sections are new combinations: Calamovilfa, Crypsis, and Spartina; four subsections are new combinations: Calamovilfa, Crypsis, Ponceletia, and Spartina; seven subsections are new: Actinocladi, Alterniflori, Floridani, Helvoli, Pyramidati, Spicati, and Subulati; 30 new combinations in Sporobolus: S. aculeatus, S. advenus, S. alopecuroides, S. alterniflorus, S. angelicus, S. arcuatus, S. bakeri, S. borszczowii subsp. acuminatus, S. borszczowii subsp. ambiguus, S. brevipilis, S. coarctatus, S. cynosuroides, S. densiflorus, S. factorovskyi, S. foliosus, S. hadjikyriakou, S. xlongispinus, S. maritimus, S. megalospermus, S. michauxianus, S. minuartioides, S. niliacus, S. pumilus, S. rigidus, S. rigidus var. magnus, S. spartinus, S. schoenoides, S. xtownsendii, S. hirkestanicus, and S. vericolor; and five new names in Sporobolus: S. arenicola, S. xeatonianus, S. hookerianus, S. mobberleyanus, and S. vaseyi are made. Lectotypes are designated for Crypsis factorovskyi, Heleochloa ambigua, and Torgesia minuartio ides.