A molecular phylogeny of Pennisetum and Cenchrus (Poaceae) based on the trnL-F, rpl16 chloroplast markers

DONADÍO, S.; GIUSSANI, L. M.; KELLOGG, E. A.; MORRONE, O.

San Isidro, Buenos Aires

Congreso; VII Reunión Argentina de Cladística y Biogeografía; 2007

Instituto de Botánica Darwinion

Pennisetum Rich. (80-140 species) and Cenchrus L. (16-22 species) are very closely related genera of Paniceae, both concentrated in the tropics and subtropics of the world (Clayton & Renvoize, 1986). The main difference between these genera lies on their involucral bristles: in Cenchrus the bristles always show some degree of fusion at the base, while in Pennisetum the bristles are usually free (DeLisle, 1963; Clayton & Renvoize, 1986; Crins, 1991). Moreover, the involucral bristles in Cenchrus are commonly flat, stiff, spiny and retrorsely barbed, while Pennisetum exhibits filiform, and antrorsely scabrous bristles (DeLisle, 1963; Clayton & Renvoize, 1986; Wipff, 2003). However, Pennisetum and Cenchrus are not sharply separated due to a morphological gradation between them, with species difficult to include in a definite genus (DeLisle, 1963; Clayton & Renvoize, 1986; Crins, 1991, Wipff, 2001). No satisfactory modern treatment is available for Pennisetum as the revisions made by several authors differ on the circumscription of sections and subsections (Stapf & Hubbard, 1934; Roshevits, 1937; Brunken, 1977; Clayton & Renvoize, 1986). DNA sequence data from two non-coding chloroplast markers (rpl16 and trnL-F region) were analyzed to test Pennisetum and Cenchrus monophyly and the relationships between both genera; infrageneric classifications were also evaluated. Phylogenetic analyses of individual genomic regions (trnL-F & rpl16), and a combined dataset were analyzed under maximum parsimony using TNT (Goloboff et al., 2003). Uninformative characters were removed prior to all analyses and informative characters were considered unordered and equally weighted. Informative gaps corresponding to indels were treated as missing data, although they were coded as binary characters following the "simple indel coding" method (Simmons & Ochoterena, 2000). Jackknife and Bremer support were used to estimate branch support. The ingroup included twenty species of Pennisetum and seven species of Cenchrus. The outgroup included a total of ten species of subfamily Panicoideae: six species belonging to the "Bristle Clade"; three species of the Paniceae x = 9 clade, and one species of the tribe Andropogoneae. One species of subfamily Centothecoideae was used as root for the analyses. Geographical, chromosomal and morphological characters of taxonomic interest were optimized onto the molecular phylogeny. In agreement with previous studies (Gómez-Martínez & Culham, 2000; Zuloaga et al., 2000; Duvall et al., 2001; Giussani et al., 2001; Doust & Kellogg, 2002; Doust et al., 2007), our results suggest that all species of Pennisetum and Cenchrus belong to the "Bristle Clade", although none of them are monophyletic. Within the Bristle clade, Pennisetum and Cenchrus form, with the exception of P. lanatum, a well-supported group. Pennisetum is revealed as a polyphyletic genus because it includes species of Cenchrus. The Pennisetum-Cenchrus clade showed an independent reduction (x = 5, 7, 8) or duplication (x = 17) of the basic chromosome number from the ancestral x = 9. Within Pennisetum, sections Pennisetum and Gymnotrix are polyphyletic.