Phylogenetic studies favour the unification of Pennisetum, Cenchrus and Odontelytrum (Poaceae): a combined nuclear, plastid and morphological analysis, and nomenclatural combinations in Cenchrus

Abstract

Backgrounds and aims: Twenty-five genera having sterile inflorescence branches were recognized as the bristle clade within the x = 9 Paniceae (Panicoideae). Within the bristle clade, taxonomic circumscription of Cenchrus (20-25 species), Pennisetum (80-140) and the monotypic Odontelytrum is still unclear. Several criteria have been applied to characterize Cenchrus and Pennisetum, but none of these has proved satisfactory as the diagnostic characters, such as fusion of bristles in the inflorescences, show continuous variation.

Methods: A phylogenetic analysis based on morphological, plastid (trnL-F, ndhF) and nuclear (knotted) data is presented for a representative species sampling of the genera. All analyses were conducted under parsimony, using heuristic searches with TBR branch swapping. Branch support was assessed with parsimony jackknifing.

Key results: Based on plastid and morphological data, Pennisetum, Cenchrus and Odontelytrum were supported as a monophyletic group: the PCO clade. Only one section of Pennisetum (Brevivalvula) was supported as monophyletic. The position of P. lanatum differed among data partitions, although the combined plastid and morphology and nuclear analyses showed this species to be a member of the PCO clade. The basic chromosome number x = 9 was found to be plesiomorphic, and x = 5, 7, 8, 10 and 17 were derived states. The nuclear phylogenetic analysis revealed a reticulate pattern of relationships among Pennisetum and Cenchrus, suggesting that there are at least three different genomes. Because apomixis can be transferred among species through hybridization, its history most likely reflects crossing relationships, rather than multiple independent appearances.

Conclusions: Due to the consistency between the present results and different phylogenetic hypotheses (including morphological, developmental and multilocus approaches), and the high support found for the PCO clade, also including the type species of the three genera, we propose unification of Pennisetum, Cenchrus and Odontelytrum. Species of Pennisetum and Odontelytrum are here transferred into Cenchrus, which has priority. Sixty-six new combinations are made here.

Fig. 1.

Phylogenetic relationships of Pennisetum, Cenchrus and Odontelytrum. (A) Strict consensus tree of 2316 MPTs from the combined analysis of the plastid markers (ndhF + trnL-F). (B) Strict consensus tree of the 3264 MPTs from the combined analysis (ndhF + trnL-F + cytology and morphology). Numbers below branches represent jackknife branch support. Optimization of the basic chromosome number (Appendix 2, character 1) is shown above the branches and, when necessary, next to the species names. Optimization of the degree of fusion of the bristles (Appendix 2, character 21) is shown as follows: black and white oval, not connate; grey oval, connate below; white oval, connate up to half the total length; black oval, connate up to two-thirds the total length. Bars represent principal clades as discussed in the text. Pennisetum glaucum 1 corresponds to voucher PI 326520 (sequences downloaded from GenBank) and P. glaucum 2 to voucher Caxambu 375 (see Appendix 1).

Fig. 2.

A single tree depicting relative branch lengths for one of the 2316 MPTs based on a combined analysis using two plastid markers: ndhF and trnL-F.

Fig. 3.

A single tree depicting relative branch lengths for one of the 30 000 MPTs based on the nuclear marker knotted1.

Fig. 4.

Strict consensus tree of 30 000 MPTs obtained from parsimony analysis of the nuclear marker knotted1. Numbers below the branches represent jackknife branch support. Letters following the species names represent different clones; letters in bold represent the reported ploidy/ies: Di, diploid; An, aneuploid; Po, polyploid. Bars represent principal clades as discussed in the text. Pennisetum glaucum 1 corresponds to voucher PI 326520 (sequences downloaded from GenBank) and P. glaucum 2 to voucher Caxambu 375 (see Appendix 1).