Molecular phylogeny of African Rytidosperma–affiliated danthonioid grasses reveals generic polyphyly and convergent evolution in spikelet morphology

Abstract

Of the 90–95 species included in the Rytidosperma clade of grasses (Danthonioideae: Poaceae) about 20 are native to southern Africa, most of these belonging to the genera Karroochloa, Schismus and Tribolium. Whereas morphological data identify all three genera as being monophyletic, the robustness of this pattern is compromised by high levels of homoplasy. Here we use molecular data (plastid trnL–trnF and nuclear ITS sequences) to re–evaluate phylogenetic relationships within the Rytidosperma clade, with the specific aim of testing the monophyly of the African genera. Separate parsimony analyses of the two partitions reveal low levels of incongruence, which may be attributable to incomplete lineage sorting or past hybridization. Analyses of the combined data produce well resolved, strongly supported topologies that are virtually identical, regardless of the inference method used (parsimony or Bayesian inference) or whether or not "conflict" taxa are included. Our data support the monophyly of the Rytidosperma clade (synapomorphies include a punctate–ovate hilum, a large embryo, and lodicule microhairs) as well as the monophyly of the Australasian–South American Rytidosperma s.l. However, the monophyly of Karroochloa, Schismus and Tribolium is strongly contradicted, all three genera being polyphyletic as currently defined. Although not implemented here, a minimum of five new combinations would be required to rectify this situation. Character reconstructions reveal extensive convergence in a number of morphological features, especially those previously used to define genera. We suggest that the convergent pattern of evolution displayed by these traits, most of which describe spikelet/diaspore morphology, is linked to their functionality in seed dispersal and burial, and that this has compromised their utility as phylogenetic markers.

A Generic Classification of the Danthonioideae (Poaceae)1

“…However, the present delimitation of the clade dates from the early molecular phylogenetic work of Barker et al (1995). At the same time, Verboom et al (1994) were able to show that the clade could also be delimited by the possession of haustorial synergids. The subfamily was formally erected by the Grass Phylogeny Working Group (2001).…”

A plastid tree can bring order to the chaotic generic taxonomy of Rytidosperma Steud. s.l. (Poaceae)

“…We sampled globally, expanding on the existing datasets of Barker et al (2000Barker et al ( , 2003Barker et al ( , 2007, Verboom et al (2006) and Pirie et al (2008). Main obstacles to achieving complete taxon sampling were geographical inaccessibility and rarity or possible extinction of species.…”

“…We filled in the gaps in the datasets used by Verboom et al (2006) and Pirie et al (2008) for the non-coding regions atpB-rbcL, the rpl16 intron, trnL-trnF and protein coding regions ndhF, matK and rbcL of chloroplast DNA (cpDNA) and ITS of nuclear ribosomal DNA (nrDNA) (Appendix A). With the aim to improve resolution within the Rytidosperma clade we sampled two further non-coding cpDNA regions: trnT-trnL, which has been shown to be useful at the species level in the Danthonioideae and trnD-psbM-ycf6-trnC, which has been recommended for use at the species level based on trials on diverse plant groups (Shaw et al, 2005) (Appendix A).…”

“…into genera would not be straightforward. Verboom et al (2006) discussed the implications of recognising the entire Rytidosperma clade at generic rank. This large genus would be defined by having a punctate-ovate hilum, a synapomorphy for the clade.…”

“…There is an increasing trend toward the use of nucleotide sequence data in taxonomic studies at the generic rank . Danthonioideae is no exception: a string of recent molecular phylogenetic studies have addressed generic delimitation of certain groups (Cortaderia: Barker et al, 2003; African members of the Rytidosperma clade: Verboom et al, 2006; Pentaschistis and allies: Galley and Linder, 2007) or have assessed generic limits in the subfamily as a whole (Pirie et al, 2008). However, none of these studies has provided resolution of the Rytidosperma clade that is informative enough to serve as a guideline for a generic classification.…”