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A focus on Pycreus and its laterally flattened pistils: the challenging taxonomy and evolution of C4 Cyperus (Cyperaceae)

Book - Dissertation

Cypereae form one of the largest and most complex tribes of the sedge family (Cyperaceae). Recently, two clades have been revealed within the tribe, the largest of which includes the giant genus Cyperus and its closest allies. However, thirteen genera of the generally accepted classification of Goetghebeur (1998) appear to be nested within Cyperus. The taxonomic status of many of these taxa has been under discussion since they are based on different combinations of a limited set of derived characters. Pycreus, the largest of these segregate genera, is characterised by laterally compressed dimerous pistils of which the derivation from the general trimerous situation was not yet understood. It shares this pistil with Kyllinga and Queenslandiella that both are, as is Pycreus, embedded in the Cyperus clade which uses C4 photosynthesis. The recent insights from molecular phylogenetics make a reevaluation possible of the taxonomic status of the thirteen different segregate genera of Cyperus and of the taxonomic value of the characteristics that have been used to delimitate these taxa. This is currently tackled in a joint international research effort, using a combination of molecular phylogenetics, ontogeny, anatomy and morphology, to understand evolutionary patterns in Cyperaceae and to build a modern classification of sedges. This research strategy is situated on three taxonomic levels: family to tribal level (macro-scale), tribal to generic level (meso-scale) and infrageneric level (micro-scale). The current thesis is embedded in this international research context and focusses mainly on meso-scale objectives (C4 Cyperus and the position and taxonomic state of its segregate genera, including Pycreus) and micro-scale objectives (the infrageneric taxonomy of Pycreus). At first, a complete nomenclatural survey is presented of all generic and subdivisional names that have been published for the taxa now included in the Cyperus clade (around 350 names), along with an evaluation of their validity, legitimacy and priority. Types are indicated and where necessary lectotypes are designated. This nomenclatural survey serves as a base for the selection of representative taxa in the molecular, ontogenetic, anatomical and morphological studies. In addition it forms an essential tool when building a modern revision for the clade. In the current thesis only names for taxa in which Pycreus species have been placed are included. Next, to be able to reevaluate the taxonomical value of derived pistils in the Cyperus clade, especially the laterally compressed dimerous pistils of Pycreus, an elaborate ontogenetic study of Pycreus and Cyperus species was performed. This study shows that both taxa follow the general ontogenetic patterns of spikelets and flowers found throughout Cyperoideae. In addition, the ontogeny and anatomy of the different types of pistils was reviewed with addition of new ontogenetic and anatomical data. These demonstrate that in Cyperoideae the pistil wall starts from an annular primordium (which evolved from congenitally fused carpels) on top of which the stigma primordia develop. The development of the central ovule is decoupled from the ovary wall development. Vascular patterns follow the development of the primordia and vascular bundles are formed where necessary. The presence of an annular gynoecial wall primordium appears to have opened new possibilities for the development of the stigma primordia in new positions independent from the constraints of individual carpels. An elaborate molecular phylogenetic study was performed on the C4 Cyperus clade using ETS1f, rpl32-trnL, trnH-psbA. Although relationships within the C4 Cyperus clade are still largely unresolved in a large polytomy, early emerging branches show better resolution than in previous studies. Pycreus appears to be para- or polyphyletic and in addition no relationships have been found between Pycreus, Kyllinga and Queenslandiella. Therefore, we have to admit, laterally compressed dimerous pistils have most likely originated multiple times in the clade. Subsequently, the most appropriate classification strategy for these taxa is sinking them into Cyperus. This also seems to be the most appropriate strategy for all other segregate genera based on a reevaluation of the taxonomical value of their key characters. Only for the C4 Cyperus clade (accommodated in C. subgenus Cyperus), which is nested within a grade of species using C3 photosynthesis (accommodated in C. subgenus Anosporum), an evolutionary classification strategy has been adopted. This is based on the evolutionary value of the origin of C4 photosynthesis which had led to a major radiation of species. On the micro-scale, it is not yet possible to present a modern classification for Pycreus since molecular phylogenetic relationships are largely unresolved. Therefore, results are presented as several case studies. First, in an elaborate SEM study, the taxonomical value of the nutlet epidermis was reevaluated. Next, the reestablishment of P. sect. Tuberculati is discussed. Finally, the new classification strategy for the Cyperus clade was applied on Pycreus and necessary combinations and nomina nova under Cyperus are listed along with some critical notes on synonymisations of several taxa.
Publication year:2013