Costularia subgenus Lophoschoenus (Stapf) Kük.

Larridon, I., Verboom, G. A. & Muasya, A. M., 2018, Revised delimitation of the genus Tetraria, nom. cons. prop. (Cyperaceae, tribe Schoeneae, Tricostularia clade) *, South African Journal of Botany 118, pp. 18-22 : 20

4. Taxonomic history of the species of Costularia subgenus Lophoschoenus

The first species of Costularia subgenus Lophoschoenus to be described was Schoenus arundinaceus Sol. ex Vahl from New Caledonia ( Vahl, 1805). In the following decades, this species was moved around from one genus to the other, i.e., it was placed in Chaetospora R.Br. and Cyclocampe Steud. , two genera placed in synonymy of Schoenus , and in Asterochaete Nees , a synonym of Carpha Banks & Sol. ex R.Br. It took over a century before the genus Lophoschoenus Stapf was described based on four sedge species, i.e., Lophoschoenus arundinaceus (Sol. ex Vahl) Stapf (basionym: Schoenus arundinaceus ), L. comosus (C.B.Clarke) Stapf (basionym: Schoenus comosus C.B.Clarke ), L. hornei (C.B.Clarke) Stapf (basionym: Schoenus hornei C.B.Clarke , nom. cons. prop.; Larridon et al., 2017b), and L. urvilleanus (Gaudich. ex Boeckeler) Stapf (basionym: Carpha urvilleana Gaudich. ex Boeckeler ) from New Caledonia, Borneo and the Seychelles ( Stapf and Turrill, 1914).

In 1939, Kükenthal united Lophoschoenus with Costularia C.B.Clarke , while upholding both names at subgeneric level and also describing a third subgenus Chamaedendron . In his study of the New Caledonian Costularia species, Raynal (1974) only maintained two subgenera ( Costularia and Lophoschoenus ), based on the arrangement of the leaves. A recent molecular phylogenetic study revealed a lack of monophyly for the genus Costularia ( Larridon et al., 2018) . Larridon et al. (2018) recognised four separate evolutionary lineages within the species previously placed in Costularia . They recircumscribed Costularia , with type species Costularia natalensis C.B.Clarke , to include only species from south-eastern Africa, Madagascar, the Mascarenes, and a single species from the Seychelles ( Costularia xipholepis (Baker) Henriette & Senterre ; Henriette et al., 2015). This meant that the species which form part of the three other lineages needed a new name. Larridon et al. (2018) elevated a slightly recircumscribed Costularia subgenus Chamaedendron to genus level (as the genus Chamaedendron Larridon ) including five species. This left the species previously placed in Costularia subgenus Lophoschoenus .

According to online species databases ( Govaerts et al., 2017; The International Plant Names Index, 2017; Tropicos.org, 2017), a total of 12 names have been published in Lophoschoenus by various authors ( Stapf and Turrill, 1914; Pfeiffer, 1927; Däniker, 1932 –33; Guillaumin, 1934 –35, 1957). One of these names conforms with a Costularia s.s. species ( Larridon et al., 2018), i.e., Lophoschoenus elongatus (Kunth) H.Pfeiff. (= Costularia melicoides (Poir.) C.B.Clarke ). Two further heterotypic names, i.e., Lophoschoenus falcifolius Däniker (1932 –33), and Costularia xyridioides (Däniker) Kük. ( Kükenthal, 1939) based on the basionym Schoenus xyridioides Däniker (1932 –33), conform to a single taxon recently placed in the newly described unrelated genus Chamaedendron ( Larridon et al., 2018) . More confusion surrounds the use of the epithet “ neocaledonicus ” in Costularia s.l. Rendle (1921) originally described Costularia neocaledonica Rendle , however, Clarke (1906) had already published a different species under the name Schoenus neocaledonicus C.B.Clarke. In 1927 , Pfeiffer included the wrong species into Lophoschoenus , i.e., Lophoschoenus neocaledonicus (C.B.Clarke) H.Pfeiff. Guillaumin (1957) later published a new species Lophoschoenus montis -fontium Guillaumin, which is now accepted as a heterotypic synonym of Rendle's species ( Govaerts et al., 2017). Despite these two names in Lophoschoenus, Larridon et al. (2018) placed this species in their new genus Chamaedendron .

According to Larridon et al. (2018), seven of the eight accepted species of Costularia subgenus Lophoschoenus , i.e. Costularia arundinacea (Sol. ex Vahl) Kük. , Costularia breviseta J.Raynal , Costularia comosa (C.B.Clarke) Kük. , Costularia pubescens J.Raynal , Costularia setacea J.Raynal , Costularia sylvestris J.Raynal , and Costularia stagnalis (Däniker) Kük. , form a monophyletic lineage together with the species of reticulate-sheathed Tetraria . Although the placement of the eighth species, i.e. Costularia pilisepala (Steud.) J.Kern (the only Costularia subgenus Lophoschoenus which is not endemic to New Caledonia but instead occurs in Borneo and New Guinea), could not be confirmed in their molecular study, this species shares with this group noded culms and a reticulate tunic surrounding the culm base. Kükenthal (1939) placed one further species in Costularia subgenus Lophoschoenus , i.e. Costularia hornei (C.B.Clarke) Kük. Larridon et al. (2018) showed that the latter species is not part of the reticulate-sheathed Tetraria clade, but is instead more closely related to the genera Morelotia Gaudich. and Tetrariopsis C.B.Clarke ( Fig. 1 ). Larridon et al. (2018) placed Costularia hornei in a new genus Xyroschoenus Larridon. The relationships between the species of Morelotia and Tetrariopsis need further study.

Figures

Fig. 1. Time-calibrated Bayesian phylogeny estimate inferred with BEAST showing tribe Schoeneae s.s. (adapted from Larridon et al., 2018). The six major clades and their approximate divergence times in millions of years are shown in A.Details as given of the Tricostularia clade, with the species previously placed in Epischoenus or Costularia highlighted in light grey.