Revision of the Afro-Madagascan genus Costularia (Schoeneae, Cyperaceae): infrageneric relationships and species delimitation

Abstract

A recent molecular phylogenetic study revealed four distinct evolutionary lineages in the genus Costularia s.l. (Schoeneae, Cyperaceae, Poales). Two lineages are part of the Oreobolus clade of tribe Schoeneae: the first being a much-reduced genus Costularia s.s., and the second a lineage endemic to New Caledonia for which a new genus Chamaedendron was erected. The other two lineages were shown to be part of the Tricostularia clade of tribe Schoeneae. Based on morphological and molecular data, the genus Costularia is here redelimited to represent a monophyletic entity including 15 species, which is restricted in distribution to southeastern Africa (Malawi, Mozambique, South Africa, Swaziland, Zimbabwe), Madagascar, the Mascarenes (La Réunion, Mauritius), and the Seychelles (Mahé). Molecular phylogenetic data based on two nuclear markers (ETS, ITS) and a chloroplast marker (trnL-F) resolve the studied taxa as monophyletic where multiple accessions could be included (except for Costularia laxa and Costularia purpurea, which are now considered conspecific), and indicate that the genus dispersed once to Africa, twice to the Mascarenes, and once to the Seychelles. Two endemic species from Madagascar are here described and illustrated as new to science, as is one additional species endemic to La Réunion. Two taxa previously accepted as varieties of Costularia pantopoda are here recognised at species level (Costularia baronii and Costularia robusta). We provide a taxonomic revision including an identification key, species descriptions and illustrations, distribution maps and assessments of conservation status for all species.

A new classification of Cyperaceae (Poales) supported by phylogenomic data

“…Oreobolus Clade: Larridon et al (2018a) found that Costularia s.l. was composed of four distinct evolutionary lineages with two lineages being part of the Oreobolus Clade: (i) a much‐reduced genus Costularia (Larridon et al, 2019a) and (ii) a small New Caledonian endemic genus Chamaedendron . The circumscription of the other genera in this clade (Fig.…”

The evolutionary history of sedges (Cyperaceae) in Madagascar

“…Costularia includes 15 species, 11 of which are endemic to Madagascar (Larridon et al, 2018, Larridon et al, 2019). Most species grow in high elevation ericoid vegetation.…”

“…The Oreobolus Clade of tribe Schoeneae is the only clade with an important in situ radiation in Madagascar associated with a positive shift in diversification rates. The Oreobolus Clade includes Costularia which is largely endemic to Madagascar (Larridon et al., 2019).…”

“…Distribution data were obtained from Govaerts et al. (2020) and recent literature (Galán Díaz et al., 2019; Larridon et al., 2019). Biogeographical analyses were carried out in the R (R Core Team, 2019) package BioGeoBEARS (Matzke, 2013).…”

The genera of Cyperaceae of Madagascar

“…Although much taxonomic work remains at the species level, we have a good understanding of species diversity per genus and the geographic distribution of species (Jung et al 2016;Semmouri et al 2019;Muasya and Larridon 2021;Larridon 2022a). However, in the case of Madagascar, the only taxonomic treatment of the family, Chermezon's Flore de Madagascar, dates from 1937, although research has been done since then on a few genera (Díaz et al 2019;Larridon et al 2019), the taxonomic treatment of the Cyperaceae of Madagascar Table 1. Overview of the genera occurring in Madagascar and their subfamily and tribal placement following Larridon et al (2021b), and ordered according to Larridon (2022a).…”