Dismantling the genus Tarenna (Rubiaceae: Pavetteae) in Africa and Madagascar, with the description of three new genera.

De Block, Petra; Razafimandimbison, Sylvain G.; Rakotonasolo, Franck; Janssens, Steven B.

Dismantling the genus Tarenna (Rubiaceae: Pavetteae) in Africa and Madagascar, with the description of three new genera.

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De Block, Petra

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Razafimandimbison, Sylvain G.

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Rakotonasolo, Franck

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Janssens, Steven B.

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September 16, 2025

Abstract

As presently circumscribed, the Paleotropical genus Tarenna, belonging to the tribe Pavetteae (Rubiaceae), comprisesabout 200 species and has its centres of diversity in tropical Asia and Africa. Previous molecular phylogenetic studies of the triberevealed the polyphyly of Tarenna, suggesting that the genus needs to be recircumscribed. However, a robust phylogeny of Pavetteaeusing a sufficient sampling of Tarenna and a comparative assessment of the morphological variation of its members throughout theentire geographic range of the genus, needed for such a taxonomic adjustment, are currently lacking. In this study, a new phylogeny ofPavetteae was reconstructed based on four plastid markers (rps16, trnT-F, petD, accD-psa1) and two nuclear markers (nrITS, MADS-box PI). Our objectives were (1) to produce a robust molecular phylogeny based on a large sampling of Tarenna; (2) to undertakemorphological comparisons between the continental African, Madagascan and Asian/Australasian/Pacific Tarenna; and, (3) to recir-cumscribe Tarenna so that the genus is less polyphyletic and to transfer the segregated species to their correct placement within thetribe Pavetteae. Our results support the polyphyly of Tarenna as presently delimited, as its sampled species are resolved in five mor-phologically distinct and geographically segregated lineages: three restricted to continental Africa, one to Madagascar and the West-ern Indian Ocean islands and one including the type T. asiatica confined to Asia, Australasia and the Pacific. We propose a narrowcircumscription of Tarenna to include the Asian/Australasian/Pacific species but excluding the continental African and Madagascanspecies. Thirty-eight continental African Tarenna species are transferred to the hitherto small genus Cladoceras, which is character-ized by exotestal cells shaped like puzzle pieces in surface view (versus polygonal exotestal cells in Asian/Australasian/Pacific taxa).The new genus Alatostigma is described to accommodate four continental African species (A. agnata, A. funebris, A. hutchinsonii,A. jolinonii) characterized by longitudinally winged abaxial stigma surfaces and polygonal exotestal cells. The new genus Sonbridia,sister to the Paleotropical genus Pavetta, is established to accommodate two continental African species (S. bipindensis,S. warburgiana, the latter previously known as Tarenna precidantenna), characterized by short anthers born on long filaments,pedicels thickened below the fruit, and exotestal cells shaped like puzzle pieces in surface view. The species from Madagascar, theComoros and the Seychelles are transferred to the new genus Nesotarenna, characterized by lax inflorescences, long pedicels, brac-teoles borne low on the pedicels and polygonal exotestal cells. The new genera are described in detail and the necessary nomenclaturalchanges are made to transfer the continental African and Madagascan species to the new genera.