Novelties in Oxypetalum (Apocynaceae: Asclepiadoideae): a new species and revalidation of the name O. megapotamicum Liede-Schumann

Based on a specimen collected in Santa Catarina state, Brazil, the new species Oxypetalum kassneri is described and illustrated. Moreover, a molecular phylogenetic analysis allows to propose the revalidation of O. megapotamicum , a species currently placed in the genus Ditassa . The neotype of D. megapotamica and the lectotype of D. oxypetala are designated here.


Introduction
Oxypetalum R. Br. is the richest genus of Oxypetalinae (Apocynaceae, Asclepiadoideae, Asclepiadeae) with over 130 taxa distributed from Argentina to Mexico, and the Caribbean, but with a center of diversity in Argentina, Brazil, and Paraguay (Ezcurra et al. 2008;Farinaccio & Mello-Silva 2006;Fontella Pereira et al. 2004;Keller & Funez 2017). Although a twining habit is the predominant growth form, around 45% (ca. 60 taxa) are constituted of erect or creeping plants which typically inhabit open environments such as grasslands and marshes.
Out of the 87 species of Oxypetalum recorded in Brazil (Flora do Brasil 2020 under construction), Fontella  recorded 21 species of Oxypetalum R. Br. in Santa Catarina State, being only nine growing as erect plants. Analysis of a recently collected specimen from São Joaquim (SC), revealed an unprecedented combination of morphological characters among the erect species of the genus, allowing the description of a taxon new to science.
During the study of the Asclepiadoideae of Misiones, Argentina, the morphological evidenced lead to the conclusion that the species known as Ditassa megapotamica (Spreng.) Malme does not fits the concept of Ditassa R. Br. as currently understood in Konno & Fontella Pereira (2004). The basally cordate leaves and the reflexed, slenderly triangular and twisted, yellowish corolla lobes of D. megapotamica are not found elsewhere in Ditassa, but are typical of Oxypetalum, the genus under which the species was firstly described (Sprengel 1827). The availability of a molecular sample allowed to reassess the generic affiliation of the species, confirming the morphological interpretations and the name Oxypetalum megapotamicum Spreng. is restablished and neotypified here. The conspecifity of D. oxypetala Decne., established by Malme (1911) is confirmed and a lectotype for the name is designated here.

Materials and Methods
A high resolution digital camera and stereoscopic microscope were used to obtain images and to study the material of O. kassneri H.A. Keller & Funez and to compile the illustrations.

A new Species from Brazil
The epithet is dedicated to the enthusiastic naturalist and botanist Anderson Kassner-Filho (Inventário Floristíco Forestal de Santa Catarina), who collected the holotype of this species.
Only known from the type locality, in São Joaquim municipality, Santa Catarina state, southern Brazil. This species inhabits open grasslands with rocky soil, growing between rocky outcrops. The presence of cattle and recent fire was observed in the locality. The high altitudinal grasslands of "Aparados da Serra Geral" region, between Santa Catarina and Rio Grande do Sul states, are extremely rich in vascular plant endemics (Hassemer et al. 2015). However, they are rapidly giving way to destructive livestock, Rodriguésia 72: e01692019. 2021  agriculture, and silviculture practices. The lack of legal protection in most of this region is a crucial aspect that, if not urgently changed, will lead to the extinction of some of the endemic species.
The only collection of this species shows floral buds, flowers and fruits during the summer, in February.
Some erect Oxypetalum R. Br. species have prominent corona lobes that completely hide the body of the gynostegium, but reveal part of the style-head appendage, which protrudes above the apex of the corona lobes. From the species with such characteristics, the one that most closely resembles O. kassneri is O. crispum, as both species possess broad leaves, manyflowered, long-pedunculate inflorescences, toothed caudicles and a style-head appendage divided at the end into two connivent processes.  Iconographies and extended descriptions were observed in Fournier (1885: 251, tab. LXXI fig. 1), Meyer (1944: tab. LXXV), Fontella Pereira et al. (2004: 155-156, Plate 38).
Oxypetalum megapotamicum was described on samples not specified by Sprengel (1827), but whose location corresponds to "Rio Grande, Brazil". However, the only specimens from this area still extant are Gaudichaud 698 ([P00252619], P) and 699 ([P00252620], P), that were later used by Decaisne (1844) to describe Ditassa oxypetala Decne., which is lectotypified here on the the specimen [P00252620] bearing a handwritten description. Therefore, the specimen [NY00279078], that bears an original stamp of "Mus. bot. Berol.", the herbarium in Berlin (B), and might have been seen by Sprengel, in whose collection Sellow specimens were found (fide Stafleu & Cowan 1976-1988, online edition: <https://www.sil.si.edu/DigitalCollections/tl-2/ browse.cfm?vol=5#page/820>), is selected as neotype here. A duplicate of this specimen is housed in K, [K000095572], but it only bears the label, not the stamp of B, and is therefore regarded as an isotype.
Some decades later, Malme (1911: 270) proposed the combination Ditassa megapotamica (Spreng.) Malme. and put Ditassa oxypetala Decne. into synonymy. A sequenced DNA sample now documented that the species is to be placed in Oxypetalum R. Br. (Fig. 3); so here we propose to restore the name originally established by Sprengel (1827).
The absence of style-head appendages is rare in the genus Oxypetalum R. Br. and has been used as specific epithets such as O. muticum E. Fourn. or to the conformation of other recently synonymized genera such as Rhyssostelma Decne. (Liede-Schumann & Meve 2015). However, in the resulting phylogenetic tree (Fig. 3), O. megapotamicum forms a well supported group with O. sylvestris Hook. & Arn. and O. pentasetum (Rusby) Goyder & Rapini. These species were included in Schistogyne Hook. & Arn., a genus of eight species now combined in Oxypetalum by Rapini et al. (2011). Schistogyne is characterized by stigmatic appendages with 5-7 filiform branches and untoothed caudicles (Ezcurra et al. 2008;Fontella Pereira et al. 2004;Hechem & Ezcurra 2006;Meyer 1944) so that the absence of stigmatic appendages in O. megapotamicum is possibly due to a reversal of the apomorphic filiform branches of the style-head.