Hemipogon trilobatus sp. nov. (Apocynaceae: Asclepiadoideae), a new microendemic from Chapada dos Veadeiros, Central Brazil

Hemipogon s. str. (Apocynaceae: Asclepiadoideae) currently consists of three species sharing an erect herbaceous habit, narrow leaves and corona-less flowers with urceolate, internally bearded corolla, that are mainly distributed in savannahs of the Cerrado biodiversity hotspot, South America. Here, we describe and illustrate a new species of Hemipogon, H. trilobatus Bitencourt & Rapini sp. nov., from an open savannah in Chapada dos Veadeiros, Central Brazil. Hemipogon trilobatus sp. nov. differs from the other species of the genus mainly by the presence of a reduced staminal corona with 3-lobed lobes, but also by opposite leaves and triangular anthers. Distribution and habitat data, as well as a key and a comparative table to distinguish the four species currently accepted in Hemipogon s. str., are provided. Based on criteria B2ab(i,ii,iii,iv) of the International Union for Conservation of Nature (IUCN), the species is provisionally assessed as Critically Endangered.


Introduction
Hemipogon Decne. (Apocynaceae Juss.: Asclepiadoideae Burnett) belongs to the neotropical subtribe Metastelmatinae Endl. ex Meisn. and originally comprised erect perennial herbs with narrow leaves and urceolate, internally bearded, corona-less flowers (Fournier 1885). The genus was enlarged with the inclusion of twining plants with corona-less flowers classified in the American Astephanus R.Br. and erect herbs with corona-bearing flowers segregated from Metastelma R.Br. (Rapini et al. 2001;Rapini 2002). Phylogenetic studies (Ribeiro et al. , 2014Silva et al. 2012), however, showed that these two groups form a lineage predominantly distributed through the campos rupestres (rocky grasslands on quartzite outcrops) of the Espinhaço Range and are not closely related to Hemipogon acerosus Decne., the type of the genus. The phylogenetic inference indicates that the corona was lost several times within Metastelmatinae and thus Hemipogon should be treated under its narrow concept, comprising only three species, H. acerosus, H. setaceus Decne. and H. irwinii Fontella & Paixão, or four, if H. acerosus var. platyphyllus Hoehne would be recognised at the species level (Bitencourt 2019). Using this narrow circumscription, Hemipogon is predominantly distributed in savannahs, and less often in campos rupestres of the Cerrado domain, in Central Brazil, reaching Bolivia, Peru and Paraguay.
Cerrado is a biodiversity hotspot (Myers et al. 2000) and the world's most diverse savannah, with around 6910 species of flowering plants, approximately 3815 of which are endemic (Silva & Bates 2002;Flora do Brasil 2020). Despite its great importance, about half of the area originally occupied by Cerrado has been lost due to the implantation of pasture and commercial crops, mainly soybean and corn, and only 10% of its original area is protected in conservation units (Beuchle et al. 2015). Chapada dos Veadeiros in the Brazilian Cerrado houses savannahs in lowlands and patches of campos rupestres over mountaintops up to 1683 m a.s.l. (topographic map, https://www.arcgis.com/), and is considered a conservation priority area due to the high level of unique evolutionary diversity (Carvalho et al. 2015;Fenker et al. 2020).
Describing the biodiversity is an essential step to understand evolutionary and biogeographic patterns, and establish conservation strategies (Brach & Boufford 2011;Ulloa et al. 2017). This task is especially relevant in areas under high pressure, such as the Cerrado hotspot. Herbaria are important source of new species (Prance 2001;Joppa et al. 2010;Bebber et al. 2010) and most undescribed species are discovered a long time after a first specimen was collected (Bebber et al. 2010). Here, we describe a new species from Chapada dos Veadeiros first collected 28 years ago. It fits the narrow concept of Hemipogon and is here provisionally assessed as Critically Endangered.

Material and methods
As part of a large study which aims to propose a new circumscription for the genus Hemipogon, specimens from different species of South American savannahs were examined in herbaria and collected in situ. Morphological measurements for this study were taken from specimens in the herbaria BHCB, CEN, HBR, HRB, HUEFS, IBGE, LPB, MBM, MO, NY, R, RB, UB, UFG, UFMT and USZ (acronyms according to the Index Herbariorum, Thiers continuously updated: http://sweetgum.nybg.org/science/ih/). Morphological terms follow Beentje (2016) and Endress et al. (2018). A provisional species conservation status was inferred based on IUCN (2019), according to the categories, criteria and conditions outlined in the Red List Guidelines. We used the GeoCAT tool (Bachman et al. 2011) to calculate the area of occupancy (AOO) using a 2 × 2 km 2 cell resolution.

Etymology
The epithet ʻtrilobatusʼ refers to the apically 3-lobed corona lobes.

Distribution, habitat and phenology
Hemipogon trilobatus sp. nov. is known from a small population nearby the São Bento waterfall, in Chapada dos Veadeiros, Goiás, Brazil (Fig. 2). It grows in open savannah and wet grasslands locally known as "várzea" and was collected with flowers in August and September. The species has not been found since a fire burned the type locality (M.A. da Silva, pers. com., 2015).  nov. in the Cerrado domain (shaded area at reference map in A) and its occurrence (black star) nearby the Chapada dos Veadeiros National Park. Images and distribution maps were built and exported using ArcGIS online (https://www.arcgis.com). © Esri and its licensors, all rights reserved.

Provisional conservation status
Hemipogon trilobatus sp. nov. is known from only two collections, both near the São Bento waterfall, in Chapada dos Veadeiros. Specimen labels provide no information about population density. However, the species is known to occur in a tourist location, surrounded by farms and cattle raising, has an area of occupancy (AOO) estimated at 8 km 2 , probably under continuing decline in geographic range and habitat quality. During fieldwork in the type locality, we did not find any specimen of H. trilobatus sp. nov. and the new species has not been recollected since 1994. Based on criteria B2ab(i,ii,iii,iv) of the International Union for Conservation of Nature (IUCN 2019) Red List assessment, we provisionally classify the species as Critically Endangered (CR).

Taxonomic note
Hemipogon trilobatus sp. nov. has narrow leaves, urceolate corolla with bearded lobes and was found in a savannah of Central Brazil. Together, these features support its classification in Hemipogon s. str. It most closely resembles H. acerosus based on the acicular leaves and sessile cymes, but differs from all other taxa in the genus by the opposite leaves and flowers with reduced corona lobes, as presented in the key below and Table 1.

Discussion
Asclepiadoideae is a cosmopolitan subfamily with approximately 180 genera and 3200 species (Endress et al. 2018); around one third of this diversity is represented in the Americas (Good 1952;Rapini 2012). The proportion of narrowly distributed species and genera in the subfamily is high (Good 1952), and many species are known only from the type locality (Endress et al. 2018). Brazil harbours 35 native genera of Asclepiadoideae, only five of them including more than 15 species. Narrowly distributed species of Asclepiadoideae are common in the country (Rapini et al. 2001(Rapini et al. , 2009Rapini 2010) and several Brazilian microendemics have been described in the last years (e.g., Bitencourt et al. 2020). This contributed to the increase of the number of Asclepiadoideae species in Brazil from 376 (Rapini et al. 2005) to 400 (Flora do Brazil 2020) in the last 15 years.
Some of the Asclepiadoideae microendemics are poorly known due to the limited number of collections.
Hemipogon abietoides E.Fourn., described in Flora Brasiliensis (Fournier 1885) from a specimen collected by Riedel during the legendary expedition headed by Langsdorff through the Espinhaço Range of Minas Gerais, for instance, was recollected only 175 years after its original collection . Molecular phylogenetic analyses (Ribeiro et al. , 2014Silva et al. 2012