A new genus of Compositae (Eupatorieae, Piqueriinae) from Peru, named Centenaria to honour the 100 th anniversary of the Natural History Museum of the National University Mayor of San Marcos

Abstract A little herb from central Peru is recognised as a new species of a new genus. Centenariarupacquiana belongs to the tribe Eupatorieae, subtribe Piqueriinae. It has asymmetrical corollas with two inner lobes smaller, a flat and epaleaceous receptacle and the presence of pappus. In Peru, Centenaria is related to the genera Ferreyrella and Ellenbergia, but Ferreyrella is different by having no pappus and a paleate receptacle; and on the other hand, Ellenbergia is different by having symmetrical corollas.


Introduction
The Compositae has more than 30000 accepted species in more than 1900 genera (Nordenstam et al. 2009). Currently, twelve subfamilies are recognised, of which Asteroideae (Cass.) Lindl. comprises over 60% of the species in the family, placed in ca. 1229 genera and 20 tribes and is the largest subfamily of Compositae ). One of these tribes is Eupatorieae Cass., the fourth largest tribe in the subfamily and the sixth largest in the family (Funk et al. 2009, Pelser and. The tribe Eupatorieae contains 19 subtribes, 185 genera and 2200 species (as of Hind andRobinson 2007, Rivera et al. 2016); and one of these subtribes is Ageratinae Less. with 26 genera and between 300 and 350 species (Hind andRobinson 2007, Robinson et al. 2009), although these numbers of genera and species will certainly change since, recently, it has been shown that the earlier concept of Ageratinae was polyphyletic (Rivera et al. 2016). For this reason, the subtribe Piqueriinae is treated here as separate (following Robinson et al. 2009).
In Peru, the tribe Eupatorieae is one of the largest with 46 genera and 325 species (Dillon and Sagástegui 2002). The subtribe Ageratinae contains seven genera in Peru (Brako and Zarucchi 1993, Ulloa Ulloa et al. 2004, Hind and Robinson 2007, of which four belong to the narrower concept of Piqueriinae and three (Ascidiogyne Cuatrec., Ellenbergia Cuatrec. and Ferreyrella S.F. Blake) are endemic to this country (Beltrán et al. 2006).
On 15 April 2018 during a field trip, as part of a botanic course of the "Universidad Nacional Mayor de San Marcos", to an area east to the city of Lima, we found a little plant of Compositae growing along the trail between Pampas and Rupac (Lima, Peru). It was here that serendipity brought us and the new species face to face. After two weeks, we concluded that it was a member of what is here treated as Piqueriinae that had not been previously reported for the Peruvian flora (Brako and Zarucchi 1993, Ulloa Ulloa et al. 2004, 2017, Beltrán et al. 2006, Dillon and Sagástegui 2002, Vásquez et al. 2002, Linares et al. 2010, Gonzáles et al. 2016. For this reason, on 7 May, we decided to make a new trip to look for more populations and to make more collections for the study of this plant. This previously unknown dwarf member of the tribe Eupatorieae from the higher elevations of the Department of Lima in Peru shows a combination of characteristics unmatched in other previously known genera of the tribe. The entity is described here as the new genus Centenaria in honour of the centennial of the establishment of the Natural History Museum of National University Mayor of San Marcos (28 Feb 1918).
As shown in the study of the tribe Eupatorieae of the Asteraceae by King and Robinson (1987), the tribe contains many genera with a dwarf habit and a reduced or absent apical appendage on the anther. These genera are scattered geographically from Mexico southwards to the Andes of South America and eastwards to Brazil. Most genera show ex-imbricate involucres with broad and blunt-tipped phyllaries and corollas with narrow bases and broadly campanulate limbs. Such genera seem mostly to be related to the Piqueria group in the subtribe Piqueriinae of the Eupatorieae (Robinson et al. 2009, Rivera et al. 2016. The new genus differs from all of these genera by the presence of a pappus of lanceolate scales on only the inner florets of the capitulum. Zygomorphic corollas of the plant are an unusual characteristic in the tribe, but are also seen in the genus Microspermum Lagasca of Mexico, in one of the species of Iltisia S.F.Blake of Costa Rica, in one of the species of Ferreyella of Peru and in the following members of the monophyletic subtribe Praxelinae (Rivera et al. 2016), the Brazilian and Bolivian turf-forming genus Piqueriopsis G.M.Barroso, the Brazilian Eitenia R.M. King & H.Rob. and a few members of the mostly Brazilian polyphyletic Praxelis Cass. (Rivera et al. 2016). The latter genera can be distinguished by having no pappus or a pappus of many capillary setae. Two genera, Pigueriopsis R.M.King of Mexico and Iltisia of Costa Rica are distinct in having 4-lobed corollas. Of the above genera, the more northern Microspermum and Iltisia also seem to have a more remote relationship because of their narrower and more pointed phyllaries.
Of most interest for purposes of comparison are the presumably related dwarf genera that occur in Peru, Ascidiogyne, Ellenbergia, Ferreyrella and Guevaria R.M.King and H.Rob. Of these, Ferreyrella differs by its paleate receptacle, Ellenbergia differs by its pappus of many narrow segments, Guevaria differs by its total lack of pappus, its conical receptacle and prostrate to procumbent habit and Ascidiogyne differs by its carnose nature and prostrate habit with clusters of short peduncles arising from short leafy lateral branches. These genera are distinguished more completely in the key below.
Distribution. Known only from the type locality in Rupac, northeast from Lima Department.
Etymology. The genus is dedicated to the centennial of the institutional foundation of the Natural History Museum of National University Mayor of San Marcos , for their hard work on the research, conservation, preservation and diffusion of the biodiversity of the country. All these actions are steadily increasing our knowledge of the flora and fauna of our native land. The specific epithet refers to Rupac, a small village with archaeological remains from the Atavillos culture, where the only two populations of this species are known from this place.
Conservation status. Centenaria rupacquiana is only known from the type collection and is therefore assessed as Data Deficient (DD) according to the UICN (2012UICN ( , 2017 criteria. However, we recommend it should be considered critically endangered (CR), as it is only known from a single locality (Criterion B1a) with a continuing decline of its quality of habitat inferred from the intensive livestock in the area (Criterion B1b). Furthermore, is only known from two populations with an estimated number fewer than 250 mature individuals (Criterion C1a(ii)).
Discussion. Centenaria belongs to the subtribe Piqueriinae of the Eupatorieae, being considered related to the genera Microspermum and Iltisia from Mexico and Central America and Ferreyrella of Peru, all having asymmetrical corollas with the two inner lobes smaller than the rest; however, Microspermum, Iltisia and Ferreyrella have no pappus or only a few capillary setae (Hind and Robinson 2007). Furthermore, Centenaria is evidently related to the genus Ellenbergia of Peru, which has a pappus of many segments but has symmetrical corollas.
The species described here is very similar to Ferreyrella; both have strikingly asymmetrical corollas, from which it differs primarily in having a flat epaleaceous receptacle (vs. slenderly conic paleaceous receptacle) and the presence of pappus (vs. lack of pappus). Although this distinction is usually important in the Eupatorieae, as it is in Compositae generally, it becomes merely a specific or even, in one case, a varietal character in Ageratum (Blake 1957). The flat receptacle and the pappus of lanceolate squamellae with densely scabrid margins in Centenaria are very suggestive of those of Ellenbergia, although the pappus is not so extremely united at the base into a cup-shaped piece, an additional significant feature not found in Ellenbergia. Robinson et al. (2009) commented about a group formed by small, mostly rather ephemeral Eupatorieae, which usually have many-flowered heads. At this time, Guevaria, Ferreyrella, Ellenbergia and Centenaria would also fit in this group despite having fewer flowers.
Key to the dwarf genera of the Eupatorieae in Peru