Phyllanthus chacoensis (Phyllanthaceae): new record for the Brazilian Amazon and its lectotypification

The first record of Phyllanthus chacoensis for the Brazilian Amazon is presented, based on a specimen from Rondônia state. The species can be recognized by its cauliflorous inflorescence, staminate and pistillate flowers with 4 sepals, devoid of a floral disk, 4 stamens, 2-carpellary ovary, drupaceous, ellipsoidal fruits, with one seed per locule. The species was previously known only from the central-western and northeastern regions of the Caatinga and Pantanal domains in Brazil. We provide a list of synonyms, a detailed description, an updated geographic distribution map in Brazil, data on habitat and conservation status, illustrations of its reproductive characters, and detailed photographs. We also propose a lectotype for the species.

The species richness of the genus in Brazil is almost certainly underestimated, since the last major revision was published in the Flora Brasiliensis by Müller Argoviensis in 1873 and included 71 species. More recently, floristic studies by Webster (2002) and Silva and Sales (2007) made important contributions to the understanding of the morphology of several taxa. The floristic knowledge of the Amazon basin is even more precarious, as pointed out by Hopkins (2005) and Ter Steege et al. (2013) and, consequently, the diversity of Phyllanthus in the region is probably underestimated, as exemplified here with the recognition of a new record of a VOL. 51(1) 2021: 52 -57 ACTA AMAZONICA Phyllanthus tree species in the Brazilian Amazon, Phyllanthus chacoensis Morong.
Taxonomic advances in the Amazon region are hampered by low numbers of herbaria and deposited specimens, low collection rates, and relatively few specialized taxonomists (Barbosa et al. 2003), which helps explain the relatively few studies and the low capacity for determining the real diversity of Phyllanthus (e.g., Rodrigues 1971;Webster 2004;Secco 2013;Secco and Rosário 2015;Secco and Silveira 2016). Additionally, Phyllanthus is one of the most morphologically complex groups of Phyllanthaceae due to its variable habits (herbs to trees), phyllanthoid or non-phyllanthoid branching, and tiny gamosepalous pistillate flowers with disks commonly entire, and disks segmented and alternisepalous in staminate flowers Sales 2004, 2007).
As a result of increased collection efforts and extensive analyses of herbarium specimens as part of a revision of Phyllanthus conducted by the first author, an indeterminate specimen (with duplicates deposited at INPA, NY and RB herbaria) that had been collected 58 years ago in what is now Rondônia state (northern Brazil) was encountered. We identified the collection as P. chacoensis, representing a new record for the Amazonian flora. In addition to the description of the new record, we propose its lectotypification and discuss its conservation status.

Phyllanthus chacoensis
Conservation status: According to IUCN (2012) criteria, the species is classified as endangered (EN) -ENB1ac (iii), as it has an extent of occurrence (EEO) of 204,000 km 2 (including the new record). Phyllanthus chacoensis should therefore be the target of collection efforts to assist in the selection of priority areas for its conservation. Most of the collected specimens in Brazil were encountered in riparian vegetation, such as along the banks of the São Francisco River (northeastern region), the Paraguay River (midwestern region), and Guaporé River (northern region), which suffer from constant anthropic impacts, including dense urban occupation and the discharge of untreated domestic sewage, resulting in biodiversity loss. Taxonomic notes: Thomas Morong (1892) described Phyllanthus chacoensis based on syntypes "In the Gran Chaco, opposite Asuncion (355)". = Balansa 1712, Fendler Panama 140, Fendler 323". The collection Morong 355 is the most likely to have been used by the author in the description. However, four specimens were found in the author's main herbarium (NY). The material selected as a lectotype is deposited under registration (NY00273054; Figure 3), as it is the best conserved and has flowers of both sexes.

DISCUSSION
The Amazon region represents a distinct phytogeographic province characterized by a humid tropical forest of enormous biomass, heterogeneity, and diversity (Braga 1979). Collection efforts such as the "Projeto Flora Amazônica (CNPq/NSF)", initiated in 1976 (Secco 2018), have made significant progress in increasing our knowledge of the flora of the Brazilian Amazon, with many expeditions and a large number of plant specimens deposited in regional herbaria (with duplicates distributed among national and international herbaria). However, considering the extent of the region, the collection rate in the region is still the lowest in Brazil (Hopkins 2019). The new record of P. chacoensis for the Brazilian Amazon reinforces the importance of these botanical collections, as the species had not been collected in the region since 1962. We have not found any more recent collection in the region. However, collections of P. chacoensis are much more numerous in northeastern and central-western Brazil (ca. 33 and six specimens respectively), collected since 1984.
Phyllanthus chacoensis is the only species in the subsection Aporosella (Chodat) G.L.Webster (P. sect. Cicca (L.) Müll.Arg. and P. subg. Kirganelia (A.Juss.) Kurz) (Webster 1957(Webster , 2001Bouman et al. 2018), and can be differentiated from other Brazilian species by the absence of a floral disk in staminate and pistillate flowers, and by having four sepals and stamens, cauliflorous inflorescences, a 2-carpellar ovary, and fruits with one seed per locule (Silva and Sales 2007;Melo et al. 2013). Phyllanthus chacoensis is morphologically similar to P. elsiae Urb., which is found in Argentina (Silva and Sales 2007), although the former can be distinguished by being monoecious (vs dioecious in P. elsiae), by having a 2-locular ovary (vs 3-locular), styles deeply 2-fid, curved downwards (vs joined in a column and descending) and ellipsoidal fruits (vs oblate-spheroidal).

CONCLUSIONS
A new record of Phyllanthus chacoensis in the Brazilian Amazon expands the distribution of the species in Brazil and confirms its preference for swampy habitats close to water bodies. It also reinforces the need for investments in botanical collections in northern Brazil, especially in the Amazon region, as well as the training of specialized human resources, as the specimen examined here had lain for more than 50 years in a herbarium without being identified. The conservation status of P. chacoensis, associated with its occurrence in highly threatened sites, indicates the need to protect this species.