Aa (Orchidaceae) of Southern Central Andes: Taxonomy, Nomenclature, and a New Species with Dark Flowers

Abstract

The genus Aa Rchb.f. (Orchidaceae: Cranichidinae) includes about 25 Neotropical species distributed in the Andean region, although Aa paleacea (Kunth) Rchb.f. reaches the mountains of Costa Rica (Trujillo and Vargas 2011; Szlachetko and Nowak 2014). In South America, this genus is a component of different kinds of vegetation, such as open scrub, woodlands, damp mountain meadows, and paramo of the northern Andes above 2600 m a. s. l. (Wood 2003; Szlachetko and Nowak 2014); as well as the dry Puna desert coast of Peru (Trujillo and Vargas 2011), and forest and montane grasslands of Yungas in the central Andes above 1000 m a. s. l. (Cocucci 1954; Schinini et al. 2008; Mart´ın and Scrocchi 2016). This genus is characterized by its elongated peduncle, with tubular and hyaline-diaphanous sheaths, generally greenishwhite (rarely black or orange) and non-resupinate flowers, dorsal sepals and petals that are free from column, cucullate lips with involute margin, and disc with a pair of subglobose basal calli (Trujillo and Vargas 2011; Szlachetko and Nowak 2014). However, this genus has low morphological variation and an unstable systematic status, since it has been transferred to Altensteinia Kunth or Myrosmodes Rchb.f. and returned to Aa several times by Reichenbach (1854, 1878), Schlechter (1912, 1920a, 1920b), and Schweinfurth (1958). Recent molecular studies showed that some species of Myrosmodes are nested in Aa (Alvarez-Molina and Cameron ´ 2009), while the morphological and ecological evidence supports the separation of these genera in current taxonomic studies. Thus, a new molecular phylogeny with a large taxonomic sampling is needed to delimit both these genera (Trujillo and Vargas 2011). In the Southern Central Andes (Argentina), the taxonomy of Aa is controversial due to the lack of a comprehensive taxonomic treatment. For example, for the Argentine flora (Correa 1996) and for the Cono Sur vascular flora (Schinini et al. 2008), five species were recognized, but Mart´ın et al. (2018) showed that some specimens cited by these authors under Aa paludosa Rchb.f. are clearly Myrosmodes gymnandra (Rchb.f.) C. A. Vargas and M. paludosa (Rchb.f.) P. Ortiz. In addition, in these former taxonomic works, Aa lorentzii Schltr. and Aa schickendanzii Schltr. were recognized as synonyms of Aa hieronymi (Cogn.) Schltr.; nevertheless, the diagnostic morphological characters (e.g. number of lip lobes, presence of hairs on the scape, rachis, and ovary) are incongruent among the type specimens of these taxa. Therefore, we present a taxonomic revision of the genus Aa from the Southern Central Andes including the description of a new species; we proposed two new synonyms under Aa achalensis; we designed a lectotype for Aa hieronymi, Aa lorentzii, Aa schickendantzii (5 Aa achalensis); and we proposed a neotype for Aa achalensis and Aa fiebrigii. In addition, a taxonomic key is included.