Psidium or Myrcia? — The problematic lectotypification of Mitranthes O.Berg (Myrteae, Myrtaceae)

The genus Mitranthes (Myrteae, Myrtaceae) was described with five species, but no indication of a generic type. These five species are now known to belong to the independent generic lineages Psidium and Myrcia of the tribe Myrteae. This paper traces the taxonomic and nomenclatural history of Mitranthes, showing that the most appropriate type species is currently accepted in Psidium. As a result, Mitranthes should correctly be treated as a synonym of Psidium and its recent synonymisation in Myrcia must be discounted.


Introduction
Mitranthes O.Berg has a tortuous taxonomic history. It was described as a New World genus with five species, but no indication of a generic type, by Berg (1856) in his Revisio Myrtacearum Americae. Unfortunately, under modern circumscription, species that have been attributed to Mitranthes belong to distantly related genera, each pertaining to a different subtribe of Tribe Myrteae (Lucas et al. 2019): Eugenia L. (subtribe Eugeniinae), Myrcia DC. (subtribe Myrciinae), Myrceugenia O.Berg (subtribe Luminae) and Psidium L. (subtribe Pimentinae). These subtribes are difficult to recognise based only on flowering material, as embryo morphology is strongly diagnostic. Eugenia has a bean-like 'eugenioid' embryo in the classification originally established by de Candolle (McVaugh 1968), with a minute hypocotyl and well-developed plano-convex cotyledons that are either free or connate. Myrcia and Myrceugenia have similar 'myrcioid' embryos i.e., a C-shaped hypocotyl that encircles the well-developed, crumpled cotyledons, and Psidium has a 'pimentoid' embryo, i.e., a C-shaped hypocotyl with rudimentary, appendage-like cotyledons (McVaugh 1968). The distinction between these subtribes becomes particularly difficult if only flowering material is available, particularly if the flowers are isolated and the calyx closed in the floral bud. Isolated flowers occur in the four subtribes mentioned above, although they are exceptional in the Myrciinae (Lucas et al. 2019). Complete calyx closure in the floral bud is found sporadically in almost all Myrteae subtribes; its low value as a generic marker has only recently become clear (Landrum 1984;Proença 1990;Parra-O. & Bohórquez-Osorio 2016;Giaretta et al. 2018;Lucas et al. 2018).
Berg noted that his new genus Mitranthes included species previously assigned to Calyptranthes Sw. and Psidium by other authors, but believed Mitranthes to have eugenioid affinities, putting it after Myrcianthes O.Berg and before Calycorectes O.Berg (both eugenioid genera). The distinguishing characters given were a closed, calyptrate floral bud, solitary flowers and a pauci-ovulate, bilocular to tetralocular ovary; Mitranthes fruits were unknown (Berg 1855).
Mitranthes ottonis O.Berg was the only species to be described by Berg (1855)  Due to this taxonomic confusion, lectotypification strongly affects Mitranthes' placement within tribe Myrteae as its species are not closely related. There were three candidates for lectotypification, since Mitranthes gardneriana and M. ovalifolia were nomina nuda when the genus was described. Mitranthes has been lectotypified three times by three different authors. We will discuss these in reverse chronological order for reasons that will become clear. McVaugh (1956b) Welker et al. 2021) as a result of their being overlooked for more than 100 years, during which time other taxonomic actions were taken and lectotypes proposed. Some of these steps now must be overturned as previous manoeuvres take priority, often to the dismay of those seeking taxonomic stability; it is likely that more such corrections will be seen in other taxa in the future.
Unfortunately, the forementioned lectotypification of Mitranthes by M. ottonis (Burret 1941), a Cuban species of recognised 'myrcioid' affinities, has been accepted by botanists for the past 50 years with several consequences. Legrand in Legrand & Klein (1977a, 1977b erected the genus Neomitranthes to accommodate several Brazilian species that were similar to Mitranthes but had 'eugenioid' embryos. Since Legrand believed Mitranthes to be lectotypified by a 'myrcioid' species as stated by McVaugh (1968), a new genus to accommodate these 'eugenioid' species was necessary. Neomitranthes closely resembles Mitranthes in its closed floral buds, prolonged hypanthial tube and bilocular ovaries, hence Legrand's choice of name. Neomitranthes was considered by Legrand to belong to Subtribe Eugeniinae but is now included in Subtribe Pliniinae due to molecular research (Lucas et al. 2019). Mitranthes has been cited in several recent papers as a synonym of Myrcia, also based on the belief that the lectotype of the genus is Mitranthes ottonis O.Berg = Myrcia ottonis (O.Berg) Flickinger, e.g., Lucas et al. (2018). The purpose of this paper is to correct this error. There is no reason to discredit the first lectotypification by Pfeiffer (1873), that is accepted by Index Nominum Genericorum and has been widely applied to other botanical families (Silva & Nelson 2008;Sennikov et al. 2011;Welker et al. 2021).
Therefore, we have applied article 10.5 of the International Code for the Nomenclature of Plants, Fungi and Algae, that states: 'The author who first designates a type of a name of a genus or subdivision of a genus must be followed [unless] the author used a largely mechanical method of selection'. The genus Mitranthes should henceforward be treated as a synonym of Psidium, as we have recently done (Proença et al. 2022), where it has been recognised as Psidium section Mitranthes (O.Berg) Tuler & Proença.

Conflict of Interest
The authors declare that they have no conflict of interest.
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