We describe here a new species of Casearia from Panama based on both morphological and molecular data. Casearia isthmica sp. nov. is restricted to the mid-elevation cloud forests of Central Panama and presents morphological similarities with two more widespread species, C. sanchezii from high elevation areas of El Salvador and Mexico and C. tremula from the Caribbean, Central America, and Northern South America. Casearia isthmica differs in presenting pedunculated and congested inflorescences with up to 20 flowers, as well as flowers with 12 stamens and a pubescent style. Phylogenetic analysis based on selected plastid (petD, trnK-matK, rpl16 and rps4-trnLF) and nuclear (GBSSI and ITS) markers shows that the new species belongs to subclade B3 of Casearia, a lineage that encompasses species from Central America, Mexico and the Caribbean. Results of the morphological and molecular analysis were congruent and allowed a broader understanding of this new taxon, especially regarding its relationships to other Casearia.

Molecular phylogeny, Neotropics, new species, Samydeae

Casearia Jacq. is a pantropical genus of more than 200 species, therefore being the most species rich genus of the subfamily Samydeae (Salicaceae). In the Neotropics, it is widely distributed and reported from every biome, such as the Amazonian rainforests, the Brazilian Cerrado, the Caribbean and the savannas (Sleumer 1980; Correll and Correll 1982; Liogier 1994; Gutiérrez 2000). Sleumer (1980) provided a taxonomical revision of the whole family Flacourtiaceae (within which Casearia was then placed) for the Neotropics, therefore providing insights into its morphological variability. He divided the genus, based on morphological characters, into six sections which were not found to be monophyletic by a recent molecular study (Mestier et al. 2022). Nevertheless, the study showed nine monophyletic subclades. Recently, many new taxa have been described in the Neotropical regions from both South America (Marquete and Mansano 2010, 2013; Alford 2015; Marquete and Torres 2022) and Mesoamerica (Castillo-Campos and Medina Abreo 2003; Linares and Angulo 2005). Here, we describe a new species of Casearia from Central America, endemic to Central Panama. Casearia species are trees or shrubs that present pellucid dots or lines on the leaves. The limb possesses a pinnate nervation and mostly serrate, crenate, or subentire margins. Flowers are apetalous, with four to five sepals, sometimes more, and mostly uniseriate stamens, alternating with staminodes. Casearia usually presents eight to ten stamens, although some species possess a higher number (Warburg 1895; Hutchinson 1967; Sleumer 1980).

Speciation processes are often associated with morphological changes (Losos and Miles 1994; Givnish 2010) and traditionally, species have been described following a morpho-species concept (Stuessy 2009). Nevertheless, providing a phylogenetic framework for a newly described taxon provides valuable insights regarding its relationships with species of the same genus and support for the accuracy of the species delimitation (Padial and De La Riva 2010). Here, we described a new species of Casearia endemic to Panama and establish a phylogenetic tree of the genus with the newly described taxa, therefore providing insights into its phylogenetic position within Casearia.

Results of both the molecular and morphological analysis highly support the recognition of Casearia isthmica as a new species. Based on the molecular trees (Figs 1, 2), C. isthmica can be hypothesized as sister to C. laetioides (A. Rich.) Northr. (=Zuelania guidonia (Sw.) Britton & Millsp) and both are related to taxa with a largely central American distribution such as C. tremula (Griseb.) Griseb. ex C. Wright, and C. sanchezii J. Linares & Angulo.

Morphologically, Casearia laetioides, C. tremula, and C. sanchezii share similarities with C. isthmica, such as the general shape of the leaves, the presence of more than ten stamens with pubescent filaments, and the size of the fruit. However, C. laetioides differs from C. isthmica in many aspects, mainly in the petiole, leaf blades, inflorescences, fruits, and other floral aspects related to the style and number of stamens and staminodia (Table 1). Character states such as essentially glabrous leaves (sometimes puberulent along the major veins), glabrous and glossy fruits shared between C. isthmica, C. tremula and C. sanchezii (Table 1) could be of plesiomorphic nature but the latter two differ mainly in the type of inflorescence, number of flowers per inflorescence and the number of stamens. Taken together, morphological differences support the recognition of C. isthmica as a new species.

Casearia laetioides was historically classified within the genus Zuelania A. Rich., which has recently been shown to be nested within Casearia, on the basis of morphological and molecular characters (Samarakoon and Alford 2019; Mestier et al. 2022). C. laetioides is widely distributed in the Caribbean region, in Mexico, in Central America and Northern South America (Sleumer 1980). Zuelania was distinguished from Casearia by having a high number of stamens and a sessile and peltate stigma, whereas most Casearia species possess up to 12 stamens and a capitate stigma with a style. However, it seems that the high number of stamens is a homoplastic character state, as within clade B3, C. isthmica is the only species presenting 12 stamens.

From a geographical and ecological viewpoint, Casearia isthmica and C. laetioides are both present in Panama, although not in the same ecosystem as C. isthmica, which occurs at higher elevation in cloud forest, whereas C. laetioides occurs at lower elevation, below 600 m, usually in wet or dry habitats such as seasonal forests (Table 1). Further research will be needed to clarify if C. laetioides is monophyletic and a vicariant to Casearia isthmica and to clarify the position of the lineage including C. isthmica and C. laetioides within subclade B3 of Casearia. It will be relevant to a better understanding of the speciation history of C. isthmica as an example for the origin of cloud forest species in Panama, but also with respect to the origin of the flora of the Caribbean, for which studies in recent years underscored the close affinities between Central America, Mexico and the Caribbean islands (Cervantes et al. 2016; Mestier et al. 2022).

Some specimens from Panama previously identified as C. tremula were re-identified and included in this work as Casearia isthmica (Galdames et al. 4317 and Correa et al. 11430). However, there is currently a representative specimen (Carrasquilla 2072 MO, PMA) that confirms the occurrence of C. tremula in the country, which was collected in the dry seasonal lowland forests from the Pacific slope of Panama.

We would like to thank Joana Sumich (SCZ) for assistance during the field trips and Gino Fatacioli (PMA) for providing specimen scans. We are grateful Ministerio de Ambiente de Panamá (Ministry of the Environment of Panama) for granting the scientific collection and export permits (SC/P-1-18, SE/P-3-20, and PA-01ARB-0112-2022). We are also very thankful to the Central Laboratory Service at the BGBM (Berlin) for their support generating the molecular data. The authors gratefully acknowledge the scholarship received for Mr. Cedeño-Fonsecaʼs doctoral studies from Deutscher Akademischer Austauschdienst (DAAD) Research Grants – Doctoral Programmes in Germany (No. 57507871). The authors also thank IFARHU (Institute for Training and Development of Human Resources) from the Panamanian government, which jointly with SENACYT (National Secretariat for Science, Technology and Innovation) gave a scholarship for doctoral studies to Mr. Ortiz. Finally, we would like to thank two anonymous reviewers and Mac Alford (University of Southern Mississippi) for providing useful comments on that manuscript, as well as Thomas Borsch (Botanical Garden Berlin) and the editors of the journal. We acknowledge support by the Open Access Publication Fund of Freie Universität Berlin.