Struthanthus ibe-dzi sp. nov. is a new species described and illustrated from the cloud and pine-oak forests of the Sierra Madre del Sur in Oaxaca, Mexico. This species shares similarities of leaf shape and inflorescence type with S. deppeanus, S. quercicola, and S. ramiro-cruzii. However, S. ibe-dzi can be recognized by its glaucous branches, leaves and inflorescences; compressed nodes; convoluted distal half of styles in pistillate flowers; and staminate flowers with asymmetrical thecae and an extended connective forming an apiculate horn in both anther series. A distribution map and an identification key are provided to separate S. ibe-dzi from morphologically similar congeners present in the region.

Struthanthus ibe-dzi sp. nov., es una nueva especie descrita e ilustrada de los bosques de niebla y de pino-encino de la Sierra Madre del Sur en Oaxaca (México). La nueva especie se puede confundir con S. deppeanus, S. quercicola, y S. ramiro-cruzii por la forma de la hoja y el tipo de inflorescencia. Sin embargo, S. ibe-dzi se distingue por sus ramas, hojas e inflorescencias glaucas; sus nodos comprimidos; flores pistiladas con el estilo convoluto desde la mitad hacia la parte distal; y flores estaminadas con tecas asimétricas y un conectivo extendido apiculado en forma de cuerno en la serie baja de las anteras. Se incluye un mapa de distribución y una clave de identificación para separar S. ibe-dzi de sus congéneres regionales morfológicamente similares.

Endemic, hemiparasitic, mistletoes, taxonomy

Struthanthus Mart. is a neotropical genus in the Loranthaceae family with approximately 60–70 species (Caires and Dettke 2020) distributed from Northern Mexico to Argentina (Kuijt and Hansen 2015). The genus comprises hemiparasitic aerial shrubs or climbing plants which are dioecious (Kuijt 2009a; Kuijt and Hansen 2015; Nickrent et al. 2019). Struthanthus is commonly recognized by the presence of thick epicortical roots at the base of the plant and along stems. Additional important characters include stems teretes, sub-teretes or quadrangular (when young), with lenticels and a glabrous, striated, or sulcate texture (Kuijt 2009a; Kuijt and Hansen 2015; Caires and Dettke 2020); leaves opposite, subopposite, or rarely alternate, sometimes with stiffly recurved leaves and prehensile petioles (Caraballo-Ortiz and Acevedo-Rodríguez 2021; Maldonado 2021); axillar and indeterminate inflorescences in racemes, spikes, or head-like with pedunculate or sessile triads; flowers small (4–10 mm long), hexamerous, pedicellate or sessile; staminate flowers with dimorphic stamens, dorsifixed (Kuijt 2009b; Kuijt and Hansen 2015) but basifixed in at least some of the Mexican species (Maldonado 2021); and pistillate flowers with monomorphic staminodes and styles straight to strongly convolute in some Mexican species (Kuijt 1975a, 2003a; Maldonado 2021).

Despite the lack of a comprehensive taxonomic treatment for Struthanthus, new species have been described during the last 20 years (Kuijt 2003a, b, 2009a, 2014; González and Morales 2004; Martínez-Ambriz et al. 2017). Also, several generic transfers have occurred, including Cladocolea Tiegh. (Kuijt 1987, 1991b), Panamanthus Kuijt (Kuijt 1991a), Passovia H. Karst. (Kuijt 2011), Peristhetium Tiegh. (Kuijt 2012), Phthirusa Mart. (Kuijt 2009a) and Pusillanthus Kuijt (Caires et al. 2012).

Regarding generic affinities, Struthanthus has been found to be closely related to Cladocolea. This relationship has been supported by pollen morphology as described in Grímsson et al. (2018), where both genera were found to share type B pollen grains, showing an affinity to Peristethium. Molecular sequence data also support a relationship between Struthanthus and Cladocolea based on plastid DNA sequences (matK, rbcL, and trnL-F) and ribosomal nuclear cistron (SSU rDNA and LSU rDNA) (Vidal-Russell and Nickrent 2008). However, Kuijt and Lye (2005) analyzed foliar sclerenchyma across multiple species of Struthanthus and Cladocolea and found a wide morphological diversity in both genera, suggesting that they are polyphyletic as currently defined. This idea is supported by surveys of additional morphological characters from multiple species of Struthanthus, where taxa exhibit a wide variation typically observed in distantly related lineages (Kuijt 1975b, 1981, 2012; Kuijt and Hansen 2015).

About 14 species of Struthanthus have been reported from Mexico (Nickrent 2020; Maldonado 2021), with five of them endemic to the country. In the state of Oaxaca, seven species have been documented: S. capitatus Lundell, S. crassipes (Oliv.) Eichler, S. deppeanus (Schltdl. & Cham.) Blume, S. hartwegii (Benth.) Standl., S. interruptus (Kunth) G.Don, S. matudae Lundell, and S. quercicola (Schltdl. & Cham.) Blume. Here we describe a new species of Struthanthus from Oaxaca that shares morphological affinities with S. deppeanus, S. quercicola, and S. ramiro-cruzii Martínez-Ambr. & Sor.-Benítez. These affinities are based on both vegetative (stems and leaf shape) and reproductive (raceme inflorescence) characters.

While preparing a floristic checklist for Struthanthus in Mexico, herbarium material and specimens collected during fieldwork were examined. The description provided below is based on our collections from Sierra Madre del Sur in the state of Oaxaca as well as herbarium specimens from ANSM, CHAPA, CIIDIR, ENCB, HUAA, HUAP, HUMO, IEB, MEXU, RSA, SLPM, UAMIZ, UAS, UAT, USON, XAL, and ZEA (acronyms follow Thiers (2021, continuously updated)).

Specimens were collected, pressed, and dried, with three to five duplicates per number. Vouchers were deposited at HUAP, HUMO, MEXU, and UAMIZ. All specimens gathered had reproductive structures (flowers and fruits) present. Morphological characters were measured from dried specimens and described using the terminology presented by Kuijt (1987, 2003a, b). Hosts and a distributional map are provided for S. ibe-dzi, as well as a taxonomic key and a comparative table to differentiate it from its three morphologically closest congeners in Mexico (S. deppeanus, S. quercicola, and S. ramiro-cruzii). The map was generated with QGIS (QGIS Development Team 2023) v 3.16. with points based on data from our field work and herbarium specimens from other collectors.

Among the currently accepted genera of Loranthaceae, Struthanthus is one of the most taxonomically challenging since no monographic treatment exists (Kuijt 2016; Robles et al. 2016). Numerous specimens in herbaria lack reproductive structures, have immature pistillate or staminate inflorescences, or contain only fruits, raising difficulties regarding their classification. Moreover, the shape of leaf blades may vary within the same individual from young to old branches (Heide-Jørgensen 2008), posing additional identification challenges when specimens are scant or sterile. In fact, many species of Struthanthus have been described from specimens presenting only pistillate or staminate individuals, resulting in numerous synonyms in the genus (Caires and Dettke 2020). Also, specimens only collected with fruits are not always possible to separate from the related genus Passovia (Kuijt 2014). Therefore, fertile specimens from the same locality with both pistillate and staminate mature flowers are essential to facilitate identification of Struthanthus at both generic and species levels.

Struthanthus ibe-dzi morphologically resembles S. deppeanus, S. quercicola, and S. ramiro-cruzii. All these four species have epicortical roots on stems and inflorescences in racemes. Struthanthus deppeanus, S. ibe-dzi, and S. quercicola have similar leaf shape ranging from ovate to lanceolate with a long acute apex (Table 1). Pistillate mature flowers in S. quercicola (Fig. 4A) have a straight to sigmoid style, whereas it is sigmoid to slightly convolute in S. deppeanus (Fig. 4B), convolute in S. ibe-dzi (Fig. 4C), and strongly convolute in S. ramiro-cruzii (Fig. 4D). These stylar convolutions have been previously described for other Mexican species of Struthanthus and Cladocolea (Kuijt 1975a, b, 2003b, 2009a, 2014; Kuijt and Hansen 2015) as well as in three species of Peristethium and Struthanthus from Ecuador and Peru: P. polystachyum (Ruiz & Pav.) Kuijt, P. tortistylum (Kuijt) Kuijt, and S. ophiostylus Kuijt (Kuijt 2012, 2014). Besides the above-described characters, these four species of Struthanthus present geographical structure by being reported for the province of Sierra Madre del Sur. However, S. deppeanus and S. quercicola are also widely distributed across Mexico and Central America, while S. ibe-dzi and S. ramiro-cruzii are restricted to this province (Fig. 3). In terms of elevational ranges, S. deppeanus occurs at elevations from 950 to 2,900 m a.s.l., S. ibe-dzi from 1,960 to 2,650 m a.s.l., S. quercicola from 1,100 to 2,100 m a.s.l., and S. ramiro-cruzii is distributed from 1,600 to 2,200 m a.s.l.

We are grateful to C.S. Caires, E. Martínez-Ambriz, and M.A. Caraballo-Ortiz for their comments and suggestions, especially to the latter for his valuable help to improve the manuscript. This work is part of the M.S. thesis of the first author and is financially supported by the National Council of Science and Technology (CONACYT) through grant 286670. We thank the curators from the herbaria cited in the Materials and Methods section for allowing access to their collections, particularly to IEB, MEXU, and XAL for processing specimen loans. We are also grateful to the Secretaría de Medio Ambiente y Recursos Naturales for granting a collecting permit (SGPA/DGGFS/712/1132/1809/N1-0081/04/18), and to the people from Río Ocotepec, Tlaxiaco, Oaxaca for providing facilities to collect plants specimens.

All authors contributed to the preparation of the manuscript by providing data and reviewing and editing the text. MGMB, LGGG, and RCT conducted the fieldwork, RCT provided photos of living plants, and MGMB prepared stereoscopic photographs and the taxonomic key. MGMB and RCT made the descriptions (measurements and morphological data) of the new species.