﻿A new species of Struthanthus Mart. (Loranthaceae) from Oaxaca, Mexico

﻿Abstract Struthanthusibe-dzisp. 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.

Despite the lack of a comprehensive taxonomic treatment for Struthanthus, new species have been described during the last 20 years (Kuijt 2003a(Kuijt , b, 2009a(Kuijt , 2014González and Morales 2004;Martínez-Ambriz et al. 2017). Also, several generic transfers have occurred, including Cladocolea Tiegh. (Kuijt 1987(Kuijt , 1991b 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(Kuijt , 1981(Kuijt , 2012Kuijt and Hansen 2015).
About 14  -Benítez. These affinities are based on both vegetative (stems and leaf shape) and reproductive (raceme inflorescence) characters.

Materials and methods
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 (1987Kuijt ( , 2003a. 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. Diagnosis. Struthanthus ibe-dzi morphologically resembles S. deppeanus and S. quercicola in having epicortical roots on stems, similar leaf shapes, and inflorescences in racemes. However, the new taxon differs by its compressed nodes, stems, leaves, and inflorescence glaucous; leaf blade with base cuneate to oblique; staminate flowers 6-9 mm long with asymmetrical thecae and an extended horn-shaped apiculate connective in both anther series; and pistillate flowers with distally convoluted styles.
Etymology. The epithet ibe-dzi refers to the common name given to any mistletoe in Zapotec language (Ibë-dzí) in the San Juan Mixtepec region, which means "hair(s) on top of ", where "ibé" denotes "hair(s)" and "dzi" indicates "on top of ".
Conservation status. Struthanthus ibe-dzi is only known from the western part of the state of Oaxaca, near the border with the state of Guerrero (Fig. 3). This species has an estimated area of occupancy of ca. 36 km 2 (criterion B1 < 500 km 2 ) and has been recorded in four localities (condition a: ≤5 locations). The specimen from Reyes 1064 (MEXU, UC/JEPS) from 1988 was collected in a cloud forest. The areas within 1 km of this locality have been actively transformed into crop fields (personal observations from 2019-2021), leading us to consider that condition b(iii) is appropriate for this case, which refers to a projected decline in area, extent, and quality of habitat. Therefore, following the guidelines to the IUCN criteria (IUCN 2022), S. ibe-dzi should be classified as Endangered [EN B1ab(iii)].
Being a hemiparasitic plant with a complete dependence on hosts, mistletoe populations are vulnerable to the indirect effects of logging important host trees such as oaks (Quercus spp.), and habitat modification and fragmentation for livestock and agriculture (Ávalos and Nixon 2004). In addition, most of the Quercus species from Mexico have not been evaluated to determine their conservation status (Valencia-Ávalos and Morales-Saldaña 2016). Press and Phoenix (2005) indicated that the local extinction of a preferred host may lead to population declines and subsequent extinctions of associated parasites, highlighting the importance of hosts for the long-term survival of mistletoes such as S. ibe-dzi, and the perpetuation of populations. Furthermore, parasitic plants have been historically stigmatized and have not received full attention in terms of conservation priorities, even though mistletoes are particularly sensitive to environmental stress and considered keystone species in forests (Watson 2001;Fontúrbel et al. 2018;Crates et al. 2022;Watson et al. 2022).
Notes. Herbarium specimens of S. ibe-dzi have been previously identified as S. deppeanus, S. quercicola, or Struthanthus sp. However, the new taxon differs from S. deppeanus and S. quercicola by its compressed or bicarinate nodes and by having glaucous stems, leaves, and inflorescences which are covered by a whitish wax (observed in both fresh and dried specimens). In addition, S. ibe-dzi has one inflorescence raceme per axil, with peduncled triads with a sessile central flower and pedicellate lateral ones. The bracts and bracteoles are caducous, forming visible scars. Staminate flowers have asymmetrical thecae and an apiculate connectival horn in both series, while pistillate flowers have a convolute style with ± 3 longitudinal folds (Fig. 4C).
Struthanthus ibe-dzi, as other mistletoes in the San Juan Mixtepec region of Oaxaca, are locally known as birdlime vine, and known for the sticky substance (i.e., viscin) produced in the fruits, which is used to catch birds (Hunn 2008).

Discussion
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.