Struthanthus ibe-dzi Mald. & Cerros, 2023

Maldonado-Borja, Maria Guadalupe, Cerros-Tlatilpa, Rosa & Galvan-Gonzalez, Luis Gil, 2023, A new species of Struthanthus Mart. (Loranthaceae) from Oaxaca, Mexico, PhytoKeys 225, pp. 69-81 : 69

Struthanthus ibe-dzi Mald. & Cerros sp. nov.

Type.

Mexico. Oaxaca: Tlaxiaco, Río Ocotepec, en el arroyo Yute kuini ( San Juan del Rio Cuquila ), carretera Tlaxiaco-Putla , 17°10'26.48"N, 97°46'17.29"W [17.174022°N, - 97.771469°W], 1962 m a.s.l., 30 Mar 2021, M.G. Maldonado, L.G. Galván G. & R. Cerros T. 21 (♀ fl, fr) (holotype: HUMO-39855!, isotypes: MEXU!, UAMIZ!) .

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.

Description.

Aerial hemiparasitic woody shrub, pendulous, perennial, with epicortical roots present at the base of main trunk; branches pendant. Stems green when young, brown with lenticels when mature; nodes glaucous, bicarinate and compressed, especially when young; internodes terete, with epicortical roots. Leaves opposite or subopposite; petioles 0.25-1.2 cm long, twisted, forming a shallow channel from the raised edge of leaf blade; blades ovate to lanceolate, rarely elliptical, 5.0-12.2 × 1.4-5.0 cm, papyraceous when dried; apex acute to acuminate, base cuneate to oblique, margin entire to repand, hyaline, venation pinnate. Inflorescences a solitary raceme of triads, indeterminate and axillar; bracts and bracteoles caducous at or after anthesis, cymbiform; rachis subterete to compressed, nodes compressed, triads opposite or subopposite, decussate, green, glabrous, glaucous. Staminate inflorescence 2.0-6.8 cm long, peduncle 0.2-0.8 cm with 6-16 (19) triads, triad peduncle 0.10-0.58 cm long. Pistillate inflorescence 2.0-5.0 cm long, peduncle 0.2-1.0 cm long with 6-12 (15) triads, triad peduncle 0.18-0.86 cm long. Staminate flowers hexamerous, rarely pentamerous, flower buds clavate with rounded apex; central flower of triad sessile, lateral pedicels 0.28-1.3 mm long; mature flowers 6.0-9.0 × 2.0-2.3 mm, petals linear, reflexed near the apex, 4.4-7.9 × 0.8-1.2 mm, anthers basifixed (not versatile) in two series, theca asymmetrical; prominent connectival apiculate horn in both the lower and upper series; calyculus irregularly dentate, whitish, vestigial ovary 1.5-2.2 mm; pistiloid straight to sigmoid 1/3 near the apex, 2.8-6.5 mm long, stigma undifferenced; nectary thick with six protuberances surrounding the pistiloid base. Pistillate flowers hexamerous, flower buds cylindrical, rounded at the apex; central flower of the triad sessile, pedicels of lateral flowers 0.18-0.8 cm long, slightly accrescent when bearing fruit; mature flowers 5.8-7.2 × 1.6-2.2 mm, linear petals 5.2-5.6 × 0.8-1.0 mm, staminodes in one series; calyculus whitish, irregularly dentate, inferior ovary 1.7-2.0 mm; style convolute 4.0-5.2 mm long ( ± 3 longitudinal folds) from the middle to the apex, stigma capitate; nectary thick with six protuberances surrounding the style. Fruit a one-seeded berry, ovoid, 3.85-6.0 × 6.52-8.60 mm. Seeds ovoid, 3.0-4.7 × 5.1-7.4 mm. Figs 1 , 2 .

Additional specimens examined

(Paratypes). Mexico. Oaxaca: San Juan Mixtepec, Yucu Shúun ( Monte de Tesoro ) a 16 km S de San Juan Mixtepec, 17°13'37.46"N, 97°47'54.5"W [17.22707°N, - 97.78484°W], 2,500 m a.s.l., 8 Nov 1988 (♀ fr), J. Reyes S. 1064 (MEXU, UC/JEPS); Putla Villa de Guerrero, 2.1 km después de Santo Domingo Chicahuaxtla, hacia Putla, [17.142761°N, - 97.848589°W], 2,050 m a.s.l., 5 Feb 1993 (♀ fr), M. Cházaro B. 7088 (CHAPA, ENCB, MEXU, XAL); Santiago Juxtlahuaca, El Manzanal, senda para la parcela del Sr. Hemeterio, entrada por Santa Rosa-San Miguel Cuevas, Distrito Juxtlahuaca, 17°13'13.20"N, 98°3'38.30"W [17.22033°N, - 98.06063°W], 2,060 m a.s.l., 13 Sep 1996 (♀ fl, fr), J.I. Calzada 21381 (MEXU); San Juan Mixtepec , Camino a Santos Reyes Tepejillo, 1 km antes de la desviación al Capulín y Tinuama de Zaragoza , 17°19'23.4"N, 97°54'13.2"W [17.32317°N, - 97.90367°W], 2,650 m a.s.l., 9 Apr 2019 (♂ fl), L.G. Galván G. & R. Cerros T. 474 (HUMO); Putla Villa de Guerrero, orilla de la carretera, km 92, 2.8 km antes de San Andres Chicahuaxtla, 17°10'48.20"N, 97°49'32.48"W [17.18014°N, - 97.82569°W], 2,363 m a.s.l., 13 Feb 2020 (♀ fr), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 13 (HUMO); ibid, 13 Feb 2020 (♀ fr), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 14 (HUMO); Tlaxiaco, Rio Ocotepec , en el arroyo Yute kuini ( San Juan del Rio Cuquila ), carretera Tlaxiaco-Putla, 17°10'26.48"N, 97°46'17.29"W [17.17402°N, - 97.77146°W], 1,962 m a.s.l., 30 Apr. 2021 (♂ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 19 (HUMO); ibid, 30 Apr 2021 (♀ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 20 (HUMO); ibid, 30 Apr 2021 (♀ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 22 (HUMO); ibid, 30 Apr 2021 (♀ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 23 (HUMO); Putla Villa de Guerrero, km 92, carretera Tlaxiaco-Putla, 200 m antes de La Cañada Tejocote, 17°10'48.38"N, 97°46'31.94"W [17.18010°N, - 97.82554°W], 2,374 m a.s.l., 30 Apr 2021 (♂ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 24 (HUMO); ibid, 30 Apr 2021 (♀ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 25 (HUMO); ibid, 30 Apr 2021 (♂ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 26 (HUMO); Putla Villa de Guerrero, Orilla de carretera de Tlaxiaco-Putla, en San Andres Chicahuaxtla, 17°09'42.16"N, 97°50'11.85"W [17.16171°N, - 97.83662°W], 2,473 m a.s.l., 30 Apr 2021 (♂ fl), M.G. Maldonado, R. Cerros T. & L.G. Galván G. 27 (HUMO) .

Distribution, habitat, and hosts.

Struthanthus ibe-dzi is endemic to Oaxaca, Mexico, where it is only known from cloud and oak-pine forests with secondary vegetation in the Sierra Madre del Sur ( Morrone et al. 2017) in the municipalities of Putla Villa de Guerrero, San Juan Mixtepec, Santiago Juxtlahuaca, and Tlaxiaco (Fig. 3 ) at elevations between 1,962 to 2,650 m a.s.l. Recorded hosts to date include Alnus spp. ( Betulaceae ), Quercus spp. ( Fagaceae ), and Salix spp. ( Salicaceae ).

Phenology.

Flowering from March to April and in September; fruiting in February to April and September to November. Individuals can be found bearing flowers and fruits on different branches.

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 km2 (criterion B1 <500 km2) 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).

Figures

Figure 1. Struthanthus ibe-dzi Mald. & Cerros, sp. nov. (Loranthaceae). Holotype: M. G. Maldonado, L. G. Galvan G. & R. Cerros T. 21 (♀ fl, fr), HUMO- 39855.

Figure 2. Struthanthus ibe-dzi (Loranthaceae) A base of stem and disk of secondary haustoria (sh) B epicortical root (er) and swollen stem of host plant by the penetration of secondary haustoria (sh) C staminate inflorescences with caducous bracts (bra) and bracteoles (brc) D dissected petals of staminate flower showing two series of stamens (S 1, S 2), with anthers in the lower series (S 1) displaying a prominent connectival horn (cn); receptacle showing an undifferentiated pistilloid and a nectary (ne) surrounded by the calyculus E pistillate inflorescences and compressed node (arrow) F dissected pistillate flower showing vestigial staminodes (st) opposite to petals, and an inferior ovary with convoluted style and calyculus (ca) G upper view of ovary in a pistillate flower showing its calyculus (ca) and nectary (ne) H immature fruits crowned by the calyculus.

Figure 3. Distribution map of Struthanthus deppeanus, S. ibe-dzi, S. quercicola, and S. ramiro-cruzii in Oaxaca and surrounding states of central Mexico.

Figure 4. Style types in four species of Struthanthus (Loranthaceae) from Mexico A S. quercicola (straight to sigmoid) B S. deppeanus (sigmoid to slightly convoluted) C S. ibe-dzi (convolute) D S. ramiro-cruzii (strongly convolute).