Friday, June 5, 2015

[Botany • 2015] Hoya undulata • A New Myrmecophytic Species (Apocynaceae, Asclepiadoideae) from Borneo (West Kalimantan, Indonesia)

Fig. 1. Hoya undulata S.Rahayu & Rodda (from M. Rodda MR650, SING)
A. Buds. B. Inflorescence, top view. C. Inflorescence from underneath. D. Domatia; E. Leaf, abaxial, with a magnification of the minutely undulate margin. F. Leaf, adaxial, with a magnification of the basal colleters.
(Photos: A, B, E, F, Michele Rodda; D, Surisa Somadee)

 In this paper we document the discovery of a new Hoya species from Borneo, Hoya undulata S.Rahayu & Rodda. The leaves of the new species form domatia, specialised structures harbouring ants, previously only observed in Hoya mitrata Kerr and H. darwinii Loher. The corona is unique among Bornean Hoya species as its lobes develop two lateral hooked appendages, also present in H. griffithii Hook.f, a species from mainland Asia. A lectotype for Hoya darwinii is also selected. 

Keywords. Ant plant, domatia, heath forest, Hoya mitrata, Indonesia, myrmecophyte, West Kalimantan

Hoya undulata S.Rahayu & Rodda sp. nov. Similar to Hoya mitrata Kerr and H. darwinii Loher when sterile because it has specialised convex leaves forming multileaved domatia. Easily separated when fertile because the flowers have a rotate corolla formed by almost completely free corolla lobes, while Hoya mitrata and H. darwinii have reflexed corollas with a tube almost as long as the lobes. 
– TYPE: Indonesia, West Kalimantan, Putussibau, 300–380 m, on a slope above a stream, February 2014, Sulaiman Hasim s.n. (holotype BO). (Fig. 1, 2)

Distribution. Only known from the type locality in West Kalimantan, close to Putussibau. A second unlocalised collection is widely available in cultivation. Etymology. The specific epithet refers to the characteristic undulate leaf margin. 

Habitat and ecology. Observed in lowland heath forest at 300–380 m above sea level on a slope above a stream, epiphytic on small tree trunks about 1.5 m above ground, growing in about 80% humidity and 50% sunlight. (Sulaiman Hasim pers. comm.). According to the epiphytic zonation by Johansson (1975), the species grows in zone B. The zonation of epiphytes is mainly correlated with their light, nutrients and water requirement. Some species are restricted to strongly illuminated sites, some to shady sites, while some avoid both strong light and deep shade and yet others have a wide range of tolerance (Benzing, 1990). Usually, occupying the B zone means that the species is adapted to moist shady habitats. The leaves of Hoya undulata can adapt to high light levels, but they have been observed to develop a purple colour in intense sunlight. At the type locality, the plants were rooting in ant nests and, on casual observation, the leaf domatia harboured ants. 

Provisional IUCN conservation assessment. Known from only one locality, the preliminary conservation status of Hoya undulata is Data Deficient (DD, IUCN 2014). Ex situ collections are present in Bogor Botanic Gardens (from the type locality) and in Singapore Botanic Gardens (SBG acc. no. 20132428). 

S. Rahayu, U. Meve and M. Rodda. 2015. Hoya undulata (Apocynaceae, Asclepiadoideae), A New Myrmecophytic Species from Borneo, and typification of H. darwinii. Gardens’ Bulletin Singapore. 67(1): 85–94. doi: 10.3850/S2382581215000101