Two new species of Hoya R.Br. (Apocynaceae, Asclepiadoideae) from Borneo

Abstract Two new Hoya R.Br. species from Borneo are described and illustrated. The first, Hoya ruthiae Rodda was collected in Sabah on Bukit Baturong, a limestone outcrop. It is one of the few species in the genus to have clear exudate. It is compared with the morphologically related Hoya uncinata Teijsm. and Binn. The other, Hoya bakoensis Rodda, was collected in the kerangas forests of Bako National Park. It belongs to Hoya section Acanthostemma (Bl.) Kloppenb., a section with numerous members in the Philippines but under-represented in Borneo.


Introduction
Hoya R.Br. is a large and complex genus with more than 500 published species names (IPNI 2015). Kleijn and van Donkelaar (2001) estimated that these names might represent 200-300 species. However their estimate was very conservative. Since 2001 more that 200 new Hoya names have been published (IPNI 2015) and the species number estimate for the genus may therefore now lie in between 350 and 450, taking into account a large number of synonyms expected in a horticulturally signifi cant genus. Th e genus is particularly diverse in the island of Borneo where Nutt (2001) listed 21 species while more recent research bring up the number to an estimated 60-70 species for Sabah alone (Lamb et al. 2014). In Brunei a preliminary checklist comprises 27 species including three undescribed taxa (Rodda 2014).
Numerous papers have been recently published documenting new Hoya species from Borneo (Green and Kloppenburg 2014;Rodda and Nyhuus 2009;Rodda and Simonsson 2010;2011a;b;Rodda and Simonsson Juhonewe 2013a;b;Rodda et al. 2011;2014a;b;Trân et al. 2011). More notably, Lamb et al. (2014) published eight new taxon names in the genus Hoya all described based on materials from Sabah originally collected sterile and brought into cultivation at Kipandi Park (Kampung Kipandi, Moyog, Sabah) where they bloomed and were identifi ed as new taxa. An extensive introduction on the morphology and ecology of Bornean Hoya is also found in Lamb et al. (2014).

Hoya ruthiae
Etymology. Th is species is named after Ruth Kiew (1946-), tropical botanist based at the Forest Research Institute Malaysia and author of numerous publications on Begonia L. and Gesneriaceae, among others.
Distribution and ecology. Hoya ruthiae is only known from Bukit Baturong, Sabah, Malaysia, where it was found growing on limestone.
Conservation status. Th e distribution area, population size and possible threats to the habitat of H. ruthiae are not known, as it is only known from the type specimen and unlocalised cultivated material. It is therefore considered Data Defi cient (DD) (IUCN 2014).
Etymology. Th e species is named after the collection locality of the holotype, Bako National Park (Sarawak, Malaysia).
Distribution and ecology. Hoya bakoensis is only known from Bako National Park, where it is common along Tajor Trail in moist, shady kerangas heath forest near a stream. Th e species is strictly epiphytic and it was found mostly germinating from the opening of small ant nests located inside hollow tree trunks (Fig. 4). Th e plants observed were forming small clumps of weakly climbing stems near the germinating point or more rarely were climbing towards the tree canopy potentially in response to low light.  Most of these species can be easily separated from H. bakoensis because their fl owers are larger (>5 mm in diameter) or their infl orescence is markedly concave (in H. beccarii and H. revoluta). As mentioned in the diagnosis H. aeschynanthoides has white, only slightly larger fl owers than H. bakoensis (ca. 5 vs. 3.5-4 mm in diameter). Additionally the bilobed outer lobes of H. aeschynanthoides appear to be less pronounced than those of H. bakoensis, but this observation is based solely on the examination of the drawing attached to the type of H. aeschynanthoides as only small buds are present on the specimen while no other specimens have been found at present. Th e fl owers of H. pubera are also comparable in size with those of H. bakoensis, but they are yelloworange, the corolla lobes are only apically revolute and the corona is more markedly raised in the centre. Lastly, H. minutifl ora has the smallest fl owers among all Bornean Acanthostemma (2.6-2.8 mm in diameter).