Hoya of Sumatra , an updated checklist , three new species , and a new subspecies

Abstract. The list of the species of Hoya R.Br. occurring on the Indonesian island of Sumatra is updated and the type citation is clarified. Sixteen taxa are added to the latest checklist. Hoya danumensis subsp. amarii S.Rahayu & Rodda subsp. nov., H. rigidifolia S.Rahayu & Rodda sp. nov., H. solokensis S.Rahayu & Rodda sp. nov., and H. sumatrana S.Rahayu & Rodda sp. nov. are newly described. Hoya parviflora Wight, H. parvifolia Schltr., and H. purpureofusca Hook. are lectotypified.


Introduction
Indonesia has been predicted to have the most diverse array of species of Hoya R.Br (Brown 1810;Kleijn & van Donkelaar 2001). However, a full inventory of Hoya occurring in Indonesia is yet to be completed. Indonesia is an extremely large and diverse country and our plan is to complete an inventory and revision of the Indonesian Hoya working on seven separate geographical areas: Sumatra, Java, Kalimantan, Lesser Sunda Islands, Sulawesi, Maluku, and Indonesian New Guinea.
Recent papers on Indonesian Hoya were published by Kleijn & van Donkelaar (2001), who revised the genus in central Sulawesi, where 13 species occur; by Rahayu & Wanntorp (2012), who published a checklist and a key to the Hoya of Sumatra that included 27 species; and lastly by Lamb & Rodda (2016), who published a checklist of the Bornean species including 72 species, 34 of which occurring in Kalimantan. The most recently published Hoya from Sumatra is H. fauziana subsp. angulata .

Etymology
The new species is named after Amar Husein Sitompul, who collected the type specimen.

Distribution and habitat
Hoya danumensis subsp. amarii subsp. nov. is only known from the type locality in northern Sumatra, Indonesia, ca 900 m a.s.l.

Conservation status
Known only from a single collection and lacking information on the distribution area, the population size and the possible threats to the habitat, H. danumensis subsp. amarii is considered as Data Defi cient (DD) (IUCN 2012).

Notes
Hoya danumensis subsp. amarii subsp. nov. can be separated from H. danumensis Rodda & Nyhuus (2009) subsp. danumensis because of the shape and size of the corolla that is shallowly campanulate, 20-23 mm in diameter and deeply campanulate, vs 25-35 mm in the latter; in the shape and size of the corona lobes that are ovate-oblong and 5.5-6 × 2.4-2.6 mm in H. danumensis subsp. danumensis vs ovate and 4.5-5 × 2.3-2.5 mm in the new subspecies.

Remarks
This name is not recorded in Tropicos (http://www.tropicos.org, accessed on 24 Jul. 2018) and IPNI (http://www.ipni.org, accessed on 24 Jul. 2018) only records it as a nomen nudum (Teijsmann & Binnendijk 1866). The species was validly published by Teijsmann & Binnendijk (1863). Original material has been searched at BO, KRB, L, and U, but none has been found and it is possible that the species was described based on a live plant only.

Distribution and habitat
Hoya rigidifolia sp. nov. is only known from the type locality in the lowland forest of Pulau Siberut, southern Sumatra, Indonesia.

Conservation status
Known only from a single collection and lacking information on the distribution area, the population size and the possible threats to the habitat, H. rigidifolia sp. nov. is considered as Data Defi cient (DD) (IUCN 2012). Both species are, however, easily separated based on leaf characters. Hoya fi nlaysonii has lanceolate to oblong leaves with pinnate venation, usually darker than the rest of the lamina; H. rigidifolia sp. nov. also has pinnate venation, but additionally it has two basal secondary veins running along the entire length of the lamina. The leaf venation of H. erythrina Rintz (1978) is similar to that of H. rigidifolia sp. nov.; however, both species can be separated based on fl ower charaters: H. erythrina has fl owers with corolla > 15 mm in diameter when fl attened, long-pubescent inside, and lasting > 2 days, while H. rigidifolia sp. nov. has fl owers with corolla < 12 mm in diameter when fl attened, glabrous inside, and lasting one day.

Distribution and habitat
Hoya solokensis sp. nov. is only known from the type locality in Solok, Sumatra, Indonesia, at ca 1000 m a.s.l. It was collected in an evergreen forest, where it was growing epiphytically.

Conservation status
Known only from a single collection and lacking information on the distribution area, the population size and the possible threats to the habitat, H. solokensis sp. nov. is considered as Data Defi cient (DD) (IUCN 2012).

Notes
Hoya solokensis sp. nov. is one of the few species of Hoya that exhibits a non-climbing habit but is instead an epiphytic shrub.

Distribution and habitat
Hoya sumatrana sp. nov. is only known from the type locality in in Lampung, Sumatra, Indonesia. It was collected in a coastal swamp forest, where it was growing as an epiphyte.

Conservation status
Known only from a single collection and lacking information on the distribution area, the population size and the possible threats to the habitat, H. sumatrana sp. nov. is considered as Data Defi cient (DD) (IUCN 2012).

Notes
Hoya sumatrana sp. nov. is a new species in H. sect. Peltostemma Schlechter (1916: 14), characterized by upright corona lobes, long linear anther appendages extending well above the style-head and pollinaria with well developed caudicle wings. This section includes six species, two of which, H. soidaoensis Kidyoo (2013) and H. phuwuaensis Kidyoo (2016), have been recently published. Hoya sumatrana sp. nov. can be separated from all other members of this section because its fl owers are much smaller; for example, the corolla of H. sumatrana sp. nov. is 7-9 mm in diameter, while the corolla of the second smallest species, H. fl agellata Kerr (1940: tab. 3407), is > 10 mm in diameter, as well as the corona of Hoya sumatrana sp. nov. is 2.8-3.2 mm in diameter, while that of H. fl agellata is > 4 mm in diameter. Another useful character to separate H. sumatrana sp. nov. from all other members of the section is the inner corona lobe process, that is bilobed in H. sumatrana sp. nov., while the other species have acute or rounded processes.

Type material
Lectotype (designated by Veldkamp et al. 1995)
The total count of taxa of Hoya known from Sumatra now stands at 43 and includes 41 species and two subspecies.