Two new species of Hoya (Apocynaceae, Asclepiadoideae) from Sulawesi, Indonesia

Abstract

We document the discovery of two new species of Hoya from Sulawesi, Indonesia, H. sulawesiana S.Rahayu & Rodda and Hoya surisana Rodda & S.Rahayu. Hoya sulawesiana is similar to Hoya isabelchanae Rodda & Simonsson in flower morphology because both species have very pubescent globose corolla but they can be separated based on leaf morphology and corolla indumentum type. Hoya surisana is very unusual because its flowers display a combination of large calyx lobes, a white, pubescent, campanulate corolla, and dark purple corona lobes so far not observed in any other species of the genus.

Full text

Taiwania 65(2): 209‒215, 2020
DOI: 10.6165/tai.2020.65.209
209
Two new species of Hoya (Apocynaceae, Asclepiadoideae) from Sulawesi,
Indonesia
Michele RODDA1,*, Sri RAHAYU2
1. Singapore Botanic Gardens, National Parks Board, 1 Cluny Road, Singapore, 259569, Singapore.
2. Bogor Botanic Gardens, Indonesian Institute of Sciences, Bogor, Indonesia.
*Corresponding author’s email: rodda.michele@gmail.com
(Manuscript received 23 January 2020; Accepted 30 March 2020; Online published 7 April 2020)
ABSTRACT: We document the discovery of two new species of Hoya from Sulawesi, Indonesia, H. sulawesiana S.Rahayu &
Rodda and Hoya surisana Rodda & S.Rahayu. Hoya sulawesiana is similar to Hoya isabelchanae Rodda & Simonsson in flower
morphology because both species have very pubescent globose corolla but they can be separated based on leaf morphology and
corolla indumentum type. Hoya surisana is very unusual because its flowers display a combination of large calyx lobes, a white,
pubescent, campanulate corolla, and dark purple corona lobes so far not observed in any other species of the genus.
KEY WORDS: Acanthostemma, Celebes, Cystidianthus, Hoya, Makalangkan Mountain, Mamuju, Marsdenieae.
INTRODUCTION
In recent years taxonomic research on the genus
Hoya R.Br. in Indonesia has intensified, with the aim to
eventually complete a revision of the genus for Indonesia.
First the species of Borneo were investigated in detail by
Lamb and Rodda (2016), who recorded 72 species, 34 of
which occur in the Indonesian southern part of the Island.
Subsequently Rahayu and Rodda (2019) investigated the
diversity of Hoya of Sumatra, where 43 taxa were found,
(41 species and two subspecies). These numbers are far
from final and instead are just a starting point for further
research: new species from Borneo and Sumatra have in
fact already been published since (Rahayu and Rodda,
2017; Rahayu and Astuti, 2019; Rodda et al., 2018).
We have recently extended our research to Sulawesi,
an island in the centre of the SE Asian ecoregion of
Wallacea. Sulawesi, unlike Borneo or New Guinea, has
long been separated from the continental region and
therefore is characterised by a highly unique array of
plants (Monk et al. 1997, Irawati and Widyatmoko 2019).
In contrast to the high plant diversity, Sulawesi is one
of the most poorly studied islands in Southeast Asia (de
Vogel, 1989; Cannon et al., 2007). The density of
herbarium specimens collected from Sulawesi is
relatively low, with 24 specimens per 100 km2, similar
to that of Sumatra (22) but much less than Borneo (35)
or New Guinea (46) (Campbell and Hammond, 1989).
With an estimated 5972 plant species, 37.3% of which
endemic, Sulawesi has a similar number of estimated
endemics as Kalimantan in Borneo (39.5%)
(KPPN/Bappenas 2016; Middleton et al., 2019).
The first species of Hoya described from Sulawesi
was Hoya multiflora Blume (as Centrostemma
micranthum Blume). The genus Hoya in Sulawesi was
first revised by Miquel (1856), who listed two species,
Hoya multiflora Blume (as Cyrtoceras micranthum
(Blume) Miq.), and Hoya ariadna Decne (likely a
synonym of Hoya coronaria Blume). Subsequently in
1863 Hoya maxima Teijsm. & Binn. was added
(Teijsmann and Binnendijk 1863) (= Hoya imbricata
Decne.). Four species were described from Sulawesi by
Schlechter (1908, 1916): Hoya gracilis Schltr., Hoya
dolichosparte Schltr., Hoya incurvula Schltr. and Hoya
minahassae Schltr.
The most recent and comprehensive study on Hoya
of Sulawesi was published by Kleijn and van Donkelaar
(2001), who focussed on central Sulawesi, where 13
species were found. They estimated the number of
species of Hoya for Sulawesi at between 15 and 20.
Three new species have since been published (Rodda
and Simonsson Juhonewe, 2016). Based on preliminary
data, Sulawesi has 15 species of Hoya, about 70 % of
which are endemic to the island. It is likely that the
number of species for Sulawesi will increase steeply as
research progresses as has been observed for Borneo,
Sumatra and New Guinea (Lamb and Rodda, 2016;
Simonsson Juhonewe and Rodda, 2017; Rahayu and
Rodda, 2019), and will far exceed the estimate of species
given by Kleijn and van Donkelaar (2001).
As noted by Rahayu and Rodda (2019), unidentified
Hoya species are often posted on social media for
identification. In 2018, Indonesian Hoya enthusiasts
posted on Facebook pictures of two unidentified Hoya
species from Sulawesi. Later the species became
available in the international horticulture market. After
the original source of the plants was traced by SR,
samples for study were obtained and deposited at BO
herbarium allowing the description of the new species.

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TAXONOMIC TREATMENT
Hoya sulawesiana S.Rahayu & Rodda, sp. nov.
Fig 1
Similar to Hoya isabelchanae Rodda & Simonsson
in inflorescence, corolla and corona morphology, but
differing in lamina shape (orbicular-ovate (to elliptic),
convex, to 7 cm long in H. isabelchanae vs. lanceolate
or elliptic in H. sulawesiana 7–30 cm long), and in type
of corolla inner surface pubescence (hairs straight and
pointing outwards in H. isabelchanae vs. recurved
towards the corona in H. sulawesiana).
Type: INDONESIA, Sulawesi, West Sulawesi,
Mamuju, 100 m elev., originally collected by Andarias
Sambokaraeng, vouchered in cultivation at Bogor Botanic
Garden, 17 June 2019, Rahayu 1215 (holotype, BO).
Epiphytic climber with white latex in all vegetative
parts. Stems slender yet stiff, to 10 m long, twining,
creeping and hanging on trees and tree branches,
internodes (2)5–20(30) cm long, 2–3 mm in diameter,
dull green or brown purplish and pubescent when young,
dull green to greyish and glabrous when mature, surface
rough; adventitious root sparsely produced along the
stem and just under the nodes where they are usually
paired. Leaves: petiole straight or recurved, cylindrical,
6–12 mm long, 2–4 mm in diameter, pale green, purplish,
greyish or brownish, sparsely pubescent to glabrous,
surface rough (when dry); lamina of two types , the first
lanceolate, very stiff and fleshy, flat, 7.5–30 × 1.5–3.5
cm, up to 3 mm thick, base acute, apex acuminate,
abaxially paler green, sometime with reddish or purplish
hue, adaxially dark green or purplish, sparsely pubescent
when young and glabrous when mature, secondary veins
barely visible above, not visible below, 5–8 each side of
the midrib, slightly sunken; the second type elliptic, flat
to sometime slightly carinate especially when young,
fleshy and stiff, 7–9.5 × 2.4–3.5 cm base sub-acute, apex
acute, abaxially mid to dark green, glabrous, adaxially
pale to mid-green green or sometime reddish, sparsely
pubescent turning glabrescent above, secondary veins
not visible; colleters 1 or 2 at lamina base, triangular to
ovate 0.3–0.5 mm long. Inflorescence positively
geotropic, pseudo-umbelliform, flat to slightly concave;
peduncle extra axillary, 7–12(–25) cm long, 1.5–2 mm
in diameter, dull green to brown, sparsely pubescent
when young, turning glabrous, papillose; rachis
indeterminate. Flowers (4)6–10(15) per inflorescence;
pedicel variable in length, inner ones 1–1.5 cm long,
outer ones 2.5–3 cm long, 1.2–1.5 mm in diameter,
pinkish cream, pinkish at apex and base, glabrous. Calyx
lobes triangular, 1.5–2 × 1–0.8 mm, apex narrow with
blunt tips, white or cream, glabrous; basal colleter one at
each calyx lobe sinus, ovate, 300–400. × ca. 100 μm.
Corolla rotate with strongly reflexed lobes, 10–15 mm in
diameter, ca. 20 mm in diameter when flattened; corolla
lobes reflexed with revolute tips; tube 2–3 mm long, dark
purple, abaxially glabrous, adaxially shortly pubescent,
denser towards the centre; lobes ovate, 7–9 × 5–6 mm,
apex acuminate, basally dark purple turning pale salmon
pink at the tips, abaxially glabrous, adaxially covered
with dense long, silvery hair recurved towards the
corolla centre (excluding glabrous tip and sparsely
pubescent base). Corona staminal, 7–10 mm in diameter,
4–6 mm high; corona lobes broadly ovoid, 3.5–4 × 2–2.5
mm, slightly ridged above, underneath sulcate, inner
process apex acuminate, outer process apex divided in
an upper round part and a lower bilobed part, inner
process and upper part of outer process pale pink, lower
outer process bilobed part reddish purple. Anther
appendages broadly triangular, c. 1.2 × 1.2 mm. Pollinia
oblong, 380–420 × 140–160 μm, base rounded, apex
lunate, divergent, pellucid margin all along the outer
edge of the pollinium; corpusculum oblong, 120–150 ×
50–60 μm; caudicle broad, spathulate, 100–120 × 110
μm at the widest point. Style-head 5 angled in cross
section, c. 2.5 mm in diameter, apex conical; ovary
conical, 1.2–1.5 mm long, each carpel ca. 0.6 mm wide
at the base, pale yellow, glabrous. Fruit and seed not
observed.
Distribution. Only known from South Sulawesi in
Towuti and West Sulawesi in Mamuju at low elevation
(100–300 m).
Etymology. Hoya sulawesiana is named after the
island of Sulawesi, where it is endemic.
Habitat and ecology. Observed in lowland forest at
100–300 m above sea level and above a stream in Towuti,
growing epiphytically on small tree trunks about 5 m
above ground, growing in about 80% humidity and 50–
70% sunlight at the time of collection (Aspar, pers.
comm.). Hoya sulawesiana can grow in low as well as
high light levels, but the leaves develop a purple colour
when exposed to intense sunlight. At the type locality in
Mamuju, the plants were rooting in ant nests. (Andarias
Sambokaraeng, pers.com).
Notes. Hoya sulawesiana is most similar to Hoya
ischabelchanae. Both species belong to Hoya section
Acanthostemma (Blume) Kloppenb. because they have
revolute corolla lobes, corona lobes with bilobed outer
apices and pollinaria with broad, spathulate caudicles.
Both species are endemic to Sulawesi: Hoya
ischabelchanae is from Gorontalo in the north part of the
island, while Hoya sulawesiana is from the south and
west part of Sulawesi. Their similarities lie in the
positively geotropic, convex inflorescences, rotate
corollas with revolute pubescent lobes and corona lobes
with bilobed outer processes. Further, the coronas of the
two species are of similar size: in H. isabelchanae the
corona is 7–8 mm in diameter, 3–3.5 mm high, while in
H. sulawesiana it is 7–10 mm in diameter, 4–6 mm high.
Hoya sulawesiana differs from Hoya isabelchanae
in both vegetative and fertile parts. The leaves of H.
ischabelchanae are orbicular-ovate (to elliptic), convex,

2020 Rodda & Rahayu: New Hoya from Sulawesi
211
Fig. 1. Hoya sulawesiana S.Rahayu & Rodda. A, inflorescence; B, inflorescence, side view; C, flower, top view; D, flower, side view;
E, corona, side view; F, corona, from underneath; G, leaves: right, from above; left, from underneath; H, pollinarium. photos, A-G by
Surisa Somadee; H by Michele Rodda.
(1.5–)2–4(–7) × (1–)1.5–2.5 cm, with a cuneate (round)
base, and an obtuse (round) apex, while the leaves of H.
sulawesiana are much larger, lanceolate or elliptic, flat
or slightly carinate, 7–30 × 1.5–3.5 cm, with an acute
base, and an acute or acuminate apex. The corolla lobes
of H. sulawesiana are covered on the outside with dense
long hair recurved towards the corona inside while the
hairs on the corolla lobes of H. isabelchanae are straight
and point outwards.
Proposed IUCN category. Known from only two
localities at Mamuju – West Sulawesi and Towuti – South
Sulawesi with unknown population size, the preliminary
conservation status of Hoya sulawesiana is Data Deficient
(IUCN 2012). Ex situ collections are present in Bogor
Botanic Gardens (from the type locality).
Additional specimen examined. Cultivated in Thailand,
Nakhorn Si Thammarat, May 2019, S. Somadee in M. Rodda MR1956
(SING).

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Hoya surisana Rodda & S.Rahayu, sp. nov.
Figs 2 & 3
Similar to Hoya mappigera in corolla and corona
shape and colour, but differing in number of flowers, size
and shape of the calyx and in corolla pubescence.
Type: INDONESIA, Sulawesi, West Sulawesi,
Mamasa, Pebassian, Makalangkan mountain, ca 500 m
elev., originally collected by Andarias Sambokaraeng,
vouchered in cultivation at Bogor Botanic Garden, 5
August 2019, Rahayu 1221 (holotype, BO).
Terrestrial climber with white latex in all vegetative
parts. Roots fibrous, basal; adventitious roots absent,
unless in direct contact with substrate. Stems terete, 1.5–
3 mm in diameter, mid green when young, later dark
green or greyish brown pubescent turning glabrescent;
internodes (2–)5–15(–25) cm long. Petiole cylindrical,
slightly channelled above, 4–10 mm long, 2–3 mm in
diameter; lamina coriaceous, oblong or elliptic-
lanceolate, (5–)8–12 × (1.5–)3–6 cm, base round
(cuneate), apex acuminate, abaxially pale green,
adaxially mid green, venation pinnate, main vein
depressed on adaxial surface, evident on abaxial surface,
secondary veins, 5–10 each side, more evident abaxially,
anastomosing; tertiary venation reticulate, adaxially
prominent on immature leaves only; colleters at lamina
base 3–6, conical, 0.5–0.8 mm long. Inflorescence
umbelliform, convex, positively geotropic, up to 10-
flowered; peduncle extra-axillary, terete, persistent, 1.5–
2.5 cm long, c. 3 mm in diameter, glabrous, rachis
indeterminate; pedicel terete, 1.8–2.8 cm long, 1–1.2
mm in diameter, sparsely pubescent. Calyx lobes ovate-
lanceolate, spatulate, 14–17 × 7–9 mm, apex acuminate,
greenish white, outside glabrous, inside very sparsely
pubescent, margins ciliate. Basal colleter 1(0) at each
calyx lobe sinus, oblong, 0.2–0.3 × c. 0.1 mm. Corolla
broadly campanulate, membranous, 1.5–2 cm in
diameter, with lobes sharply bent outwards; tube broadly
campanulate, 7–9 mm long, white, outside glabrous,
inside very finely pubescent; lobes triangular, sharply
bent outwards, 7–9 × 9–10 mm, white, abaxially
glabrous, adaxially very finely pubescent. Corona
stipitate, stipe ca 1 mm long, ca 2.5 mm in diameter;
corona lobes with a basal, spreading globose process ca
2 mm in diameter with basal revolute margins, dark
purple, and an apical lunate process 3–3.5 mm long,
apically acute, dark purple. Guide rails ca 3 mm long,
yellow. Style head conical ca 2 mm in diameter, ca 1.5
mm high, covered by the anther appendages. Pollinia,
oblong, 900–1100 × 150–200 μm, without pellucid
margin, corpusculum ovate, 700–750 × 480–520 μm,
caudicles triangular c. 120 μm long. Anther appendages
ovate, ca 2.5 × 1.5 mm, membranous, hyaline, apically
acuminate. Ovary oblong, ca 3 × 1 mm, glabrous. Fruit
a single follicle, fusiform, 15–18 × 1–1.5 cm, base with
pedicel laterally attached, apex acuminate, seeds 3.5–4.5
× c. 1.5 mm, long comose.
Distribution. only known from West Sulawesi,
Makalangkan mountain, at about 500 m elev.
Etymology. Hoya surisana is named after Surisa
Somadee, Thai Hoya expert and grower.
Habitat and ecology. Hoya surisana is found in pine
forest on steep slopes. This species was seen rooted in
the ground and then producing long climbing stems
growing onto the nearby trees.
Notes. Hoya surisana is an unusual species of Hoya
that combines characters so far not observed in such a
combination in any other species of the genus. It is
therefore extremely difficult to pinpoint a similar species
for morphological comparison. The leaves are similar to
these of Hoya thuathienhuensis T.B.Tran et al., a species
endemic to Vietnam. The similarities are particularly
evident in the very visible venation of the young leaves,
that are paler than the rest of the lamina, and become
darker as the leaf matures. Both species are also
terrestrial climbers. The mature plain mid green glabrous
leaves are also comparable to these of Hoya ischnopus
Schltr. from New Guinea, which are almost
indistinguishable. Both H. thuathienhuensis and H.
ischnopus have corollas with reflexed lobes, while H.
surisana has campanulate corollas.
The most prominent feature of the flower of H.
surisana is the rather large, broadly campanulate, white
corolla. This has been observed in numerous species
such as Hoya campanulata Blume, Hoya danumensis
Rodda & Nyhuus, H. mappigera Rodda & Simonsson
and Hoya wallichii (Wight) C.M.Burton from Sundaland.
All these species have glabrous corolla, while the corolla
of H. surisana is very finely pubescent inside.
Additionally, these species have small calyx lobes that
do not extend beyond the corolla lobe sinuses while in H.
surisana the calyx lobes are very prominent and extend
beyond the corolla lobe sinus. Very conspicuous and
large calyx lobes have been observed in Hoya griffithii
Hook.f. and Hoya thailandica Thaithong from
continental Southeast Asia and some forms of Hoya
calycina Schltr. from New Guinea. However, none of
these species have a campanulate corolla. The corona of
H. surisana is somewhat similar to that of Hoya
mappigera from peninsular Malaysia and Borneo. In
both species the corona lobes have a distinct swollen
basal process and an erect elongated process. As
mentioned above the two species can be separated based
on the corolla pubescence and the vastly different size of
the calyx. Further, H. mappigera produces
inflorescences that produce only one flower at a time
while H. surisana inflorescences produce up to 10
flowers opening concurrently.
Additional specimen examined. Cultivated in Thailand,
Nakhorn Si Thammarat, May 2019, S. Somadee in M. Rodda MR1955
(SING).

2020 Rodda & Rahayu: New Hoya from Sulawesi
213
Fig. 2. Hoya surisana Rodda & S.Rahayu. A, inflorescence; B, buds, almost ready to open; C, inflorescence back view, showing the
large calices; D, leaves: right, from underneath; middle, from above; left, from above and immature; photos, A-C by Surisa Somadee;
D by Michele Rodda.
ACKNOWLEDGMENTS
We thank Mr. Aspar from Towuti, South Sulawesi, Andarias
Sambokaraeng from Mamasa, West Sulawesi and Surisa
Somadee from Thailand who provided material of the new
species for study and cultivation in Bogor Botanic Gardens and
Singapore Botanic Gardens; We also thank the curators of BCU,
BK, BKF, BM, BO, BRUN, FI, K, KEP, L, LAE, P, SAN, SAR,
SNP, SING, UC and UPM herbaria for allowing access and/or
for providing high quality images of herbarium specimens.
Finally we would like to thank two anonymous reviewers for
their valuable comments on the manuscript.
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Fig. 3. Hoya surisana Rodda & S.Rahayu. A, flower; B, flower, side view; C, calyx; D, E, corona, side view; F, corona, from underneath;
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