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E D I N B U R G H J O U R N A L O F B O T A N Y 69 (1): 83–93 (2012) 83
ÓTrustees of the Royal Botanic Garden Edinburgh (2012)
doi:10.1017/S0960428611000412
TAXONOMY OF HOYA LYI,
HOYA YUENNANENSIS
AND HOYA MEKONGENSIS
(APOCYNACEAE – ASCLEPIADOIDEAE)
M. RODDA
Hoya lyi H.Le
´v. and H. yuennanensis Hand.-Mazz. (Apocynaceae – Asclepiadoideae),
described at the beginning of the 20th century from specimens collected in South West
China, have often been confused with each other and with H. carnosa R.Br. Currently
Hoya yuennanensis is considered a synonym of H. lyi but after the selection of lectotypes
and the comparison of extensive herbarium materials from the region two species are once
again recognised. Emended descriptions and illustrations are provided. The recently
described Hoya mekongensis M.G.Gilbert & P.T.Li is synonymised with H. yuennanensis.
Keywords. Lao PDR, lectotypification, South West China, taxonomy, Vietnam, Yunnan.
Introduction
Hoya R.Br. (Apocynaceae) is a monophyletic genus well rooted within Asclepiadoi-
deae (Wanntorp et al., 2006). It was last revised in De Candolle’s Prodromus
(Decaisne, 1844) and is in urgent need of revision (Meve, 2002).
The International Plant Names Index lists more than 600 Hoya species names but
estimates of the true number of species are extremely variable, ranging between
200 and 300 (Forster et al., 1998; Kleijn & Van Donkelaar, 2001). These are very
conservative estimates and likely many more species are still undescribed.
Apart from the lack of a complete revision, the broad Indomalesian-Australian-
Western Pacific distribution also makes an accurate estimate of the number of species
difficult. The regions with the highest known species diversity are the Philippines and
New Guinea but many parts of the distribution of the genus are still undercollected
(Forster et al.,1998).
Chinese Hoya are rather better known because they have recently been revised for
the Flora of China (Li et al., 1995) where 32 species and one variety of Hoya are
recorded. With the recently published Hoya baishaensis Shao Y.He & P.T.Li
(He et al., 2009a), H. bawanglingensis Shao Y.He & P.T.Li (He et al., 2009b) and
H. persicinicoronaria Shao Y.He & P.T.Li (He et al., 2009c), the total number of
Chinese Hoya species has been raised to 35. In these treatments Hoya yuennanensis
Herbarium, Singapore Botanic Gardens, 1 Cluny Road, Singapore 259569. E-mail: rodda.michele@
gmail.com
was considered a synonym of H. lyi although neither species has been lectotypified.
While searching for suitable lectotypes at the herbaria of B, BM, C, E, FI, GH, HBG,
HITBC, IBSC, K, L, P, S, SING, TO, W, WRSL, WU and Z it became apparent that
two separate morphological entities are present and, therefore, the name Hoya
yuennanensis should be resurrected. It also became apparent that Hoya mekongensis
(Gilbert et al., 1995) is morphologically indistinguishable from H. yuennanensis.
In the present paper I (i) discuss the taxonomic history of Hoya lyi and
H. yuennanensis, (ii) emend their descriptions, (iii) lectotypify H. lyi and H. yuennanensis,
(iv) resurrect the name H. yuennanensis,and(v)synonymiseH. mekongensis with
H. yuennanensis.
Taxonomic History and Current Study
In 1907, when Hoya lyi was published (Le
´veille
´, 1907), only two Chinese Hoya
species had previously been described, H. carnosa R.Br. and H. pottsii Traill (Dunn,
1911). In 1934, Hoya lyi was synonymised into H. carnosa (Rehder, 1934) but two
years later Tsiang (1936) resurrected it, stating ‘It cannot be confused with
H. carnosa which belongs to a different series of the section’, citing his two
collections (Tsiang 4681 and 4738) as ‘exactly matching the type’. Nonetheless,
attached to specimen 4738 there is a note by Tsiang: ‘Tsiang 4738 5Ching 6727
seems not identical with Hoya lyi, but more material will show transitions
H. Handel-Mazzetti, in litt., June 11, 1935’. On the specimen Tsiang 4681, Tsiang
annotated instead: ‘Identical with the type of H. lyi and I agree now that it is a good
species, different from H. carnosa H. Handel-Mazzetti, in litt’. Here Tsiang was
probably referring to specimen Handel-Mazzetti 7971, collected in China in 1915 and
distributed as Hoya carnosa. After revising his Chinese collections Handel-Mazzetti
(1936) published Hoya yuennanensis based on this specimen. Surprisingly, Tsiang &
Li (1977) considered Hoya yuennanensis a synonym of H. lyi.
In the most recent edition of the Flora of China Li et al. (1995) made no changes
from the earlier Chinese language version (Tsiang & Li, 1977) but did provide an
extended description of the taxon.
The name Hoya yuennanensis was recently misapplied in Wanntorp & Forster
(2007) to Micholitzia obcordata N.E.Br., a species now included in Hoya and whose
currently recognised name is H. manipurensis Deb.
Correct application of names in Hoya is still problematic due to the complex and
often confused nomenclature, partly caused by horticultural interest in Hoya species
that has led to a proliferation of synonyms.
It is clear from the examination of the lectotypes and of the cited specimens of
Hoya lyi and H. yuennanensis that there are two separate species, both ecologically
and morphologically. Hoya lyi is a small lithophytic species with variable leaves that
can be oval, elliptic or oblong. It is found only on limestone, in deep shade, growing
tightly appressed to the rock surface. Hoya yuennanensis, in contrast, is a more
rampant climber, generally with elliptic or oblanceolate leaves found on siliceous and
84 M. RODDA
limestone outcrops in exposed locations. Both species have white to pale pink
flowers of similar size but they can be easily discriminated by the shape of their
coronas. Hoya lyi has a rather flat-topped corona with rounded and flattened
slightly ascending outer corona lobes while H. yuennanensis has erect corona lobes
with rounded outer processes and a depressed stigmatic head (Fig. 1). They can also
be discriminated in their pollinia: Hoya lyi has smaller pollen masses with an
elongated retinaculum while H. yuennanensis has a massive retinaculum compared to
the size of the pollinium (Fig. 2). For these reasons Hoya yuennanensis should be
reinstated. A complete comparison between Hoya lyi and H. yuennanensis based on
all examined specimens is presented in Table 1.
Hoya yuennanensis was initially confused with H. carnosa (Handel-Mazzetti, 1927)
but the two species can be easily separated by the flat-topped corona lobes of
H. carnosa that have acute inner and outer lobes.
Often Hoya species are known from one or just a few specimens and an assessment of
their variation is, therefore, impossible. The study of an extensive number of specimens
of Hoya lyi has highlighted its morphological variation across the distribution range.
Leaves can vary greatly in shape (oval, elliptic, oblong) and size, ranging from 1.5 to
10 cm long. The calyx can have very short narrow lobes or rather broad lobes. Flowers
can be variable in colour, from fully white to white with a prominent pink or purple
centre, and corona lobes that can be either completely flat and perpendicular to the
pollen tube, or slightly ascending. The large variation may be due to the broad
distribution range. Hoya lyi appears to be closely related to H. thomsonii Hook.f. They
are both lithophytic species adapted to limestone rocks, they grow in very shady
conditions and they bear white villous corollas and rounded corona lobes.
Hoya thomsonii was described from India but is also found in northern Thailand and
southern China. Biogeographic links have been observed between the floras of Nepal
and Thailand (Pendry et al., 2009) suggesting that the distribution of Hoya thomsonii,
and possibly of H. lyi, may be more extensive than so far observed. Since the extent of
morphological variation of Hoya thomsonii is unknown, further studies are required to
establish whether H. lyi falls within the range of variation of H. thomsonii.
While searching for specimens of Chinese Hoya species, the holotype of Hoya
mekongensis was compared with specimens of H. yuennanensis. The two species
appear to be indistinguishable on morphological grounds. Furthermore, Hoya
yuennanensis and H. mekongensis are together known only from three collections
in a very restricted area along the Mekong River (Fig. 3). Pollinaria from Hoya
mekongensis have been included in Fig. 2 for comparison. It is therefore suggested
that Hoya mekongensis is a new synonym of H. yuennanensis.
Taxonomy
Hoya lyi H.Le
´v., Bull. Soc. Bot. France 54: 369 (1907); H.Le
´v., Fl. Kouy Tche
´ou 42
(1915). – Type: China, Guizhou, Tsien-Sen-Kiao, xi 1904, Ly 1879 (lecto E!
(barcode E00279452), designated here). Figs 1, 2.
TAXONOMY OF HOYA IN SW CHINA 85
FIG.1. Hoya lyi H.Le
´v. (above; a, b, c) and H. yuennanensis Hand.-Mazz. (below; d, e, f).
a, d: corolla, only shape of corolla lobes drawn; b, e: corona, from side; c, f: calyx. Drawn by
M. Rodda from Ly 1879 (Hoya lyi, E, lectotype) and Handel-Mazzetti 7971 (Hoya
yuennanensis, E, isolectotype).
86 M. RODDA
All measurements from rehydrated material.
Semi-woody, lithophytic or rarely epiphytic vine, delicate, with white latex. Leafy
stems up to 2–3 m, generally shorter, cylindrical up to 3 mm in diameter, young
stems hirsute. Internodes 1–5(–10) cm, conspicuously rooting along stems. Leaves
FIG.2. Pollinaria from herbarium specimens of Hoya lyi (above;a,b,c)andH. yuennanensis
(below;d,e,f).a:Ly 1879 (E, lectotype); b: Martin &Bodinier 1853 (E); c: Poilane 2138 (P);
d: Handel-Mazzetti 7971 (E, isolectotype); e: Soulie
´1598 (P, holotype of Hoya mekongensis);
f: Kingdon Ward 1139 (E). The pollinia of Handel-Mazzetti 7971 appear to be immature or not
well preserved.
TAXONOMY OF HOYA IN SW CHINA 87
opposite, petiolate; petiole 3–5(–10) mm long, 1–1.5 mm wide, hirsute; lamina stiff,
fleshy, very variable in shape, from oval to elliptic or oblong, (1.5–)3–5(–10) 3
1–3 cm, apex acute or rounded, base rounded, less often truncate or shortly
attenuate, margin entire, adaxially dark green, hirsute, lighter green on abaxial side,
hirsute, penninerved, main vein ridged on adaxial surface, less evident on abaxial
surface, secondary veins 3–5 pairs evident when dry, nearly at right angles from main
nerve but variable, from 60 to 120°, anastomosing near leaf margins. Inflorescences
pseudo-umbelliform, convex, 4–10(–20)-flowered; peduncle extra-axillary, perennial,
2–5 cm long, 1.5–3 mm wide, hirsute when young; pedicels 2.5–3 cm 3c.1 mm,
hirsute. Calyx 4–5 mm in diameter, lobes elliptic to ovate, 2–3 31–1.5 mm, apex
rounded, glabrous inside, sparsely pubescent outside, rugose, margins ciliate, 1–3
basal glands at the junction between the lobes. Corolla flattened to slightly convex,
white to light pink, 14–18 mm in diameter, abaxial side glabrous, adaxially villous
throughout; lobes 7–8 34–5 mm, ovate to rhomboid with acute apex. Corona
TABLE 1. Morphological comparison between Hoya lyi and H. yuennanensis
Character Hoya lyi Hoya yuennanensis
Leaf shape Very variable in shape,
from oval to elliptic
or oblong
Variable, elliptic or
oblanceolate (suborbicular)
Leaf base Base rounded, less often
truncate or shortly
attenuate
Base rounded to obtuse
Leaf apex Apex acute or rounded Apiculate to abruptly acute
Leaf length (1.5–)3–5(–10) cm 6–13 cm
Leaf width 1–3 cm 4–5 cm
Leaf nerves (number) 3–5 pairs c.7 pairs
Leaf secondary nerves
(branching angle)
90°(60–120°)60°
Flower peduncle length 2–5 cm 1–2.5(–5) cm
Flower peduncle surface Hirsute when young Pubescent
Calyx lobes shape Elliptic to ovate Broadly ovate to orbicular
Corolla surface (adaxial) Villous throughout Hirsute
Corona lobe shape Boat shaped, flattened
above
Ovoid, flattened internally
Corona lobe inner
process
Acute, about as high
as outer processes and
stigmatic head
Acute, incumbent on
gynostegium
Corona lobe outer
process shape
Rounded, slightly
ascending
Rounded, erect
Pollinarium length 900 lm 1100 lm
Pollinarium width 350 lm 450 lm
Retinaculum length 400 lm 700 lm
Retinaculum width 200 lm 350 lm
88 M. RODDA
staminal, ivory white with purple centre, 2–3 mm high, 5–7 mm diameter, lobes boat
shaped, 2.5–3 31.5 mm, flattened above; outer processes rounded, slightly
ascending, inner processes acute, about as high as outer processes and stigmatic
head; basal anther process free from the filament tube. Pollinarium (all measure-
ments approximate) 1100 3750 lm, pollinia oblanceolate, 900 3350 lm, base
obliquely truncate, apex rounded, retinaculum 400 3200 lm, caudicles thin, 100 lm
long. Ovary bottle shaped, c.2 mm long, 1 mm wide. Fruits and seeds not seen.
Distribution. The localities of the syntypes indicated in Le
´veille
´(1907) could not be
equated with modern Chinese localities. Le
´veille
´(1915) noted that the three
specimens were collected in Guizhou. More recent collections have been made in
Sichuan, Guangxi and Yunnan, as mentioned by He et al. (2009a). Zhu et al. (2003)
identified Hoya lyi as one of the characteristic species in limestone habitats in
southern Yunnan. In the present study I have also identified specimens of Hoya lyi
from Vietnam and Lao PDR, a first record for these countries (Fig. 3).
FIG.3. Distribution of Hoya lyi and H. yuennanensis based on specimens indicated by (*)
within main text. Hoya lyi is indicated by a full circle; H. yuennanensis by a full square. The
Chinese provinces of Guangxi, Guizhou, Sichuan and Yunnan are indicated in dark grey. In
lighter shades of grey are indicated Vietnam and Lao PDR. Map constructed using
SimpleMappr (www.simplemappr.net).
TAXONOMY OF HOYA IN SW CHINA 89
Ecology.Hoya lyi is a delicate climber, often lithophytic on limestone where it grows
tightly appressed to the rock surface. It prefers moderately moss-covered rocks in
very shady places, usually on the north side of cliffs or in deep gorges or where the
sunlight is filtered by an overhanging tree canopy. It can be found at the northern
limit of the distribution area of Hoya (in particular Wilson 5071) and appears to be
one of the most cold-tolerant species in the genus.
Specimens examined (* indicates specimens mapped in Fig. 3). CHINA.Guangxi: Nandan, sin.
coll. 99659* (IBSC barcode 520490); Baise Qin, Wanglao Shan, Langping, sin. coll. 618173*
(IBSC barcode 520491). Guizhou: Kouy Tcheou, Cavalerie s.n. (P); sin. loc., vii 1909, Esquirol
1590 (E barcode E00281697); Ha Hai Tre, x 1910, Esquirol 2517 (P); Na-Kan, Cheng-Feng,
Tsiang 4738* (IBSC barcodes 520503 & 520504); Na-Kan, Cheng-Feng, Tsiang 4681* (IBSC
barcodes 520505 & 520506); Gan-Pin, 20 ix 1897, Martin &Bodinier 1853 (syn E barcode
E00279451, P); Lo-Pie
´, 7 x 1897, Martin &Bodinier 1853 (syn P barcode P00634485). Sichuan:
Leibo, Zhongshan Ping, Z.T. Guan 9658* (IBSC barcode 520622); Mt. Omi, Wilson 5071*
(BM, IBSC, K); sin. loc.,Wilson 4098 (BM, K). Yunnan:sin. loc., xi 1904, Cavalerie 1879 (K);
ibid., 10 ii 1905, Ducloux 3477 (P); ibid., Xishuangbanna, 100°309E, 21°309N, 14 x 1978, Tao
Guo Da 35474* (HITBC); Yan Shan, Wang 591772* (IBSC barcode 520495); Le Tchong, 15 x
1904, Ducloux 2909 (P).
LAOS.Xieng Khouang: Tam La, 19 x 1920, Poilane 2138* (P barcodes P00700446 &
P00700534).
VIETNAM.Ha Giang: Yen Minh distr., Lao Va Chai municipality, vicinity of Lao Va Chai
village, 9 x 1999, Hiep,Binh,Averyanov,Cribb NTH 3470* (K).
Notes. There are some inaccuracies in the citation of specimens by Le
´veille
´in the first
publication of Hoya lyi (Le
´veille
´, 1907). The three specimens cited are: Gan Pin,
Martin &Bodinier s.n., 20 ix 1897; Lo Pie, Martin &Seguin 1853, 7 x 1897 and Ly
1879, xi 1904.
Martin &Bodinier s.n., the material from Gan Pin, has been located in E and P but
bearing the collection number 1853. On the E specimen there is a reference to
specimen Lo Pie, Martin &Seguin s.n., 7 x 1897 as belonging to the same species. It
is reasonable to assume that this is actually a reference to Martin &Bodinier 1853,
7 x 1897 from Lo Pie, a specimen extant in P.
Specimen Ly 1879, xi 1904 was found in E and is here selected as lectotype since it
does not present any citation incongruence with the original publication and was
collected by the person, Jean Ly, after which Hoya lyi is named. Also, the specimen is
in good condition, possessing leafy stems and two pseudo-umbels.
Hoya yuennanensis Hand.-Mazz., Symb. Sin. Pt. VII: 1001 (1936). – Type: China,
Yunnan, Lota Tanschan & Tsedjrong, ad fluvium Landsang-djiang (Mekong), 10
ix 1915 or 4 x 1915 (see notes below), Handel-Mazzetti 7971* (lecto WU (number
WU0043535), designated here; isolecto E (barcode E00281696), GH (barcode
A00076426), K, W (number 1924-0011093), WU (number WU0043536)). Figs 1, 2.
90 M. RODDA
Hoya mekongensis M.G.Gilbert & P.T.Li, Novon 5(1): 10 (1995), syn. nov. – Type:
China, Yunnan: Tse
´kou, 1895, Soulie
´1598* (holo P (barcode P00639769); iso P
(barcodes P00639770, P00639771, P00639772)). Fig. 2.
All measurements from rehydrated material.
Semi-woody, lithophytic or epiphytic climbing vine. Leafy stems cylindrical up to
5 mm in diameter, young ones hirsute, mature ones glabrescent. Internodes
(5–)10–20 cm, conspicuously rooting along stems. Leaves opposite, petiolate; petiole
flattened above, rugose, 0.7–1.6 cm long, 3–4 mm wide, densely puberulent; lamina
stiff, fleshy, variable, elliptic or oblanceolate (rarely suborbicular), 6–13 34–5 cm,
apex apiculate to abruptly acute, base rounded to obtuse, margin entire, minutely
hairy, penninerved, main vein sulcate on adaxial side, secondary veins c.7 pairs,
poorly defined, held at c.60°from main nerve, anastomosing and forming
a secondary reticulate venation. Inflorescences pseudo-umbelliform, convex, at
least 8-flowered; peduncle extra-axillary, perennial, 1–2.5(–5) cm long, c.2 mm
wide, pubescent; pedicels 2–3 cm 3c.1 mm, sparsely hairy. Calyx 4–5 mm in
diameter, lobes broadly ovate to orbicular, 1.5–2 31.5 mm, apex rounded, inside
glabrous, outside rugose with a few hairs, margin sparsely ciliate, 1–2 basal glands
at the junction between the lobes. Corolla flattened to slightly convex, white to light
pink, 17–19 mm in diameter, abaxial side glabrous, adaxially hirsute; lobes
6–8 35 mm, ovate with acute reflexed apex. Corona staminal, ivory white,
c.3 mm high, 5–7 mm in diameter, lobes ovoid, flattened internally, 2.5–3 32mm;
outer processes rounded, erect, inner processes acute, incumbent on gynostegium, just
meeting in the centre; stigmatic head depressed; anther free from the filament tube.
Pollinarium (all measurements approximate) 1350 31100 lm, pollinia oblanceolate,
1100 3450 lm, base and apex rounded, retinaculum 700 3350 lm, caudicles thin,
200 lmlong.Ovary triangular, c.2.5 mm long, 1 mm wide. Fruits and seeds not seen.
Distribution.Hoya yuennanensis has only been found in three localities along the
Mekong River, near the villages of Lota, Tse
´kou and Loudre. These localities have
been identified using the map in Handel-Mazzetti (1927) where Tse
´kou is spelled
Tseku (Fig. 3).
Ecology. Handel-Mazzetti (1927) provides a description of the habitat where Hoya
yuennanensis was collected: ‘Among other plants growing on the arid rocks, receiving
a little shade from the sclerophyllous shrubs above them, were numerous orchids,
now out of flower, and Hoya carnosa, a creeper with thick fleshy leaves; Sedum
diymarioides, a little stonecrop with numerous glands, and the large S. indicum,
Polypodium nipponicum, a fern with bluish creeping rootstocks, and a few mosses’. In
the same book there is a picture of the collection locality above Lota, along the
Mekong River (Handel-Mazzetti, 1927, cap. 18, fig. 31).
The species appears to be adapted to grow on rocks along the Mekong River and
can be quite a rampant climber. From the limited material available it does not look
TAXONOMY OF HOYA IN SW CHINA 91
like it grows tightly attached to the rock surface as is the case for Hoya lyi. Since it
grows above 2000 m a.s.l. at the northern limit of the distribution area of Hoya it is
one of the most cold-tolerant species in the genus.
Additional specimen examined (* indicates specimen mapped in Fig. 3). CHINA.Yunnan:
Loudre, 18 x 1913, Kingdon Ward 1139* (E barcode E00281698).
Notes. An attempt at lectotypification was made by D. H. Kent in 1989 who labelled
the specimen Handel-Mazzetti 7971 (W) as the lectotype of Hoya yuennanensis,
a name he considered to be a synonym of H. lyi. However, it was never published so
has no standing. The same specimen was cited in Wanntorp & Forster (2007) as the
holotype. It must be noted that Handel-Mazzetti 7971 is likely a mixed accession. On
the labels of the various duplicates studied two collection dates co-occur: 10 ix 1915
and 4 x 1915. Probably plants were collected with the same collection number at the
same locality on different days. This is also suggested by the reference on the
specimens to two notes in Handel-Mazzetti’s diary (1477 and 1569). Unfortunately
the diary is missing and these notes cannot be verified (Handel-Mazzetti, 1996).
Among the duplicates of Handel-Mazzetti 7971, the specimen here designated as the
lectotype is the only one with a single leafy shoot with attached flowers. Although it is
impossible to determine on which of the two possible dates it was collected it cannot be
a mixed accession and is, therefore, available for lectotypification. Likewise it is
impossible to state definitively which isolectotype was collected on which date and it is
possible that not all of them are, strictly speaking, duplicates of the lectotype.
Acknowledgements
I thank David Boufford, Vichith Lamxay, Boon Leng and Mark Newman for the
translation of Chinese labels and for locating old Chinese and Lao localities, and the
curatorsofB,BM,C,E,FI,GH,HBG,HITBC,IBSC,K,L,P,S,SING,TO,W,
WRSL, WU and Z herbaria for providing high quality images or for the loan of
specimens. This study is part of an ongoing research project on the systematics of Hoya.
Financial support has been received from the National Parks Board, Singapore and
the Synthesys programme, grant nos GB-TAF-5657, NL-TAF-676 and DE-TAF-675.
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Received 27 April 2011; accepted for publication 18 October 2011
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