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A further new species of Isoglossa (Acanthaceae) from the Eastern Arc Mountains of Tanzania

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Isoglossa pareensis I.Darbysh. & Hemp (Acanthaceae), from submontane moist forest at Mwala in the South Pare Mountains of northeastern Tanzania, is described and illustrated. This species is considered to be related to I. gregorii (S.Moore) Lindau and I. punctata (Vahl) Brummitt & J.R.I.Wood, which are widespread in the montane forests of eastern Africa, but it clearly differs from these species in inflorescence structure and indumentum and in anther morphology. Notes on the habitat requirements and extinction risk of this new species are provided; it is considered to be Vulnerable under IUCN criterion D2 because of its extremely limited range and a plausible future threat from wildfires. The recent discovery of the Critically Endangered acanthaceous herb Asystasia masaiensis Lindau at lower, drier elevations at the same site is also reported and the first known photograph of that species is reproduced
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KEW BULLETIN
DOI 10.1007/S12225-023-10103-1
© The Author(s), 2023
A further new species of Isoglossa (Acanthaceae)
from the Eastern Arc Mountains of Tanzania
Iain Darbyshire1 & Andreas Hemp2
Summary. Isoglossa pareensis I.Darbysh. & Hemp (Acanthaceae), from submontane moist forest at Mwala in the
South Pare Mountains of northeastern Tanzania, is described and illustrated. This species is considered to be
related to I. gregorii (S.Moore) Lindau and I. punctata (Vahl) Brummitt & J.R.I.Wood, which are widespread in the
montane forests of eastern Africa, but it clearly diers from these species in inorescence structure and indu-
mentum and in anther morphology. Notes on the habitat requirements and extinction risk of this new species are
provided; it is considered to be Vulnerable under IUCN criterion D2 because of its extremely limited range and a
plausible future threat from wildres. The recent discovery of the Critically Endangered acanthaceous herb Asys-
tasia masaiensis Lindau at lower, drier elevations at the same site is also reported and the rst known photograph
of that species is reproduced
Key Words. Conservation, extinction risk, Isoglossinae, IUCN Red List, taxonomy.
Introduction
The genus Isoglossa Oerst. (Acanthaceae: Acan-
thoideae: Justicieae: Isoglossinae; Kiel et al. 2006;
Manzitto-Tripp et al. 2021) currently comprises c. 70
species, distributed across the palaeotropics and sub-
tropics, with centres of species richness in eastern and
southern Africa (POWO 2022). It is currently delim-
ited by having the combination of a bilabiate corolla
with ascending-cochlear aestivation, two bithecous sta-
mens that lack appendages on the thecae, 4-seeded
stipitate capsules, and (usually) bipororate pollen (i.e.,
with two compound apertures, each with the ecto- and
endo-apertures both porate), circular in apertural view
and with a pronounced interapertural “girdle” of tec-
tate exine, termed “gürtelpollen” (Darbyshire et al.
2023). The limited molecular phylogenetic evidence
currently available indicates that Isoglossa s.l. is para-
phyletic, with monothecous genera of Isoglossinae
arising out of it (McDade et al. 2021). However, clear
synapomorphies to support the splitting of Isoglossa
have not yet been identied, and further molecular
data, including the use of Next Generation Sequenc-
ing techniques, is required before further decisions
on generic delimitation in Isoglossinae can be made
(Darbyshire et al. 2023).
In the Flora of Tropical East Africa account for Isoglossa,
26 species were documented in Tanzania, of which three
were undescribed and poorly known due to incomplete
material (Darbyshire et al. 2010). Isoglossa s.l. is notewor-
thy for including a signicant number of narrowly range-
restricted taxa, both in Tanzania and in wider tropical
and southern Africa and Madagascar (Brummitt 1985;
Darbyshire 2009; Darbyshire et al. 2010, 2011, 2021,
2023; Balkwill et al. 2017; Champluvier & Fischer 2020).
Although the genus occurs in a range of habitats, many
of these range-restricted species are endemic to specic
mountain ranges. For example, the Eastern Arc Moun-
tains of Kenya and Tanzania — well known for their
exceptionally high biodiversity and endemism (Burgess
et al. 2007; Gereau et al. 2016) — support seven endemic
species of Isoglossa. These are I. asystasioides I.Darbysh.
& Ensermu, I. bondwaensis I.Darbysh., I. ixodes Lindau
and I. oreacanthoides Mildbr. from the Uluguru Moun-
tains, I. candelabrum Lindau from the Taita Hills and East
Usambara Mountains, I. variegata I.Darbysh. from the
East Usambara and Nguru Mountains, and I. ventricosa
I.Darbysh. from the southwestern-most extremity of the
Eastern Arc in Lulanda Forest and the Mundi Escarp-
ment (Darbyshire 2009; Darbyshire et al. 2010).
Here, we report on a further new species of Isoglossa
endemic to the Eastern Arc Mountains in Tanzania,
found by the second author during ongoing ecologi-
cal studies of forest biodiversity on East African moun-
tains. The type locality for this new species, at Mwala
in the South Pare Mountains of Kilimanjaro Region,
Tanzania (Fig. 1), is of high botanical interest. A new
orchid species, Rhipidoglossum pareense Cribb & Hemp,
was documented recently from this site (Cribb &
Hemp 2022), and the recently described Asplenium
arcumontanum Hemp & N.R.Crouch (Aspleniaceae)
occurs here and on the neighbouring North Pare and
West Usambara Mountains (Hemp & Crouch 2018).
Furthermore, the rare Asystasia masaiensis Lindau was
encountered in the dry foothill savannas at Mwala at
ISSN 1874-933X (print)
ISSN 0075-5974 (electronic)
Accepted for publication 12 April 2023.
1 Royal Botanic Gardens, Kew, Richmond TW9 3AE, Surrey, UK. e-mail: i.darbyshire@kew.org
2 Institute of Plant Systematics, University of Bayreuth, Universitätstsr. 30, 95440 Bayreuth, Germany
KEW BULLETIN
© The Author(s), 2023
700 m elevation in December 2019 (A. Hemp #7348;
Fig. 1D). This latter species was previously known only
from two collections from Mkomazi town to the south
of the South Pare Mountains, and is assessed as Criti-
cally Endangered on the IUCN Red List (Vollesen in
Darbyshire et al. 2010; Luke et al. 2015).
The new Isoglossa species was rst collected in
November 2017 when it was found primarily in fruit;
the specimen was shown to the first author dur-
ing a research visit to the Kew herbarium, and was
found not to match any of the species documented
in the Flora of Tropical East Africa (Darbyshire et al.
2010). Flowering material was later collected and
the species was also grown from seed in the glass-
houses at the University of Bayreuth where an addi-
tional herbarium specimen was made, allowing for
conrmation that this is an undescribed species.
Based on similarities in corolla and seed morphol-
ogy, it is hypothesised to be allied to Isoglossa gregorii
(S.Moore) Lindau and I. punctata (Vahl) Brummitt &
J.R.I.Wood, which are both widespread in the moun-
tains of eastern Africa, but the new species displays
clear and consistent morphological dierences such
that they are unlikely to be confused.
Materials & Methods
This work is based on the study of both herbar-
ium collections and live plants of the new species
of Isoglossa from Mwala in the South Pare Moun-
tains of Tanzania and from cultivated plants at the
University of Bayreuth grown from seed collected
from the type locality. The herbarium material was
analysed and measured at K. Prior to dissection, a
flower was soaked in Aerosol OT 5% solution; all
other characters were measured on dry material.
Herbarium abbreviations follow Thiers (continu-
ously updated).
The species conservation (extinction risk) assess-
ment follows the Categories and Criteria of the IUCN
Red List (IUCN 2012) and the guidelines for their use
(IUCN Standards and Petitions Subcommittee 2022).
Taxonomic Account
Isoglossa pareensis I.Darbysh. & Hemp sp. nov. Type:
Tanzania, South Pare Mountains, Mwala, 28 Nov. 2017,
A. Hemp 6953 (holotype K; isotypes B, NHT, UBT).
http:// www. ipni. org/ urn: lsid: ipni. org: names: 77323 326-1
Fig. 1. Mwala, South Pare Mountains, Tanzania. A aerial view of submontane forest and associated habitats; B within the for-
est understorey with shaded rock outcrops; C ground fog within the forest, a major source of moisture at this site; D Asystasia
masaiensis recorded from the dry foothill savannas at Mwala, collected as A. Hemp 7348. photos: a. hemp.
KEW BULLETIN
© The Author(s), 2023
Slender herbs 10 – 25 cm tall, trailing and rooting
towards base, with decumbent leafy stems; stems
green, with conspicuous cystoliths, angular (sulcate
in dried material), with inconspicuous short pale ret-
rorse or mixed antrorse and retrorse hairs mainly on
two opposite sides, hairs more dense along the nodal
line. Leaves ovate or proximal leaves ovate-elliptic,
pairs somewhat anisophyllous, largest leaves 4.2 – 6 ×
2.5 – 3.9 cm, each leaf with base broadly cuneate to
obtuse or rounded or distal-most leaves shallowly cor-
date, margin entire, apex acute to shortly attenuate,
surfaces largely glabrous except for antrorse multicel-
lular hairs along midrib above and margin, or adaxial
surface with scattered multicellular hairs appearing
deated in dry state; cystoliths linear, conspicuous in
dry state; petiole 20 – 33 mm long, antrorse-pubes-
cent along adaxial groove. Inflorescence a slender
panicle-like thyrse 74 – 105 × 28 – 37 mm including
primary peduncle 26 – 32 mm long, few-branched,
the branches dichasial or partially monochasial;
inorescence axes shortly glandular-pubescent and
eglandular-puberulous to -pubescent, glandular hairs
0.1 – 0.25 mm long, sometimes more dense on the
secondary branches; bracts and bracteoles purplish,
linear-lanceolate, 1.3 – 3.6 mm long, up to 0.5 mm
wide. Calyx purple, lobes linear-lanceolate, 3.4 – 3.8
mm long in ower, 4.6 – 5.5 mm long in fruit, shortly
glandular-pubescent and eglandular-puberulous.
Corolla white suused pink and with deeper pink
markings on palate of lower lip, 10.5 – 11.5 mm long,
pubescent externally mainly on dorsal side of tube;
tube cylindrical, 6.7 – 7 mm long, with basal cylindri-
cal portion ± 1.5 mm in diam., only slightly widened
towards mouth where 2 – 2.3 mm in diam.; upper
lip 2.7 – 2.8 mm long, apex 2-lobed for ± 1.6 mm,
lobes somewhat reexed; lower lip 3.6 – 4.7 mm long,
with 3 lobes 2 – 2.8 mm long, palate somewhat raised
with central furrow, lacking prominent herring-bone
patterning, glabrous. Stamens inserted ± 4 mm from
base of corolla tube; laments free for 2.8 – 3 mm,
glabrous; anthers barely exserted or held in corolla
mouth, thecae slightly oblique to parallel, overlap-
ping for half their length, upper theca 0.65 – 0.75 mm
long, lower theca 0.6 – 0.7 mm long, muticous. Pistil
glabrous; style ± 5 mm long; stigma shortly bilobed.
Capsule stipitate with expanded fertile portion, 9.5
– 10.5 mm long, puberulous with eglandular and
occasional glandular hairs; seeds subattened, ± 3 ×
2.7 mm, with elongate, minutely glochidiate tubercles
particularly towards the rim Figs 2, 3 and 4.
RECOGNITION. Isoglossa pareensis is similar to I. gre-
gorii in foliage and to both I. gregorii and I. punc-
tata in corolla form and in having seeds with elon-
gate, minutely glochidiate tubercles, but differs
from both in the inflorescence being a panicle-like
thyrse with pedunculate dichasial or monochasial
cyme units (vs inflorescence a simple spike or, if
branched, the branches being spiciform with (sub)
sessile cyme units); in the anther thecae overlapping
for c. half their length (vs thecae fully superposed to
widely separated); in the capsule being puberulous
with eglandular and occasional glandular hairs (vs
capsule glabrous or occasionally with few eglandu-
lar hairs towards apex and/or with scattered glan-
dular and/or long eglandular hairs); and in the
inflorescence axes and calyces having a more dense
indumentum including more numerous glandular
hairs. It additionally differs from I. gregorii in hav-
ing linear-lanceolate bracts, 1.3 – 3.6 × 0.3 – 0.5
mm (vs bracts elliptic, somewhat obovate or basal
pairs ovate, (3.5 –) 4.5 – 14 × 0.7 – 6 mm) (Fig. 5;
Table 1).
DISTRIBUTION. This species is known only from the
South Pare Mountains of Kilimanjaro Region, north-
east Tanzania.
ADDITIONAL COLLECTIONS STUDIED. TANZANIA. Kiliman-
jaro Region, South Pare Mountains, Mwala, 1700 m,
. & fr., 17 Dec. 2019, A. Hemp 7355 (NHT, UBT);
cult. at Univ. Bayreuth, ., 13 June 2021, A. Hemp
8165 (K, NHT, UBT).
HABITAT & ECOLOGY. This species occurs in the herb
layer of lower montane Newtonia forests between
1600 and 1700 m, with an underlying geology of
gneisses (Geological survey of Tanzania, Quarter
degree sheet 108, Hedaru) (Fig. 1). Dominant asso-
ciated trees include Newtonia buchananii (Baker f.)
G.C.C.Gilbert & Boutique (Fabaceae), Pleiocarpa
pycnantha (K.Schum.) Stapf (Apocynaceae; newly
recorded for South Pare), Strombosia scheeri Engl.
(Olacaceae), Drypetes gerrardii Hutch. (Putranjiva-
ceae), Syzygium micklethwaitii Verdc. sensu stricto
(Myrtaceae), Dasylepis integra Warb. (Achariaceae),
Xymalos monospora (Harv.) Baill. ex Warb. (Monimi-
aceae), and Myrsine melanophloeos (L.) R.Br. ex Sweet
(Primulaceae). The dense shrub layer in these for-
ests is mainly composed of Dracaena fragrans (L.) Ker
Gawl. (Asparagaceae), Psychotria goetzei (K.Schum.)
E.M.A.Petit, Chassalia parvifolia K.Schum., C. discolor
K.Schum. (all Rubiaceae), and Maytenus acuminata
(L.f.) Loes. (Celastraceae).
Isoglossa pareensis grows on the shady forest floor
together with many ferns, particularly Asplenium
gemmiferum Schrad., A. arcumontanum, A. erectum
Bory ex Willd., A. elliottii C.H.Wright (all Asple-
niaceae), Lomariopsis warneckei (Hieron.) Alston
(Lomariopsidaceae), and Blotiella stipitata (Alston)
Faden (Dennstaedtiaceae). Other herbs belonging
to the Acanthaceae include Crossandra tridentata
Lindau and Isoglossa lactea Lindau subsp. saccata
I.Darbysh. Isoglossa pareensis is not common within
these forests; using data from 30 forest plots (A.
KEW BULLETIN
© The Author(s), 2023
Fig. 2. Isoglossa pareensis. A habit, owering branch; B ower in situ; C face view of corolla with stamens and stigma visible at mouth; D dis-
sected corolla with stamens; E partial infructescence with mature capsules; F indumentum of calyx lobe, side view, external face to left; G indu-
mentum of peduncle; H capsule with seeds, and dorsal view of capsule valve; J mature seed, with detail of sculpturing. A & D from Hemp 8165;
B & C from photographs of plants in the eld; EJ from Hemp 6953. drawn by andrew brown.
KEW BULLETIN
© The Author(s), 2023
Hemp, unpubl. data) we estimate that it occurs in
13% of the area of Mwala with perhaps 10 – 20 indi-
viduals per 0.1 ha.
The climate at Mwala is humid and foggy. Mean
annual rainfall is only about 700 mm; however, fog
water interception provides more than two times this
amount in addition (data of 5 years; A. Hemp, unpubl.
data). Consequently, mean annual relative humidity
is 94%. Mean annual temperature is 15.7°C with a
minimum temperature of 8.7°C and a maximum tem-
perature of 29.0°C. Because of the high humidity, tree
trunks and branches are densely covered by mosses
(30 – 50% coverage) and vascular epiphytes (5 – 10%
coverage) (A. Hemp, unpubl. data).
CONSERVATION STATUS. This species is known only
from the type locality, and using the recommended
2 × 2 km grid cell size for calculation, its Area of
Occupancy is 4 km2. However, the forest in which
this species is found covers an area of c. 3.5 km2,
within which suitable habitat for the Isoglossa is esti-
mated to be 1.2 km2, and as noted above, it is not
common here (A. Hemp, pers. obs.). The area where
Fig. 3. Isoglossa pareensis. AC in situ at Mwala, Tanzania, collected as A. Hemp 7355; D & E in cultivation at UBT. photos: a
c a. hemp; d, e u. meve.
Fig. 4. Fruits of Isoglossa pareensis. A fruit valve within calyx, showing dense glandular indumentum; B dehiscing capsule with
seeds; C capsule valve showing immature seeds with glochidiate-tuberculate sculpture. Scale bars = 2 mm. photos: u. meve.
KEW BULLETIN
© The Author(s), 2023
the species occurs is currently unprotected although
it has been proposed as a Forest Reserve, and dur-
ing the visit of the second author (A.H.) no illegal
logging activities or other ongoing threats were
observed. However, there is a plausible future threat
from forest fires at this site; such events have been
recorded from similar forest patches in the region
and larger burning events can be highly damaging to
the forest understorey (A. Hemp, pers. obs.). Given
the extremely small range size and this plausible
threat, Isoglossa pareensis is assessed as Vulnerable
under criterion D2 – VU D2.
ETYMOLOGY. The species epithet “pareensis” denotes
that this species is, so far as is known, endemic to the
Pare Mountains of Tanzania.
NOTES. Although this species appears to be most
closely allied to Isoglossa gregorii (Fig. 5) and I. punc-
tata based on corolla and seed morphology, it is easily
separated by the characters listed in the Recognition
section and in Table 1, and the inorescence form, in
particular, is so dierent from those species that they
are unlikely to be confused.
One further species from adjacent areas — Isoglossa
paucinervis I.Darbysh. from the Eastern Arc Mountains
(West Usambara, Ukaguru and Udzungwa) and Mt Kili-
manjaro — could potentially be confused with I. pareensis
because they share a similarly small stature, small leaves
with few lateral veins, and an open, branched inores-
cence with small bracts. However, I. pareensis is easily
separated from I. paucinervis by a range of characters,
including the capsule being puberulous (vs glabrous),
the seeds having glochidiate tubercles (vs seeds with
elongate but non-glochidiate tubercles), a markedly
more slender corolla tube and a more dense inores-
cence indumentum (see Darbyshire 2009; Darbyshire et
al. 2010). We hypothesise that these two species are not
closely related and that the resemblance between them
is likely to be supercial. Isoglossa paucinervis is included
in Table 1 for completeness.
To date, none of the Isoglossa species discussed in
this paper have been sampled in molecular phyloge-
netic studies pertaining to Isoglossinae (Kiel et al. 2006;
McDade et al. 2021), and so relationships based on mor-
phology are speculative. In their study of Justicieae using
ve molecular loci (nrITS, trnT-L and trnS-G, trnL-F and
rps16 intron), McDade et al. (2021) documented two
clades of Isoglossa s.l. within Isoglossinae. Included within
Isoglossa Clade 1 were two species from Madagascar, one
of which — I. a. justicioides Baker — is from a group
of Malagasy species that appear morphologically similar
to I. gregorii and I. punctata (I. Darbyshire, pers. obs.).
We could hypothesise, therefore, that I. pareensis may
be resolved within Isoglossa Clade 1, but this needs to be
tested with further molecular phylogenetic data, ideally
using Next Generation Sequencing phylogenetic tech-
niques to improve data resolution. With this in mind, a
RADseq analysis of the tribe Isoglossinae is now being
prepared to investigate the delimitation and evolution-
ary history of Isoglossa and its allies (C. A. Kiel & I. Dar-
byshire, in prep.).
Fig. 5. Isoglossa gregorii. A habit of owering plants; B inorescence for comparison to I. pareensis; Ngozi, Tanzania. photos: i.
darbyshire.
KEW BULLETIN
© The Author(s), 2023
Table 1. Main diagnostic characters for separation of Isoglossa pareensis from I. gregorii, I. paucinervis and I. punctata.
Character Isoglossa punctata Isoglossa gregorii Isoglossa pareensis Isoglossa paucinervis
Inorescence form spiciform or if (often) branched
then branches spiciform with
(sub)sessile cyme units
spiciform or if (more rarely)
branched then branches spici-
form with (sub)sessile cyme units
slender panicle-like thyrse with
pedunculate dichasial or mono-
chasial cyme units
slender panicle-like thyrse with
pedunculate, mainly monochasial
cyme units, or sometimes reduced
to a simple 1 – 2-owered cyme
Indumentum of inorescence axes two opposite lines of dense eglan-
dular hairs, rarely with scattered
glandular hairs
two opposite lines of dense eglan-
dular hairs, sometimes with
scattered glandular and/or long
eglandular hairs
shortly glandular-pubescent
and eglandular-puberulous to
-pubescent
eglandular-puberulous to -pubes-
cent, few glandular hairs occasion-
ally present, or axes sometimes
glabrous
Bract shape and size (mid-point of
inorescence axes)
triangular-ovate, 1.5 – 4.5 × 0.7 –
2.5 mm elliptic to somewhat obovate, (3.5
–) 4.5 – 14 × 0.7 – 6 mm linear-lanceolate, 1.3 – 3.6 × 0.3 –
0.5 mm linear-lanceolate to narrowly elliptic
or ovate, 1 – 11.5 × 0.3 – 6 mm
Indumentum of calyces, external
surface shortly ciliate, elsewhere glabrous
or rarely with scattered longer
glandular hairs
shortly ciliate or hairs more
widespread, often with scattered
longer glandular and/or long
eglandular hairs
shortly glandular-pubescent and
eglandular-puberulous shortly ciliate, with or without scat-
tered glandular and/or eglandu-
lar hairs on external surface
Corolla tube shape (sub-) cylindrical to narrowly cam-
panulate (sub-) cylindrical cylindrical (sub-) campanulate to inated
Anther thecae arrangement superposed and widely separated immediately superposed to widely
separated overlapping for half their length usually superposed and separated by
0.5 – 1.5 mm
Capsule indumentum glabrous or with minute eglandular
hairs towards apex, occasionally
with scattered longer glandular
hairs
glabrous or with minute eglandular
hairs towards apex, occasionally
with scattered longer glandular
and/or long eglandular hairs
puberulous with eglandular and
occasional glandular hairs glabrous
Seed sculpturing with elongate, minutely glochidiate
tubercles particularly towards
rim
with elongate, minutely glochidiate
tubercles particularly towards
rim
with elongate, minutely glochidiate
tubercles particularly towards
rim
with elongate, appressed tubercles
extending in length towards rim,
lacking glochidia
KEW BULLETIN
© The Author(s), 2023
Acknowledgements
We gratefully acknowledge the financial support
granted by the Deutsche Forschungsgemeinschaft
(DFG) and the research permit granted by COSTECH
and TAWIRI. We are highly thankful to Andrew Brown
for his illustration of the new species and to Ulrich
Meve (Bayreuth) for growing living material in the
greenhouse of UBT and permitting us to reproduce
his photographs. We also thank Kaj Vollesen (K) for
assistance in the identication of Asystasia masaiensis.
Declarations
Conicts of interests/Competing interests The authors
declare no conicts of interest/competing interests.
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