Isoglossa darbyshirei Champl. & Eb.Fisch., 2020

Champluvier, Dominique & Fischer, Eberhard, 2020, Isoglossa darbyshirei (Acanthaceae), a new plietesial species from the Albertine Rift (Rwanda, Burundi), Phytotaxa 438 (5), pp. 276-288 : 278-284

Isoglossa darbyshirei Champl. & Eb.Fisch. sp. nov. ( Figs. 1–5 )

Type:— BURUNDI. Bubanza, Mugumero (Rugazi), 1900 m, 1 May 1981, Reekmans 10019 (holotype BR 00000595248!, isotype K!).

Diagnosis:— Isoglossa darbyshirei is similar to I. humbertii Mildbraed (1937: 358) sharing the same wine-red corolla, but differs in the ovate leaves up to 140–145 × 65–80 mm with short acuminate apex, the larger calyx (14–15 mm long), and the larger corolla with tube 25–28 mm long and 10 mm in diameter at mouth, the upper lip 14 × 10 mm, and the lower lip 15 × 10 mm. It also resembles

Isoglossa laxiflora Lindau (1911: 306) , but this species has a cream corolla with purple spots, a tube of 11.0– 13.5 mm long and 6–11 mm in diameter at mouth, with upper lip 9 × 6 mm and lower lip 9 × 8 mm.

Straggling monocarpic shrub up to 2(–3) m tall. Stems with slightly swollen nodes. Leaves broadly ovate, base slightly asymmetric, attenuate, upper surface, margin and veins beneath pubescent, lateral veins 12–14 pairs, petiole up to 15 mm long, lamina 140–145 × 65–80 mm, margins entire. Inflorescence terminal on principal and short lateral branches, laxly cymose, with 6–8 flowers, peduncle up to 65 mm long. Bracts ovate-lanceolate, acuminate at apex, 2.8–3.0 × 1.5 mm. Pedicel up to 1.5–2.0 mm long. Calyx up to 1.5 cm long, calyx lobes linear-lanceolate, 12 × 1.2–1.4 mm, minutely pubescent. Corolla campanulate, upper and lower lip subequal, wine-red, glabrous outside, pilose within towards base, tube 28 mm long, cylindrical in lower third, 6 mm in diameter, constricted above ovary with 4 mm in diameter, upper two thirds expanded, 10 mm in diameter at mouth, upper lip hooded, 14 × 10 mm, 2-lobed, lobes minute, lower lip 15 × 10 mm, minutely 3-lobed, lobes 2–3 mm long, palate indistinct, lacking the typical “herring-bone”-pattern. Stamens with filaments attached at lower third of corolla tube, free for 10 mm, glabrous; anther thecae partially overlapping, parallel, thecae 4 mm long. Style glabrous, up to 20 mm long.

Additional specimens examined (paratypes):— RWANDA. Lacs Edouard et Kivu. Nyungwe National Park, Cyamudongo Forest , close to road towards Nyakabuye on N-exposed slope, 2001 m, 31 December 2016, Fischer 788/16 ( KOBL, BR) . BURUNDI. Lacs Edouard et Kivu. Bubanza, Mugumero (Rugazi), 1900 m, 1 May 1981, Reekmans 10000 ( BR, K) ; ibid. 2 May 1981, Reekmans 10052 ( BR, K) .

Etymology: —The species is named after Iain Darbyshire, specialist in Acanthaceae at Kew (K), who has published several papers on Isoglossa in Tropical Africa.

Habitat: —In Rwanda the species grows in the shrub layer of a submontane rainforest. The accompanying vegetation within a plot of 100 m ² consists of the following dominant species: Tree layer: Entandrophragma excelsum Sprague (1910: 180) ( Meliaceae ); Grewia mildbraedii Burret (1909: 344) ( Malvaceae ); Newtonia buchananii (Baker) Gilbert & Boutique (1952: 213) (= Piptadenia buchananii Baker 1894: 354 ) ( Fabaceae ). Shrub layer: Lepidotrichilia volkensii (Gürke) J.-F.Leroy ex Styles & White (1991: 37) (= Trichilia volkensii Gürke 1894: 33 ) ( Meliaceae ); Oxyanthus troupinii Bridson (1979: 122) ( Rubiaceae ). Herb layer: Asplenium friesiorum C.Chr. in Fries et al. (1924: 181) ( Aspleniaceae ); Culcasia falcifolia Engler (1899: 418) ( Araceae ). No detailed information is available on the locality in Burundi.

Ecology and plietesial life cycle: —The start of synchronous flowering of Isoglossa darbyshirei was recorded in Cyamudongo Forest, Rwanda, at the end of December 2016. The last flowers were observed in March 2017. In May 2017 only the remains of dead individuals were present. In June 2017 no traces of the plants could be found. In July 2019 mid-sized plants without flowers were present. Isoglossa darbyshirei thus exhibits a distinct plietesial life history ( Bremekamp 1944), i.e. it represents a perennial, monocarpic plant with simultaneous mass flowering, followed by mass death and typically simultaneous germination to start the new life cycle ( Daniel 2006). This type of life history is well observed in the genus Strobilanthes Blume (1826: 796) ( Deng et al. 2010, Kakishima et al. 2011, Tsukaya et al. 2011). This synchronous flowering and monocarpy is also observed in other Acanthaceae like Acanthopale confertiflora (Lindau) Clarke (1899: 64) (= Dischistocalyx confertiflorus Lindau 1894: 13 ) ( Champluvier & Darbyshire 2009), several species of Isoglossa ( Van Steenis 1978, Darbyshire 2009, Darbyshire et al. 2011, Poriazis & Balkwill 2008, Balkwill et al. 2017), and in various Mimulopsis Schweinfurth (1868: 677) species ( Fischer 2017). On a herbarium sheet of Isoglossa humbertii (Stauffer 1121, BR) it is noted that the species is “zahlreich in den feuchten Tälchen, viele abgestorben” (numerous in the humid valleys, many dead). Thus, there is evidence that Isoglossa humbertii also exhibits a plietesial life cycle. Brachystephanus giganteus Champl. in Champluvier & Darbyshire (2009: 139) of the Cameroon Highlands is another well-documented case displaying a plietesial life cycle, i.e. one of perennial monocarpy with gregarious growth and synchronous flowering over a 7–9 year cycle ( Thomas 1996, Cable & Cheek 1998, Cheek et al. 2000, 2004, Darbyshire 2009). This cycle is known to be synchronous with other plietesial species of Acanthaceae , e.g. Mimulopsis solmsii Schweinfurth (1868: 677) , M. arborescens C.B.Clarke in Burkhill & Clarke (1899: 57), or M. champluvierae Fischer (2017: 87) . Together, these species can dominate the montane forest understorey. Brachystephanus glaberrimus Champluvier (1997: 194) was also previously known to exhibit regular mass flowering ( Champluvier & Darbyshire 2009). In Cyamudongo Forest, a mass flowering of this species occurred in December 2016 and the species had completely vanished by June–July 2017 (pers. obs. E.Fischer).

Distribution: —Only known from Nyungwe National Park, Cyamudongo Forest in South West Rwanda and from

Kibira National Park, North West Burundi. The Albertine Rift between Northwestern Uganda and the southern tip of Lake Tanganyika is one of the biodiversity hotspots on earth (e.g. Mutke & Barthlott 2005, Kier et al. 2009, Dagallier et al. 2019). The geographically and climatically complex region is characterized by a large biome gradient situated between 250 m and 5119 m, and includes several mountain ranges and large lakes ( Plumptre et al. 2007; Mutke et al. 2011). The region started to form in its current shape during the Miocene with an increase in mountain uplifting, valley subsidence and volcanic activity during the Plio-Pleistocene ( Hamilton 1982, Durham 2008). The region served as a forest refuge during the ice ages ( Loader et al. 2014). Due to the high geographical complexity, the relatively young Albertine Rift harbours an extraordinary diverse flora (c. 5800 vascular plant species), including about 10% endemic species ( Plumptre et al. 2007). All three closely related species of Isoglossa , i.e. I. darbyshirei , I. humbertii and I. laxiflora , are considered as Albertine Rift endemics.

Conservation status: —With an area of occupancy of 18 km ², Isoglossa darbyshirei could be considered to be Endangered (EN, B1+2) according to the IUCN criteria ( IUCN 2016). The whole population, however, is located in two protected areas, i.e. Nyungwe National Park in Rwanda, and Kibira National Park in Burundi, and there are no actual threats. Thus the preliminary assessment should be Vulnerable (VU B2).

Figures

FIGURE 1. Isoglossa darbyshirei. A. Detail of habit with inflorescence. B. Inflorescence. C. Corolla, lateral view. D. Corolla, frontal view. E. Corolla, dissected. F. Stamen. G. Calyx with style and stigma.All drawn by E. Fischer from Fischer 788/16.

FIGURE 2. Isoglossa darbyshirei. A. Habit. B–D. Details of habit. Photographs: E. Fischer, Rwanda, Cyamudongo Forest, 31 December 2016.

FIGURE 3. Isoglossa darbyshirei. A. Flower, frontal view. B–F. Details of inflorescence with flowers. Photographs: E. Fischer, Rwanda, Cyamudongo Forest, 31 December 2016.