Transfer of the Malagasy genera Humbertianthus and Macrostelia to Hibiscus (Malvaceae) with description of four new species

Abstract Hanes, M.M., G.E. Schatz & M.W. Callmander (2020). Transfer of the Malagasy genera Humbertianthus and Macrostelia to Hibiscus (Malvaceae) with description of four new species. Candollea 75: 193–202. In English, English and French abstracts. Two Malagasy species of the genus Macrostelia Hochr. (Malvaceae) are transferred to Hibiscus L. and two new combinations are proposed: Hibiscus calyculatus (Hochr.) M. Hanes, G.E. Schatz & Callm. and Hibiscus involucratus (Hochr.) M. Hanes & G.E. Schatz & Callm. The monotypic genus Humbertianthus Hochr. with its single species Humbertianthus cardiostegius Hochr. was defined as a taxon in its bud stage and represented unopened flowering collections of Hibiscus laurinus Baill. Several recent herbarium collections from the eastern humid forest of Madagascar represent four new restricted range species: Hibiscus ambanitazensis M. Hanes & G.E. Schatz, Hibiscus analalavensis M. Hanes & G.E. Schatz, Hibiscus ankeranensis M. Hanes & G.E. Schatz, Hibiscus vohipahensis M. Hanes & G.E. Schatz. Each of the new species are documented by field photographs and their geographic distribution is presented. Risk of extinction assessments of the seven species discussed indicate three taxa are threatened as “Critically Endangered”, and four are “Least concern”.


Introduction
Tribe Hibisceae Rchb. has 102 species known from Madagascar including 65 described species in the genus Hibiscus L. and members attributed to at least ten other genera (Madagascar Catalogue, 2020). Furthermore, Hibiscus itself is not monophyletic (Pfeil & Crisp, 2005) and several Malagasy endemic genera are embedded in Hibiscus (Koopman & Baum, 2008). The present work focuses on the genus Macrostelia Hochr. which was described by Hochreutiner (1952) to accommodate collections in family Malvaceae from Madagascar with the unique combination of entire, pinnately-veined leaves and pendent flowers with petals proximally fused into a tube. Two species, Macrostelia calyculata Hochr. and M. involucrata Hochr., were described from escarpment forests of the east coast of Madagascar. A third taxon, M. laurina (Baill.) Hochr. & Humbert, was transferred from Hibiscus L. to Macrostelia three years later (Hochreutiner, 1955) because it shared these unusual characteristics. The geographic range of Macrostelia was extended to Australia in 1974 with the description and recognition of M. grandifolia Fryxell (Fryxell, 1974). Koopman & Baum (2008) constructed a molecular phylogeny exploring Hibisceae on Madagascar and used Macrostelia laurina as a placeholder for Malagasy species of Macrostelia. They demonstrated that the genus belongs in the Euhibiscus clade, an unranked clade name in Hibiscus as defined by Koopman & Baum (2008). Notably Euhibiscus includes the type of Hibiscus (H. syriacus L.) suggesting that members of Macrostelia are better placed in the genus Hibiscus. Furthermore, there is ample molecular evidence that the Malagasy species of Macrostelia are more closely related to other Hibiscus from Madagascar rather than to Australian taxa formerly included in Macrostelia (Fryxell, 1974;Craven & Pfeil, 2004;Koopman & Baum, 2008). Craven & Pfeil (2004) previously transferred all Australian members of Macrostelia to Hibiscus. Hochreutiner (1948) further established the monotypic genus Humbertianthus Hochr. to accommodate a new taxon from eastern Madagascar that appeared to possess a suite of characters entirely novel and unique in the tribe Hibisceae. The presence of five geniculate style branches with convergent capitate stigmas, a severely reduced staminal column, and petals that are not fused with the staminal column differentiated Humbertianthus from all other genera in Malvaceae. We believe that H. cardiostegius Hochr. was defined as a taxon in its bud stage and that the Hochreutiner's descriptions were not measured from mature, open flowers. Thus, we propose that collections previously attributed to H. cardiostegius, instead, possess unopened flowers of Hibiscus laurinus from the low altitude rainforest of Betampona.
In response to the evidence presented above, we propose to transfer all currently described Macrostelia species on Madagascar to Hibiscus in accordance with morphology, phylogenetic understanding of Hibiscus on Madagascar (Koopman & Baum, 2008) and the generic delimitation of Hibiscus (Pfeil & Crisp, 2005). We also consider Humbertianthus cardiostegius Hochr. as a synonym of Hibiscus laurinus.
Several collections from the east coast of Madagascar previously determined as H. aff. laurinus due to the unusual combination of glabrous (or nearly so) leaves with entire margins and large, axillary, solitary and pendent flowers represent new restricted species from the eastern humid forest of Madagascar. These specimens form the basis of the four new species described below.
Each of the seven species treated here are provided with notes on their morphological affinities, a distribution map, and risk of extinction assessments following the IUCN Red List Categories and Criteria (IUCN, 2012 (IUCN, 2012), with the caution that this status is highly dependent on continued effective protection.  Distribution and ecology. -Hibiscus involucratus is known from the low elevation tropical evergreen forests around Vondrozo at c. 450 m elevation (Fig. 2).
Conservation status. -Hibiscus involucratus is known only from a single location near Vondrozo, collected in 1926. It is possible that the collection has been made in what is now known as the Ambositra-Vondrozo protected area. With a single collection in the low elevation moist evergreen forests, which are highly threatened in Madagascar, this species is assigned a conservation status of "Critically Endangered" [CR B2ab(iii)] using the IUCN Red List Categories and Criteria (IUCN, 2012).
Notes. -Hibiscus involucratus can be distinguished from all other pendent Hibiscus in Madagascar by its tubular epicalyx with 6 irregular teeth that completely hides the calyx.
We consider the sheet [P00037126] as the holotype, as it is the only specimen with Hochreutiner's handwriting in P.  Notes. -Humbertianthus cardiostegius and Hibiscus laurinus were both described as small, slender trees with entire, elliptic, reticulate to pinnately-veined, briefly petiolate leaves with similar leaf sizes (4 -8 × 2.5 -4 cm). All collections of Humbertianthus cardiostegius are limited to January and March, whereas Hibiscus laurinus has been collected in flower from March through October. Pedicels on both entities are covered with ferrigenous hairs. The buds of H. laurinus resemble those of Humbertianthus cardiostegius. Perianth descriptions of H. cardiostegius have smaller proportional measurements than Hibiscus laurinus; shorter sepals, 10 mm (vs 3 cm) and shorter petals, 5 mm (vs 4 -5 cm). The convergent nature of the stigmas in Humbertianthus cardiostegius, as well as the extremely "reduced" staminal column appear to represent reproductive organs that have yet to differentiate and elongate. The stigmas in both taxa have very long, hairy style branches. The abaxial portions of petals in both taxa are covered in a coarse pubescence. Flowers of Hibiscus laurinus are a vibrant red whereas those of Humbertianthus cardiostegius have been noted in the literature to be white or pink. The first author visited Betampona in January 2005 and collected Hibiscus laurinus in bud (Koopman 224) which ranged in color from white to a brilliant red. Fruits from Humbertianthus cardiostegius were never described, though 2 -3 ovules in 5 locules within a hirsute ovary have been described, these values are consistent with Hibiscus laurinus.

Hibiscus laurinus
Three type collections of both Humbertianthus cardiostegius and Hibiscus laurinus are extant at P (respectively Réserves Naturelles 74 and Humblot 241). The most complete collections of each of the two names are designated here as lectotypes.

Holotypus
Conservation status. -Hibiscus ambanitazensis is known only by a single location outside of the Protected Area Network in a small patch of forest north of Antalaha in close proximity to recent slash and burn activity. The "table basaltique" along the east coast of Masoala to north of Antalaha are highly threatened because their rich volcanic soils are highly suitable for agriculture. The new species is assigned a conservation status of "Critically Endangered" [CR B2ab(iii)] using the IUCN Red List Categories and Criteria (IUCN, 2012).
Notes. -The new species differs from all other reddish, pendent flowered Hibiscus from the east coast of Madagascar in its petal pigmentation, purple red (vs. red). Epicalyx, calyx and style color further distinguish H. ambanitazensis from H. ankeranensis, H. calyculatus and H. laurinusus (see Table 1).
Shrub to small tree 3 -5 m tall, 3 -4 cm dbh; twigs moderately densely covered with golden stellate trichomes. Leaves 7.3 -11.9 × 3.3 -5.1 cm, narrowly obovate, the base cuneate and abruptly attenuate, the margin strongly revolute, the apex  A B C rounded to obtuse or slightly retuse, glabrous above, nearly completely glabrous below, venation brochidodromous with 6 -7 secondary veins per side, midvein raised above and below, secondary veins slightly raised above and below, tertiary venation reticulate, slightly raised above and below; petiole 4 -5 mm, c. 1 mm in diam., moderately densely covered with stellate trichomes. Flowers solitary, axillary, pendent; pedicels 30 -32 mm long, 1.4 mm in diam., densely covered with reddish-gold, erect simple, fasciculate and stellate trichomes; epicalyx with 5 lobes fused at the base for 2 mm, the lobes 15 × 6 -9 mm, broadly ovate, the apex acute, densely covered with rufous golden stellate trichomes on the outside, densely covered with golden appressed simple and fasciculate trichomes inside; calyx with 5 lobes, fused at the base for 6 -9 mm (c. half of length), the lobes 20 × 18 mm, triangular, the apex triangular, very densely covered with golden stellate trichomes outside, very densely covered with appressed curly white trichomes inside; petals 5, 50 × 35 -37 mm, obovate, the apex rounded, white with a peach blush, meanwhile they turn from pink to white with age, densely covered with light golden stellate trichomes outside, glabrous inside with evident venation; staminal column c. 30 mm long, c. 2 mm in diam., white, glabrous, with a tuft of dense, golden, erect, simple trichomes at the apex; stamens c. 70, filaments 10 mm long, white, anthers c. 1.5 × 1.5 mm; styles 5, c. 16 mm long, white, slender, moderately densely covered with white, erect, simple and fasciculate trichomes, stigma c. 0.5 × 0.8 mm, bulbous.  1 km2), there appear to be no current threats to H. analalavensis. Thus, the species is assigned a status of "Least Concern" [LC] using IUCN Red List Categories and Criteria (IUCN, 2012), with the caution that this status is highly dependent on continued effective protection.
Distribution and ecology. -Hibiscus ankeranensis is known only from a single collection near the Andrangato river at Ankerana in the eastern coast of Madagascar at c. 350 m in elevation in lowland moist evergreen forests (Fig. 2).
Conservation status. -Hibiscus ankeranensis is known only from a single collection from the newly designated Corridor Ankeniheny-Zahamena protected area. We lack information on population size but lowland moist evergreen forests (below 800 m) are fragmented and heavily deforested in the region (Gautier, 2018). Recent satellite imagery from Google earth [https://www.google.com/intl/en/earth] suggests that the collection site and the surrounding area is deforested or disturbed. Hibiscus ankeranensis is therefore assessed as "Critically Endangered" [CR B2ab(iii)] using IUCN Red List Categories and Criteria (IUCN, 2012).

A B C
Distribution and ecology. -Hibiscus vohipahensis has the most southern distribution of all species discussed here and is known from two protected patches of lowland moist evergreen forests near Vangaindrano from 10 to 135 m elevation.
Conservation status. -Hibiscus vohipahensis is known from 6 locations from two protected areas (Agnakatrika and Ankarabolava). Despite the very restricted extent of occurrence (EOO) and area of occupancy (AOO) of c. 9 km2, there appear to be no current threats to H. vohipahensis. Thus, the species is assigned a status of "Least Concern" [LC] IUCN Red List Categories and Criteria (IUCN, 2012), with the caution that this status is highly dependent on continued effective protection.