Incadendron : a new genus of Euphorbiaceae tribe Hippomaneae from the sub-Andean cordilleras of Ecuador and Peru

Abstract Incadendron esseri K.Wurdack & Farfan, gen. & sp. nov., from the wet sub-Andean cordilleras of Ecuador (Cordillera del Cóndor) and Peru (Cusco, Oxapampa) is described and illustrated. This recently discovered large canopy tree with a narrow elevational range presents an unusual combination of rare morphological characters in Hippomaneae including mucilage-secreting sheathing stipules, conduplicate ptyxis, and large, woody fruits. The broader significance of these characters in Hippomaneae is discussed. The morphology and anatomy of Incadendron were investigated, highlighting its fruit similarities with Guiana Shield endemic Senefelderopsis, and the systematics value of ptyxis variation, which remains poorly studied for the family.


Introduction
Euphorbiaceae contains about 330 genera and 6300 species, and new genera continue to be added both through taxonomic adjustments to non-monophyletic groups discovered during molecular phylogenetic studies (e.g., Karima, Cheek et al. 2016), and more rarely through their discovery as recently collected novelties (e.g., Gradyana, Athiê-Souza et al. 2015;Tsaiodendron, Zhou et al. 2017). One such novelty, described herein, has been relatively well collected over the past 15 years due to intensive floristic and ecological studies in several sub-Andean cordilleras of Ecuador and Peru. These wet, mid-elevation mountains between the main Andean chain to their west and Amazonian lowlands to their east are rich in plant and animal endemism, especially in the Cordillera del Cóndor (Neill 2005, Ulloa Ulloa andNeill 2006), which contains the northern part of the known range of this plant and has floristic ties to the Guiana Shield. However, this region is relatively sparse in Euphorbiaceae endemism and a new genus is noteworthy. Based on morphological features, the Andean plant was quickly determined to be an undescribed member of Euphorbiaceae subfamily Euphorbioideae, and affiliated with tribe Hippomaneae. The shared diagnostic features include exudate of white latex, indument lacking, staminate bracts glandular at their base, perianth small in both pistillate and staminate flowers, petals absent, and staminate flowers inclinate in bud.
Etymology. The genus is combined from "Inca" (as Inka, Quechua for "ruler" or "lord") referring to the indigenous Inca people and pre-Columbian empire that was centered in Cusco and encompassed much of the range of this taxon, and "dendron" (Greek) referring to tree, which is the habit of the plant. Some localities occur near the Trocha Unión, an ancient Inca path. The specific epithet is from "Esser", the surname of Hans-Joachim Esser (Botanische Staatssammlung München, Germany) and honors this expert on Hippomaneae who has contributed much to our understanding of the tribe and Euphorbiaceae in general.
Distribution, life history, and ecology. Incadendron is known from three wellseparated clusters of localities (hereafter referred to as Cóndor, Manu, and Oxapampa populations) on the eastern slopes of the main Andean mountain range in Peru and Ecuador, where it occurs in wet montane forests at 1800-2400 m elevation (Fig. 2). The extent of discontinuity in its range is presently unclear due to the floristically poorly known nature of the intervening areas, and it should be looked for in similar habitats between the three populations. There are minor vegetative differences including leaf apex variation with most tips distinctly acute versus more rarely rounded (i.e., Neill & Kajekai 16620, Monteagudo et al. 16929), and a larger-leafed collection (i.e., Monteagudo et al. 4458). The differences exist within the populations and presently do not suggest differentiation worthy of taxonomic recognition.
Detailed field observations were made in the tropical montane cloud forests of the Kosñipata Valley in Manu National Park (Parque Nacional del Manu), which contains the southernmost part of the known range of Incadendron. The general site characteristics are well documented (see Girardin et al. 2010, Farfan Rios 2011, Feeley et al. 2011) as part of intensive forest monitoring using permanent forests plots established by the Andes Biodiversity and Ecosystem Research Group (ABERG, http://www. andesconservation.org/) along an elevational gradient (i.e., Kosñipata transect) from the Andes to the Amazon. Incadendron has been found (i.e., Farfan et al. 1049Farfan et al. , 1090Farfan et al. , 1131 in the cloud immersion zone between 1800-2250 m at the study site. The substrate where the tree was collected is granite between 1800-2000 m, and shale at 2250 m. The soils below the thick organic surface layer are relatively poor in nutrients. At the elevations where found, Incadendron is among the taller components of the forest and its habit is a canopy tree with a spreading crown. The maximum height observed was 21.2 m, and maximum tree diameter at breast height (dbh) was 56.7 cm. When cut, the thin trunk bark has a cream-yellowish color with abundant white latex. Mean tree growth (diameter increment) at the study site was 4.02 mm yr -1 ± 0.90 (95% CI). Mean wood density is 0.55 g/cm 3 ± 1.18 (95% CI), based in field sampling. The highest population density in the network of one hectare plots was found at 2000 m of elevation, with 30 adult individuals/ha (≥10 cm dbh), making it the ninth most abundant tree in that plot (Parcela VII). The main associated species include Alzatea verticillata Ruiz & Pav. (Alzateaceae), Cyathea lechleri Mett. (Cyatheaceae), and Ilex villosula Loes. (Aquifoliaceae). The Euphorbiaceae diversity for the tropical montane forests of this region includes nine genera, of which there is notable species-richness in Alchornea Sw. (Farfan-Rios et al. 2015). The closet relatives of Incadendron (i.e., other members of Euphorbioideae) among these nine genera include Sapium spp. and Pseudosenefeldera inclinata (Müll. Arg.) Esser, with the latter occurring at lower elevations. The basal marginal leaf glands of Neill & Kajekai 16620 from the Cordillera del Cóndor are overgrown by unusual clusters of tiny, 0.1 mm diameter black globules that are fruiting bodies of a likely ascomycete fungus. While epiphyllous fungal growth such as mold growing on glandular secretions is to be expected, these unusual fruiting structures are very different and deserve further study.
Phenology. The trees are evergreen, with an observed flowering season during July-September and fruiting during August-November. Herbarium collections also indicate a spring reproductive period of January-April for the Manu and Oxapampa populations. Fruits can be abundant, they turn brown when mature (Fig. 3G-H), and due to their large, heavy nature become pendulous on the relatively long infructescence axes. They are subject to predispersal seed predation by Lepidoptera, based on caterpillar remains recovered from inside fragmented fruits of Monteagudo & Ortiz 4605. In developing fruits these moths (likely members of Phycitinae, Pyralidae: Pyraloidea; A. Solis, personal communication) hollow out the seeds, which have well-developed endosperm, and leave holes (1.75 mm dia.) in the mericarp septa and seed coats (Fig. 3I-J). Plant defenses to deter herbivory would appear to be high in Incadendron due to latex, and the thick, lignified pericarp. Seed predation by specialist moths is well known for other Hippomaneae including in Mabea Aubl., where oviposition occurs early in fruit development when the pericarp is thin and soft (De Steven 1981). One young Incadendron fruit (4 mm dia.) on Monteagudo & Ortiz 4605 (US) has what appears to be an oviposition hole at the top of the ovary that is likely a weak spot into the interior.
Conservation status. Following the criteria and categories of IUCN (2012), Incadendron esseri is given a preliminary status of Vulnerable (VU) under geographic range criteria B2 area of occupancy <2000 km 2 (B2a, known to exist at no more than 10 locations; B2b, continuing decline projected). Threats to this taxon in the Cordillera del Cóndor include mining for the underlying silica sand. Parts of its Peruvian range are protected within the Parque Nacional Yanachaga Chemillén and Parque Nacional del Manu. Discussion. Specimens of Incadendron mostly were tentatively identified by collectors either as Sapium Jacq., due to similarities in coriaceous leaves and glandular, spicate inflorescences (Fig. 3A, D), or as Micrandra Benth. (e.g., Farfan-Rios et al. 2015), due to their shared unusually large fruits and white latex. Sapium notably differs in its bistaminate flowers and red-arillate seeds, and Micrandra is florally very different as a member of subfamily Crotonoideae. Within the context of the generic key to South American Hippomaneae in Athiê-Souza et al. (2015), Incadendron would group with Sebastiania Spreng. A comparison of select genera and distinguishing characters is given in Table 1. These genera are the most morphologically and geographically similar to Incadendron but are not necessarily its closest relatives, which are presently unclear. Senefelderopsis Steyerm., however, may have a closer relationship as suggested by similar fruit structure (see below), biogeographic ties between the Andean cordilleras and Guiana Shield (Berry and Riina 2005), and isolated phylogenetic placement in the same diverse subclade H1 of Hippomaneae (Wurdack et al. 2005).
Incadendron presents a unique combination of rare characters (discussed below) within Hippomaneae including sheathing stipules, conduplicate ptyxis, leaf margins entire and with unusual glands, and large, woody fruits. None of these characters appears phylogenetically very informative because they are autapomorphic or clearly homoplasious when considered in the context of molecular phylogenies (i.e., Wurdack et al. 2005, Wurdack, unpublished). Thus, while the rare character combination serves well to distinguish Incadendron and indicates a degree of morphological disparity deserving of generic recognition, it does not inform on relationships nor provide much insight into how such characters evolved. Major floral features that are variable in Hippomaneae have broadly distributed, likely plesiomorphic, states in Incadendron, including terminal and unbranched inflorescences, single pistillate flower per bisexual inflorescence, staminate cymules that are multiflowered and glandular, and 3-merous pedicellate staminate flowers with free stamens.
Pollen morphology and ultrastructure are remarkably diverse across Euphorbiaceae, but are less variable in Euphorbioideae, and Hippomaneae in particular (Punt 1962, Park andLee 2013). The pollen of Incadendron resembles that of other hippomanoids in being tricolporate with a perforate exine (Fig. 4C-D). The seed structure of Incadendron is also similar to that of other Hippomaneae (Martin 1946, Tokuoka andTobe 2002). Its seed coat (Fig. 3L) with a thin testa of collapsed cells and palisade-like mechanical exotegmen of elongate, pitted, thick-walled cells resembles that of exotegmic genera across the entire family. Depending on exact location, the pallisadal cells vary in length (3-4× shorter at apex versus bottom and sides) and orientation (vertical, inclined, or curved). Embryos generally deserve more careful description and study given their diversity in Euphorbiaceae, although most are variants of the spathulate fully developed type (Martin 1946). The embryo of Incadendron agrees with this generalization (see Description for details, based on one intact seed) and in particular resembles that of other Hippomaneae including Senefelderopsis croizatii Steyerm. (examined here, Maas et al. 5828, US), which has a slightly longer hypocotyl-radicle axis at 3 × 1 mm (versus 2 × 1 mm) and thicker cotyledons at 0.5 mm (versus 0.1 mm).
Unusual morphological features of Incadendron. The entire hippomanoid clade (sensu Wurdack et al. 2005) is considered here for the purposes of broad discussion rather than restricted to paraphyletic Hippomaneae. This broad grouping is reflected in the nomenclaturally problematic (see Esser 2012) Hippomaneae s.l. of Webster (2014). Stipules in the hippomanoids are typically small, scale-like (Fig. 5D) or absent. Conspicuous, sheathing stipules are relatively rare and in addition to Incadendron (Fig. 1B) also characterize Conosapium Müll. Arg., Homalanthus A. Juss. (Fig. 5A), Table 1. Comparison of Incadendron with the morphologically most-similar neotropical genera. Based on primary observations with supplements from Kruijt (1996) and Esser (1995Esser ( , 2001. The circumscription of Sebastiania is controversial leading to some uncertainty in the breadth of character states.  Hura L., Neoshirakia Esser, and Pachystroma Müll. Arg. While functionally these large stipules are similar in conferring additional protection to the shoot apex, they differ in morphological details and likely are convergent size increases. The stipules in Incadendron are large, sheath the terminal bud, unusually stout in keeping with the coriaceous leaves, deciduous, and centrally attached such that an elliptic scar remains after abscission (Fig. 3E). The buds are internally mucilaginous. Serial sections of a single bud show a distinct palisade-like cell layer on the inner surfaces of the stipules that is the likely source of this secretion (Fig. 4H-I). This layer appears to differentiate late in development as it is present in the outer stipules but not on the enclosed next younger pair of the bud (Fig. 4H). The inflorescence bracts resemble a smaller version of the stipules (rarely with a few subtending glands of the type found with the staminate cymules) and are also deciduous, leaving large scars (Figs 3F,4B). Conspicuous sheathing stipules in other taxa are mostly thinner except in Pachystroma, and may be centrally attached, leaving elliptic scars (Homalanthus, Hura, Neoshirakia) or broadly attached along their entire base, leaving semi-circumferential scars (Conosapium, Pachystroma). Leaf folding (ptyxis) is variable but poorly studied for Euphorbiaceae in general (Cullen 1978). Based on recent surveys (K. Wurdack, personal observations) Incadendron appears unique in the hippomanoids in having conduplicate ptyxis (Fig. 4G). In bud and early expansion after stipule abscission, the halves of the blade are folded tightly together along their adaxial surface and the leaf in transverse section shows no curling at the edges. After the blade halves spread open, the margins recurve before hardening and finally at maturity the halves assume a flattened aspect with a slightly revolute margin (Fig. 3A, C). The terminal buds are flattened (Fig. 3E, 4G), which likely reflects the conduplicate nature of the enclosed developing leaf blades, although this point needs further study with fresh material. Other hippomanoids have varying degrees of developing leaf blade curvature including tightly rolled scrolls, rolled edges, or gently curved loops that span involute (e.g., Homalanthus, Fig. 5B) to supervolutecurved ptyxis (e.g., Senefelderopsis). Cullen (1978) (Fig. 5C) and herbarium material indicate its glandulartoothed margin is inwardly curled and the ptyxis is more accurately described as a conduplicate-involute intermediate. The hippomanoids contain a wide diversity of petiolar and leaf gland form and position. Incadendron has no acropetiolar or embedded laminar glands but it has glands along the leaf margin, which are best developed (i.e., consistent in presence and largest in size) near the leaf base. Curling of the leaf edge with age shields these basal glands (Fig. 3C), although the scattered more distal glands remain exposed. The marginal glands (Fig. 4E) are not associated with teeth or setae and are similar in morphology to those found on Gymnanthes schottiana Müll. Arg. Hippomanoid leaf margins typically have regular teeth or marginal setae, and are more rarely similar to Incadendron or entire without any associated glands. The abaxial leaf surface of Incadendron is finely striate, but the stomata are not shielded or sunken (Fig. 4F). Some high elevation hippomanoids have micro-papillose surfaces and concealed stomata (e.g., Dendrothrix Esser, Senefelderopsis).
Inflorescences in the hippomanoids are axillary and/or terminal. In Incadendron the inflorescence is terminal but appears distinctly leaf-opposed due to near simultaneous development of both the inflorescence and adjacent leaf, coupled with the start of renewal shoot growth from the axillary bud before flowering is finished (Fig. 3E). Axial growth and orientation in Incadendron branches is not deflected by inflorescence development and gives little hint of being sympodial by substitution. Specimens of Incadendron show twinned branching (Fig. 1A, 3A), due to the occasional growth of additional leaf-axil accessory buds close to the growing branch tips. Most accessory buds, even in older parts of the shoot system, remain as barely visible meristems.
The fruits of Incadendron most closely resemble those of Senefelderopsis. In addition to large size, they share many structural features including a thick pericarp with an ex-ceptionally thick woody mesocarp (≥3 mm), sharp woody apex, well-developed septa, mericarp valves connected with a basal triangle, and a thin funicle. These features individually (or in various combinations) occur in other genera, but the thick mesocarp is exceptional among the large-fruited, dehiscent species. The mesocarp of Incadendron has a prominent dorsal-suture lip, has non-vascularized raised ridges that follow underneath major exocarp venation, and is stratified with a darker layer of different structure lining the locule (Fig. 3M); these features are to varying degrees shared with Senefelderopsis (Fig. 3N). In both Incadendron and Senefelderopsis the robust septa dividing the locules are lignified and nearly complete except for a small apical gap (Fig. 3J-K, M-N) to accommodate the funicle and its attachment to the placenta. Many other hippomanoids differ in having thin, poorly lignified septa that become destroyed during dehiscence, and/or septa that are incomplete leaving a large gap descriptively called a "C-shaped cut" (Athiê-Souza et al. 2015). Fruit vascular variation, especially septal strand number, has been considered potentially informative at the generic level in Hippomaneae (Esser et al. 1997, Esser 2001, but can be difficult to observe and its functional significance is not well understood. The septal vascular strand numbers are very similar between Incadendron (single or bifurcating) and Senefelderopsis (single or rarely bifurcating). The seemingly delicate, suspended nature of the relatively large seeds in the locules of both genera is unusual. Their thin, nearly erect funicles differ in length, which allow the seeds of Incadendron to be further displaced downward in the locule from the point of attachment to the columella as compared with Senefelderopsis ( Fig. 3I-K). Many other hippomanoid taxa have better support for their seeds either through greater filling of the locule cavity by the seed that leaves little free space around it, and/or through more robust attachment to the columella via a short and thickened funicle or fusion along the raphe. The fruit and locule shape differ slightly between the genera in being subglobose in Incadendron versus elongate and more trigonous in Senefelderopsis (Fig. 3J-K, M-N). In addition, the exocarps (not examined anatomically and topographically defined here as the more or less removable outer layer) differ in gross structure with prominent vasculature at the interface between the loosely attached exocarp and woody mesocarp in Incadendron, likely supporting greater fleshiness when fresh. In Senefelderopsis there is little exocarp vascularization and tight adherence via a stiff, porous layer of apparently sclerified cells that is best developed in S. chiribiquetensis (R.E. Schult. & Croizat) Steyerm. Although there are considerable morphological differences between Incadendron and Senefelderopsis (see Table 1), the striking fruit similarities may have special significance for indicating a closer relationship between the genera.