Lecanorchis tabugawaensis (Orchidaceae, Vanilloideae), a new mycoheterotrophic plant from Yakushima Island, Japan

Abstract A new species, Lecanorchis tabugawaensis Suetsugu & Fukunaga, sp. nov. from Yakushima Island, Kagoshima Prefecture, Japan, is described and illustrated. Lecanorchis tabugawaensis is similar to Lecanorchis taiwaniana, but it is easily distinguished by the straight column, the glabrous status of the base of the column, the almost entire and narrow labellum morphology, the shorter part of the column fused with the labellum and the glabrous status of the apical part of the adaxial labellum surface. The new species appears to be restricted to two locations, each consisting of only dozens of mature individuals, and is assessed as Critically Endangered [CR D1] according to IUCN Red List Categories and Criteria.


Introduction
Th e genus Lecanorchis Blume comprises a group of mycoheterotrophic plants with an erect stem which may be either branched or unbranched (Hashimoto 1990;Seidenfaden 1978;Szlachetko and Mytnik 2000;Hsu and Chung 2010). A key characteristic of the species of Lecanorchis is the presence of a calyculus, a cup-like structure between the base of the perianth and the apex of the ovary (Cameron 2003;Hashimoto 1990;Sawa et al. 2006). Th ere are over thirty species and/or varieties in the genus Lecanorchis extending across a large area that includes Th ailand, Malaysia, Indonesia, Vietnam, the Philippines, Taiwan, Japan, and New Guinea (Hashimoto 1990;Seidenfaden 1978;Su 2000;Szlachetko and Mytnik 2000;Cameron 2003;Averyanov 2011). Precise Lecanorchis species identifi cation is often diffi cult due to close similarities in morphology and the short durations of fl owering periods (Hashimoto 1990;Averyanov 2005;Suddee and Pedersen 2011;Tsukaya and Okada 2013;Suetsugu et al. 2016a). Th is challenge remains due to the diffi culty of identifi cation at the fruiting stage even though the Lecanorchis species maintains withered plants above ground levels for longer periods compared to other mycoheterotrophic species, and the fruiting plants can be easily found in the forests. Furthermore, detailed descriptions for some species remain lacking, particularly for those fi rst described decades ago . Given such diffi culties in precise identifi cation, adequate taxonomic studies of this genus have not been conducted.

Lecanorchis tabugawaensis
Conservation. IUCN red list category: Critically Endangered, [CR D1]. To date, the distribution of Lecanorchis tabugawaensis appears to be restricted to two locations, separated by ca. 1.5 kilometers, along the Tabu  Although the lowland humid evergreen forests fl anking the rivers in Yakushima Island have previously been identifi ed as hotspots for endemic plant species, only a small proportion of the area is currently under protection (Yahara et al. 1987). Th is is in spite of the fact that 61% (20989 ha) of Yakushima Island is designated as a National Park, and 21% (10747 ha) is a World Natural Heritage site (Yahara and Tsukaya 2008). Consequently, one of the two locations of Lecanorchis tabugawaensis remains unprotected in an evergreen forest along the Tabu River adjacent to a Cryptomeria japonica (L.f.) plantation that has recently been cut. Th e area also harbors rare mycoheterotrophic plants, such as Gastrodia uraiensis T.C.Hsu & C.M.Kuo, G. takeshimensis Suetsugu, and G. albida T.C.Hsu & C.M.Kuo. Considering these mycoheterotrophs are completely dependent on their unique host fungi (e.g. Suetsugu et al. 2014b), it is important to conserve the entire ecosystem of their surrounding habitat. Further regulations restricting forest logging and construction are therefore required to conserve the fl ora, fauna, and the numerous endemic species restricted to low altitude habitats on Yakushima Island.
Etymology. Th e specifi c epithet is derived from"Tabugawa", which is the Japanese name for the Tabu River, the type locality that also harbors other rare mycoheterotrophic plants.
Taxonomic notes. Lecanorchis tabugawaensis is similar to L. nigricans Honda and L. taiwaniana S.S. Ying. Lecanorchis taiwaniana has often been treated as a synonym of L. nigricans Honda, a species known to be found in Japan, China, and Taiwan (Su 2000;Chen et al. 2009;Govaerts et al. 2016). However, this is based on the ambigu- ity of the original description by Ying (1987). Suetsugu et al. (2016b) reported that L. taiwaniana can be easily distinguished from L. nigricans by a combination of several characters (Table 1), including the longer peduncles, the longer rachis, the longer internodes, the narrower sepals and petals, the slightly 3-lobed labellum, the bright brown ascending capsules, the column that is more than half-fused with the labellum, the pubescence at the base of the column and the paler rachis coloration, based on the knowledge obtained by the newly discovered specimens. In addition, the diff erences of L. taiwaniana (junior synonym: L. amethystea) and L. nigricans have been clearly stated Table 1.
Ecology. Investigation on the column morphology suggested that the rostellum of Lecanorchis tabugawaensis is not very developed, as it does not function as a physical barrier between the stigma and the pollinia. As such, columns excised from fl owers about one day after anthesis exhibit contact between the pollinia and the stigma because the pollinia begins to drop downward onto the stigma from the clinandrium. However, autonomous self-pollination in a bud stage is unlikely to occur because columns from the buds picked about a day before fl ower opening showed that pollinia are usually compacted within the clinandrium and basally inserted behind the apex of the stigma.
Autonomous self-pollination in Orchidaceae has previously been reported in various species, including the Vanilloideae subfamily, under which Lecanorchis belongs (e.g. Suetsugu 2013b, 2015b). Autonomous self-pollination has been proposed as an evolutionary response to ensure reproductive success given a lack of pollinators when the frequency of pollination is regularly quite low (Baker 1955). Mycoheterotrophic plants are often found growing on the dense forest fl oor, shaded by woodland or scrub. It can be theorized that mycoheterotrophy developed as an adaptation for these species to survive in such low-light conditions (Bidartondo et al. 2004). At the same time, pollinators may not be particularly suited to such low-light environments (Herrera 1995(Herrera , 1997, thus indirectly creating a problem for plant reproduction if pollinators are unlikely to visit these areas. It appears that most of the mycoheterotrophic species investigated to date (especially nectarless species) have indeed abandoned an insectmediated pollination system in favor of self-pollination (e.g. Suetsugu 2013a, b; Suetsugu 2014a; Suetsugu 2015c). Th us, autogamy in L. tabugawaensis can also be considered a reproductive assurance to compensate for pollinator limitation due to their lack of nectar and pollinators' habitat preferences.