Taxonomic monograph of Oxygyne (Thismiaceae), rare achlorophyllous mycoheterotrophs with strongly disjunct distribution

Abstract

Oxygyne Schltr. (Thismiaceae) is a rare and little-known genus of achlorophyllous mycoheterotrophic perennial herbs with one of the most remarkable distributions of all angiosperm plant genera globally, being disjunct between Japan and West–Central Africa. Each species is known only from a single location, and in most cases from a single specimen. This monographic study names, describes and maps two new species, Oxygyne duncanii Cheek from cloud forest in SW Region Cameroon and O. frankei Cheek from gallery forest in the Central African Republic, representing the first new Oxygyne species described from Africa in 112 years, and raising the number of described Oxygyne species from four to six. Oxygyne duncanii is remarkable for sharing more morphological characters with two of the three Japanese species (O. hyodoi C.Abe & Akasawa, O. shinzatoi (H. Ohashi) Tsukaya) than with the geographically much closer type species of the genus, O. triandra from Mt Cameroon. Based mainly on herbarium specimens and field observations made in Cameroon and Japan during a series of botanical surveys, we provide descriptions, synonymy, mapping and extinction risk assessments for each species of Oxygyne, together with keys to the genera of Thismiaceae and the species of Oxygyne. The subterranean structures of African Oxygyne are described for the first time, and found to be consistent with those of the Japanese species. We review and reject an earlier proposal that the Japanese species should be segregated from the African species as a separate genus, Saionia Hatus. The only character that separates the two disjunct species groups is now flower colour: blue or partly-blue in the Japanese species compared with orange-brown in the African species. Studies of the pollination biology and mycorrhizal partners of Oxygyne are still lacking. Two of the six species, O. triandra Schltr. and O. hyodoi, appear to be extinct, and the remaining four are assessed as Critically Endangered using the IUCN 2012 criteria. To avoid further extinction, an urgent requirement is for conservation management of the surviving species in the wild. Since few achlorophyllous mycoheterotrophs have been successfully cultivated from seed to maturity, ex situ conservation will not be viable for these species and protection in the wild is the only viable option. While natural habitat survives, further botanical surveys could yet reveal additional new species between Central Africa and Japan.

Oxygyne: An extraordinarily elusive flower

“…Only six species have been described, but given the inconspicuous nature of Oxygyne , and the recent spate of species discovery of related Thismiaceae in Southeast Asia, it is possible that further taxa await discovery. Such discoveries are most likely to be in areas of high rainfall with exceptional species diversity and endemism, in regions where Oxygyne is already known to occur (Cheek et al, ). Two species ( O. triandra and O. hyodoi ) have not been encountered during numerous targeted searches and have been assessed as extinct (Cheek et al, ); coupled with the potential for habitat loss and degradation in areas where Oxygyne occurs, the identification and conservation of new species are an urgent priority.…”

“…Despite the unusual appearance of the flowers of Oxygyne , nothing is known about their reproductive biology. Interestingly, Japanese species are all blue, whereas African species are brownish‐orange (Cheek et al, ) (Figure a, b). All are small and inconspicuous.…”

“…For example, although the flowers of the newly described species O. yamashitae are a vibrant shade of blue, they are small (just 5 mm high and 5 mm across) and often at least partially concealed by leaf litter (Yahara & Tsukaya, ). The authors suggest that this species is self‐compatible; however, empirical data on its pollination biology (indeed for all species of Oxygyne ) are completely lacking (Cheek et al, ) and are scant across genera in the Thismiaceae more broadly. Tiputinia foetida, a recently discovered achlorophyllous mycoheterotroph in the related family Burmanniaceae (which includes the Thismiaceae in the views of some authors), attracts flies with the smell of carrion in Amazonian Ecuador (Woodward, Berry, Maas‐van de Kamer, & Swing, ).…”

“…Oxygyne (Thismiaceae) is a rare and poorly known genus with a remarkably disjunct distribution, occurring only in a few places in Japan and West Central Africa (Cheek, Tsukaya, Rudall, & Suetsugu, ). Each of the six species is known from just one location, usually from a single specimen, two are believed to be extinct, and the remaining four are Critically Endangered (Figure ).…”

“…Genera in the family Thismiaceae are renowned for their extreme scarcity (Franke, ) and many species have been discovered only recently. Oxygyne yamashitae was described in 2008 (Yahara & Tsukaya, ); indeed, the first taxonomic monograph of Oxygyne was published only in 2018 (Cheek et al, ) and raised the number of recognized species from four to six. Meanwhile, recent discoveries in the related genus Thismia include T. cornuta , T. pallida (Hroneš et al, ), and T. hexagona (Dančák et al, ) in Borneo; T. filiformis in Thailand (Chantanaorrapint, ); T. clavarioides in Australia (Thiele & Jordan, ); the rediscovery of T. neptunis —not seen for 151 years—in Borneo in 2017 (Sochor, Egertová, Hroneš, & Dančák, ); and T. kobensis found preserved in a Japanese natural history museum, and probably now extinct (Suetsugu, Nakanishi, Kobayashi, & Kurosaki, ).…”