Research

09 Jun 2015
  • Research Result

Plant that neither photosynthesizes nor blooms has been discovered

Dr Suetsugu

Dr Kenji Suetsugu, assistant professor of the Hakubi Center, Kyoto University announced the discovery of a new species of achlorophyllous mycorrhizal (mycoheterotrophic) orchid. The new species, named Gastrodia flexistyloide, was found on Takeshima Island in the Kagoshima Prefecture of Japan (not the Takeshima Island currently disputed by Korea and Japan). During his research trip, Dr Suetsugu came across approximately one hundred of these bloomless, yet fascinating flowers growing in a bamboo forest dominated by Pleioblastus linearis. Flowering was observed from mid-March to early April, and fruiting from early April to early May. The description of a new angiosperm species in Japan is itself a very rare event as the flora of this region have been thoroughly investigated. However, G. flexistyloides was a particularly special discovery because it is both completely mycoheterophic, deriving its nutrition not from photosynthesis but from host fungi, and completely cleistogamous, producing flowers that never bloom.

The most remarkable characteristic of G. flexistyloides is its trilobed column, which has a strongly incurved central lobe that facilitates autonomous self–pollination. In this respect, this species shows great similarity to G. flexistyla from Taiwan, although G. flexistyloides can be distinguished by its larger stature during flowering (9–18 cm vs 3–6 cm), its floral character (cleistogamous vs chasmogamous), smaller size (15–18 mm vs 19–24 mm), and narrower perianth tube (5–6 mm vs 11–13 mm), as well as the ratio of its petal to sepal length (ca 1:1 vs 1:1.5–2 for G. flexistyloides and G. flexistyla, respectively).

While reports of autonomous self-pollination are fairly common in the Orchidaceae, the usual mechanism by which it is achieved involves the pollinia falling from the anther onto the stigma surface, allowing contact between the pollinia and stigma. However, the autogamous system of G. flexistyloides is notable because the central lobe of the column, which is strongly incurved, allows the anther to contact the stigma directly. To date, this selfing strategy has only been reported in G. flexistyloides and the closely related species G. flexistyla, which has been found in Taiwan. The most significant difference between the two species is that G. flexistyloides produces exclusively cleistogamous flowers, while G. flexistyla always produces chasmogamous flowers that bloom.

Cleistogamy, literally meaning "a closed marriage", refers to plants that produce flowers in which self-fertilization occurs within closed buds. This mechanism of reproduction has intrigued botanists since the time of Darwin, and is now recognized as an important mechanism of self-pollination that is found in a diverse range of plant taxa. However, most cleistogamous orchids also produce chasmogamous flowers. In some cases, this has led to incidents of misclassification, where specimens bearing only cleistogamous flowers have been mistaken for new taxa. However, such "new species" were always collected in regions where equivalent chasmogamous species also existed. The discovery of G. flexistyloides is distinct from such examples because none of the individuals in the Takashima population were found to produce cleistogamous flowers (complete cleistogamy). Given that the morphological dissimilarities are clear and stable, this taxon should therefore be treated as an independent species rather than an infraspecific taxon of G. flexistyla. Furthermore, given that the completely cleistogamous condition in G. flexistyloides prevents gene flow with G. flexistyla, the two species differ according to the biological species concept, which defines a species as members of populations that can actually or potentially interbreed in nature.

Cleistogamous flowers are considered a bet-hedging strategy, since they are less costly than chasmogamous flowers, and because they can provide reproductive assurance by setting seeds in the absence of pollinators and under disadvantageous environmental conditions. In addition, cleistogamous flowers can also promote adaptation to local habitats, as both maternal sets of genes can be passed on to the progeny purging deleterious alleles. However, this is a somewhat risky strategy as the progeny are also less able to adapt to changes in spatially and temporally heterogeneous habitats.

The evolution of complete cleistogamy is therefore somewhat of a mystery. Chasmogamous flowers are an important factor in the success of most plants as even a small degree of outcrossing can result in a relatively rapid decline in linkage disequilibrium across the genome, and is sufficient to overcome the negative effects associated with an absence of effective recombination, such as the accumulation of deleterious mutations and a slowdown in the rate of adaptation. The discovery of G. flexistyloides, therefore, provides a useful opportunity to further investigate the ecological significance, evolutionary history, and genetic mechanisms underlying the evolution of complete cleistogamy.

02.jpg
"Flowering" individual of the mycoheterotrophic orchid Gastrodia flexistyloides

Paper Information

[DOI] http://dx.doi.org/10.11646/phytotaxa.175.5.5

Kenji Suetsugu
"Gastrodia flexistyloides (Orchidaceae), a new mycoheterotrophic plant with complete cleistogamy from Japan"
Phytotaxa 175 (5): pp. 270–274 published: 15 Aug 2014