Mycorrhizal structures in mycoheterotrophic Thismia spp. (Thismiaceae): functional and evolutionary interpretations

High mycorrhizal specificity in the monotypic mycoheterotrophic genus Relictithismia (Thismiaceae)

Mycoheterotrophic plants, which depend entirely on mycorrhizal fungi for carbon acquisition, often exhibit high specificity toward their fungal partners. Members of Thismiaceae are generally recognized for their extreme mycorrhizal specialization and rarity. In this study, we examined the mycorrhizal associations of Relictithismia, a recently discovered monotypic genus within Thismiaceae, and Thismia abei, a Thismia species with a similar distribution in southern Japan, by employing high-throughput DNA sequencing of the 18S rRNA gene. Our analyses revealed that both R. kimotsukiensis and T. abei are predominantly associated with two specific virtual taxa (VTX00295 and VTX00106) of the genus Rhizophagus (Glomeraceae). These shared associations may reflect either phylogenetic niche conservatism, in which the common ancestor of R. kimotsukiensis and T. abei retained the same AM fungal partners, or convergent evolution, in which the AM fungal phylotypes were independently recruited due to their potential benefits for these mycoheterotrophic plants. Furthermore, BLAST searches demonstrated that VTX00295 and VTX00106 are widely distributed globally, suggesting that highly specialized mycorrhizal interactions are unlikely to be the primary drivers of the limited distribution and rarity of R. kimotsukiensis and T. abei. Overall, our findings enhance our understanding of high mycorrhizal specificity in Thismiaceae. However, broader investigations, combining extensive sampling of Thismiaceae species with ancestral state reconstruction, are needed to determine whether the shared associations detected here reflect phylogenetic niche conservatism or convergent evolution.

Thismiamalayana (Thismiaceae), a new mycoheterotrophic species from Peninsular Malaysia

A new species of the mycoheterotrophic genus Thismia from Malaysia is described and illustrated. Thismiamalayana introduced here was found in two localities: in the lowlands of Gunung Angsi Forest Reserve, Negeri Sembilan, and in the hilly dipterocarp forests of Gunung Benom in Tengku Hassanal Wildlife Reserve, Pahang. Thismiamalayana falls into the section Thismiasubsect.Odoardoa, as it has creeping vermiform roots and free equal tepals. It is characterised by the following taxonomically important features: a sepia-brown, urceolate-curved floral tube, free equal tepals with terminal appendages, prominent bright yellow annulus and bright violet-blue stamens each bearing five appendages (one pair of club-shaped inwards-pointing, one pair of acute outwards-pointing, and one central appendage). According to the categories and criteria of the IUCN Red List, T.malayana is provisionally classified as Vulnerable.

Relictithismia kimotsukiensis, a new genus and species of Thismiaceae from southern Japan with discussions on its phylogenetic relationship

The family Thismiaceae, known as "fairy lanterns" for their urn- or bell-shaped flowers with basally fused tepals, consists of non-photosynthetic flowering monocots mainly in tropical regions, extending into subtropical and temperate areas. Here, we propose a new mycoheterotrophic genus, Relictithismia Suetsugu & Tagane (Thismiaceae), with its monotypic species Relictithismia kimotsukiensis Suetsugu, Yas.Nakam. & Tagane from Kimotsuki Mountains in the Osumi Peninsula, Kagoshima Prefecture, Kyushu Island, southern Japan. Relictithismia resembles Haplothismia Airy Shaw in having a cluster of tuberous roots, a feature previously observed only in this genus within the family Thismiaceae. However, it differs in having solitary flowers (vs. 2-6-flowered pseudo-raceme in Haplothismia), anther thecae largely separated (vs. connate), and the presence of an annulus (vs. absent). Additionally, Relictithismia differs from the geographically overlapping genus Thismia Griff. in its stamen structure and the position of the annulus. In Relictithismia, the stamens lack connectives, and its free filaments arise from the annulus located inside the perianth mouth, while in Thismia, the stamens typically have connate connectives, forming a staminal tube pendulous from the annulus located at the mouth of the floral tube. Our morphological and phylogenetic data indicated that R. kimotsukiensis holds an early-diverging position within the family, situated outside the Old World Thismia clade. This paper offers an extensive description and color photographs of R. kimotsukiensis, complemented by notes on its phylogenetic relationship and evolutionary history.