Floral structure inThismia(Thismiaceae: Dioscoreales): new insights from anatomy, vasculature and development

Abstract

Thismia is characterized by an exceptionally complicated floral morphology that is currently not understood properly. In the taxonomic literature, descriptive rather than morphological terms are often applied to parts of the flower in Thismia, relating to the general appearance of the floral organs instead of their precise homologies. Precise understanding of the floral structure is complicated by the rarity of Thismia spp. and the paucity of appropriate material. Here we provide a comprehensive study of reproductive organs of three Thismia spp. (T. annamensis, T. javanica and T. mucronata) including the first investigation of inflorescence architecture and early floral development in Thismiaceae. We found a hitherto unknown diversity of the reproductive shoots in the genus, manifested in the number of floral prophylls (two or three, in contrast to a single prophyll in the vast majority of monocots) and in the branching plane resulting in two distinct inflorescence types, a drepanium and a bostryx. We report the non-acropetal sequence of initiation of floral whorls (with stamens being the last elements to initiate), never previously described in monocots, and the gynoecium composed of completely plicate carpels, also a rare feature for monocots. Floral vasculature is relatively uniform in Thismia, but significant interspecific differences are found in tepal innervation, including the number of tepal traces; some of these differences are not immediately related to the external tepal morphology. We argue that the annulus, which acts as a roof of the hypanthium, possesses an androecium nature and represents congenitally fused bases of stamen filaments. We describe the stamens as laminar structures, which are also shortly tubular in the distal part of the supraconnective with the adaxial tubular side forming a skirt-like appendage. Finally, the placentas, which are column-like when mature, are initially parietal, becoming secondarily similar to free-central placentas through schizogenous separation from the ovary wall.

Thismia belumensis (Thismiaceae), a remarkable new species from The Royal Belum State Park, Gerik, Perak, Peninsular Malaysia

“…Morphologically, T. belumensis is strikingly different from other species by its annulus structure, tepals with an appendage, and floral colours. Nuraliev et al (2020) , describe the annulus as a large, fleshy ring-like structure. However, T. belumensis possesses an unusual form of annulus.…”

“… Thismia Griff. ( Thismiaceae ) is a genus of small mycoheterotrophic herbs, currently comprising approximately 80–90 species ( Dančák et al 2020a ; Nuraliev et al 2020 ; Shepeleva et al 2020 ). Plants of this genus are primarily distributed in the tropical regions of Asia, Australia, and South America, and extend into the subtropical and temperate regions of Japan, New Zealand, Australia, and the USA ( Merckx et al 2013 ; Dančák et al 2020b ).…”

Comparative Analysis of Plastid Genomes in the Non-photosynthetic Genus Thismia Reveals Ongoing Gene Set Reduction

“…This is an exclusively heterotrophic genus (the family is also fully heterotrophic) comprising about 90 species that are widely distributed in tropical and subtropical regions of Asia, Australasia, and America, but extending to some temperate areas. Representatives of Thismia are tiny herbs, remarkable for their unusual appearance, curious flower morphology and highly specialized pollination biology ( Merckx, 2013 ; Guo et al, 2019 ; Dančák et al, 2020a ; Shepeleva et al, 2020 ; Nuraliev et al, 2021 ). Thismiaceae may represent one of the most ancient mycoheterotrophic groups of plants: according to Merckx et al (2010) , they appear to have originated in the late Cretaceous, although Givnish et al (2018) infer a much more recent origin.…”

Thismia sitimeriamiae (Thismiaceae), an extraordinary new species from Terengganu, Peninsular Malaysia

“…Since the most comprehensive taxonomic revision of the genus Thismia in the 1930s and 40s (Jonker 1938(Jonker , 1948, the number of species described in this genus has proliferated. Recent species discoveries and investigations of the floral anatomy and evolution of the genus have brought an astonishing diversity of floral structure to our attention (Shepeleva et al 2020;Nuraliev et al 2021). The extraordinary morphology and extreme scarcity of Thismia sitimeriamiae raise interesting questions about the ecology, evolution and conservation of this and other species in this exceptional genus.…”

“…Shepeleva et al (2020) recommended a re-evaluation of the floral anatomy across the genus. The most detailed analysis of the floral architecture, anatomy and development of Thismia was carried out by Nuraliev et al (2021). This work has identified an exceptional array of reproductive structures.…”