Radula is a genus of liverwort in the family Radulaceae. The genus includes 248 species of small plants that typically grow as green, scaly patches on tree trunks, logs, or rocks in moist environments. It is distinguished from other liverworts by several unique features, including the production of root-like structures (rhizoids) exclusively from leaf surfaces and characteristic branching patterns. The plants have rounded, overlapping leaves consisting of two unequal lobes, with considerable variation in structure across species. Following a major taxonomic revision in 2022, the genus comprises five subgenera with distributions ranging from tropical to temperate regions. The oldest known fossil species, R.cretacea, found in Burmese amber, dates to the Cenomanian age, though molecular evidence suggests the genus originated in the Triassic period, around 228 million years ago.
The genus Radula was historically subject to different taxonomic interpretations. In 1936,[3] Hepstead Castle published a broad interpretation of the genus, including all species with perianths on short branches in the subgenus Cladoradula, which made the subgenus nearly worldwide in distribution. However, in the late 1970s and early 1980s, several bryologists including Eustace Wilkinson Jones (1977),[4] Kohsaku Yamada (1979),[5] and Rudolf Mathias Schuster (1980)[6] challenged this broad interpretation. These researchers advocated for a return to Richard Spruce's original narrower concept from 1885,[7] which was supported by detailed morphological characteristics including stem anatomy and patterns of leaf insertion.[8]
The classification of Radula species has been challenging historically due to the genus's high diversity combined with relatively simple morphology. The first infrageneric classification was published by Franz Stephani in 1884, dividing the then-known 92 species into 12 sections based on readily observable characteristics like leaf shape and growth habits. Almost simultaneously, Richard Spruce proposed dividing the genus into just two subgenera based on reproductive features. Stephani later revised his treatment in 1910 to include seven sections for 220 species, but did not incorporate Spruce's subgeneric divisions. Castle's 1936 work expanded on Spruce's system, placing species into either subgenus Cladoradula (15 species) or Acroradula (197 species), with the latter further divided into 11 sections. This classification was formally described by Riclef Grolle in 1970 but was criticised by Jones (1977) as being artificial and potentially misleading regarding relationships between species.[9]
In 2022, molecular phylogenetics studies led to a major taxonomic revision of Radulaceae. Two former subgenera of Radula were elevated to genus rank based on their ancient divergence times and distinct morphological characteristics. The subgenus Cladoradula, comprising seven species, was found to have diverged during the late Permian period about 263 million years ago, making it one of the oldest lineages within the family. These species, including the former R.boryana and R.perrottetii, were transferred to the new genus Cladoradula. Similarly, the monospecific subgenus Dactyloradula was also elevated to genus rank, with R.brunnea becoming Dactyloradula brunnea. Both new genera are distinguished from Radula by having a subepidermis and transverse rather than longitudinal lobule insertion. The remaining five subgenera continue to be recognised within Radula:[8]
Amentuloradula: Found primarily in Asia, Australasia, and Oceania. Many species produce distinctive amentulose (reduced-leaf) branches.
Metaradula: Primarily tropical in distribution but extends into southern temperate regions.
Odontoradula: Mostly confined to Asia, Australasia, and Oceania. Some species are characterised by acute leaf lobes and the presence of two pairs of female bracts.
Radula: Has a subcosmopolitan range.
Volutoradula: Largely limited to tropical regions but extends into southern temperate areas.
The genus shows considerable morphological and anatomical diversity across these subgenera, including variations in lobule shape, stem anatomy, oil-body morphology, shoot architecture, and reproductive structures.[8]
Description
Radula species are leafy liverworts that typically appear as a scaly, green surface on tree trunks, logs, or rocks in sheltered, moist outdoor environments. The plants have several distinctive characteristics that set them apart from other liverworts. Unlike most related genera, they lack small leaves (underleaves) on their lower surface. Instead, they produce root-like structures (rhizoids) exclusively from their leaf surfaces, a feature unique to this genus.[8]
The leaves are rounded and overlapping, consisting of two unequal lobes . The smaller lobe (lobule) is folded beneath the larger one, and the shape of these lobules varies considerably among species. Each leaf cell typically contains one or two large, brown oil bodies that nearly fill the cell.[8] The plants show a characteristic growth pattern called Radula-type branching, only producing a different type of branch (Lejeunea-type) when the growing tip is damaged. The overall architecture of the plants varies considerably among species, from simple unbranched shoots to complex, regularly branched forms. Stem structure also varies significantly between species, particularly in the arrangement and characteristics of their internal cells. When reproducing, they produce flattened protective structures (perianths) around their reproductive organs, which can vary from short to long depending on the species. The spore-producing structures (sporophytes) show considerable diversity: their capsules vary in size and shape, can split open in either straight or spiral patterns, have different types of wall thickening, and produce varying numbers of spores. The spores themselves also differ among species in size and surface patterns. This high degree of morphological variation across the genus is unusual among liverworts.[8][10]
Reproduction and evolution
Studies of Radula species have revealed patterns in how these liverworts reproduce and adapt over time. While most species reproduce with separate male and female plants (a condition called dioecy), some species have evolved to have both male and female reproductive organs on the same plant (called monoecy). This shift to combined sexes appears to have happened multiple times independently in different Radula species, suggesting it provided evolutionary advantages.[11]
Species that grow on various surfaces (facultative epiphytes) were more likely to evolve combined sexes compared to species that grow only on trees or leaves (obligate epiphytes). This may be because species growing exclusively on trees face unique challenges – they need to spread to new trees but also establish themselves successfully in an environment where moisture levels can change rapidly. These tree-dwelling species tend to reproduce by breaking off fragments of their plant body rather than producing specialised reproductive structures called gemmae. This strategy allows them to skip the vulnerable early growth stage that gemmae must go through, though it limits how far they can spread.[11]
The genus originated in the late Jurassic period, with most existing species emerging between the Cretaceous and Eocene periods. While some groups within Radula are found worldwide, others are restricted to specific regions – some groups are mostly found in Australasia, others in Central and South America, and others in tropical regions of the Old World. The current distribution of some Radula groups appears to have been shaped by the ancient breakup of the supercontinent Gondwana, as evidenced by the timing of their evolutionary splits coinciding with continental separation events. This is particularly noteworthy because liverworts typically produce tiny spores that can be carried long distances by wind, which usually erases such ancient geographic patterns. The restricted ranges of some Radula groups appear to be due to their environmental preferences being conserved over evolutionary time, rather than an inability to disperse to new areas.[12]
Three species have been identified from Baltic amber and Bitterfeld amber deposits: R.baltica, R.oblongifolia, and R.sphaerocarpoides. Baltic amber specimens date to the Eocene period (around 35–47 million years ago), while the age of Bitterfeld amber remains debated – it may be either Eocene or late Oligocene (24–25 million years ago) in age. R.oblongifolia and R.sphaerocarpoides are found in both amber deposits, suggesting these morphological forms persisted for several million years, though they may represent multiple biological species that appear similar.[15] In 2024, two additional species, R.oblongifolia and R.tikhomirovae, were reported from Rovno amber, dating back to the late Eocene.[16] The first Radula fossil ever described was R.sphaerocarpoides, identified in 1853 from Baltic amber.[15]
Another fossil species, R.steerei, is known from Dominican amber of Miocene age. Radula is the second most diverse liverwort genus found in Baltic and Bitterfeld amber, after Frullania which has nine species. However, since these amber deposits formed from conifer resin, the fossils may only represent species that grew on these trees, while other Radula species that lived primarily on other plant hosts may not have been preserved.[15]
Chemistry
Genus Radula is chemically distinct from other liverworts because it produces unique compounds called bibenzyls and their derivatives. These are organic molecules made up of two connected benzene rings with various chemical modifications. The most common compounds found in Radula species are 3,5-dihydroxy-2-(3-methyl-2-butenyl)bibenzyl and 2-geranyl-3,5-dihydroxybibenzyl, which often form the basic structure for more complex molecules in these plants.[17]
Of particular interest is the presence of compounds similar to those found in cannabis (cannabinoids) in some Radula species. For example, Radula marginata produces perrottetinene, a compound that has psychoactive properties similar to THC (tetrahydrocannabinol, the main psychoactive component in cannabis). Research has shown that perrottetinene can cross the blood-brain barrier and cause effects like lowered body temperature and reduced movement in laboratory studies.[17]
Many compounds isolated from Radula species show various biological activities. These include antimicrobial effects against various bacteria and fungi, cytotoxic (cell-killing) effects against certain cancer cell lines, antioxidant properties, anti-inflammatory effects, and activities affecting blood vessels. Some of these compounds have shown potential medical applications, though research is still in early stages.[17]
While most liverworts produce many terpenoids (a class of aromatic compounds common in plants), Radula species generally contain relatively few, with some exceptions among Portuguese species that are rich in these compounds. Many Radula species also contain α-tocopherol (a form of vitamin E), which may help protect their other chemical compounds from oxidation. The presence of α-tocopherol is particularly important because many of the other compounds in these plants are unstable and susceptible to breaking down when exposed to air.[17]
Rhododendron is a very large genus of about 1,024 species of woody plants and in the heath family (Ericaceae). They can be either evergreen or deciduous. Most species are native to eastern Asia and the Himalayan region, but smaller numbers occur elsewhere in Asia, and in North America, Europe and Australia.
Phalaenopsis, also known as moth orchids, is a genus of about seventy species of plants in the family Orchidaceae. Orchids in this genus are monopodial epiphytes or lithophytes with long, coarse roots, short, leafy stems and long-lasting, flat flowers arranged in a flowering stem that often branches near the end. Orchids in this genus are native to Asia, New Guinea, and Australia, but mostly occur in Indonesia and the Philippines.
Metzgeriales is an order of liverworts. The group is sometimes called the simple thalloid liverworts: "thalloid" because the members lack structures resembling stems or leaves, and "simple" because their tissues are thin and relatively undifferentiated. All species in the order have a small gametophyte stage and a smaller, relatively short-lived, spore-bearing stage. Although these plants are almost entirely restricted to regions with high humidity or readily available moisture, the group as a whole is widely distributed, and occurs on every continent except Antarctica.
Monoicy is a sexual system in haploid plants where both sperm and eggs are produced on the same gametophyte, in contrast with dioicy, where each gametophyte produces only sperm or eggs but never both. Both monoicous and dioicous gametophytes produce gametes in gametangia by mitosis rather than meiosis, so that sperm and eggs are genetically identical with their parent gametophyte.
Fulfordianthus evansii is a species of liverwort in the family Lejeuneaceae. It is found in Belize, Costa Rica, Guatemala, and Panama. Its natural habitat is subtropical or tropical moist lowland forests.
Radula jonesii is a species of liverwort in the family Radulaceae, first described in 1988 from specimens collected in the Anaga Mountains of Tenerife. Endemic to Macaronesia, this dark to olive-green liverwort is known from five locations on Madeira Island and one location on Tenerife, where it grows as both an epiphyte on trees such as Laurus novocanariensis and as a lithophyte on shaded rocks in old growth laurel forest ecosystems between 800–1,000 m (2,600–3,300 ft) altitude. The species is distinguished by its procumbent growth habit, pinnately branched stems measuring 10–15 mm (0.4–0.6 in) in length, and distinctive cellular features including uniformly thin-walled leaf cells. Classified as Endangered due to its restricted range and small population size, R. jonesii faces threats from climate change, habitat degradation, and tourism pressure, though its habitat receives protection through various conservation designations including UNESCO World Heritage status.
Radula visianica is a species of liverwort in the family Radulaceae. It is endemic to the European Alps. It was thought to be extinct since 1938 but was rediscovered in 2014 in Austria.
Perrottetinene is a naturally occurring cannabinoid compound found in liverworts from the family Radulaceae native to Japan, New Zealand and Costa Rica, namely Cladoradula perrottetii, Radula marginata and Radula laxiramea, along with a number of similar compounds. Its chemical structure closely resembles that of THC, the main active component of marijuana but with a cis rather than trans conformation and a bibenzyl tailchain instead of pentyl. The absolute configuration of perrottetinene was established in 2008 by an enantioselective total synthesis.
Pleurozia is the only genus of liverworts in the family Pleuroziaceae, which is now classified in its own order Pleuroziales, but was previously included in a broader circumscription of the Jungermanniales. The genus includes twelve species, and as a whole is both physically distinctive and widely distributed.
Ptilidium is a genus of liverwort, and is the only genus in family Ptilidiaceae. It includes only three species: Ptilidium californicum, Ptilidium ciliare, and Ptilidium pulcherrimum. The genus is distributed throughout the arctic and subarctic, with disjunct populations in New Zealand and Tierra del Fuego. Molecular analysis suggests that the genus has few close relatives and diverged from other leafy liverworts early in their evolution.
Lejeunea hodgsoniana is a species of liverwort in the family Lejeuneaceae. Endemic to New Zealand, it was first recognized in 1980 but not formally described until 2013. The plant forms bright green mats up to 7 centimetres in diameter on tree bark and occasionally on rocks. The species is found from the Kermadec Islands in the north to the Chatham Islands in the south, primarily in coastal and lowland areas below 100 metres elevation. It is distinguished from related species by its relatively large size, multi-celled tooth on the leaf lobule, and deeply divided underleaves with pointed tips. While showing a particular affinity for mahoe trees, it grows on various native and introduced trees and is considered "Not Threatened" under the New Zealand Threat Classification System due to its abundance within its range and ability to grow in both pristine and disturbed habitats.
Radula demissa is a species of liverwort in the family Radulaceae. It occurs in southeastern Australia and New Zealand, where it grows as an epiphyte in temperate rainforest environments.
Cladoradula perrottetii is a species of liverwort is the family Radulaceae. It is distributed across tropical and subtropical Southeast Asia, occurring in Thailand, Sumatra, Taiwan and Japan. It contains the small molecule perrottetinene, a cannabinoid, and other secondary metabolites of scientific interest including marchantin A.
Nowellia curvifolia is a species of liverwort belonging to the family Cephaloziaceae. It is a small, delicate plant that forms prostrate growths on rotting logs in forests throughout the northern temperate and subarctic regions. The species is distinguished by its distinctive billowed leaves arranged in two rows, each divided into two pointed lobes with the lower lobe forming a specialised water-holding pocket called a water-sac. The plant shows seasonal colour variation from rose to purple to brown to spring green, developing reddish-purple pigmentation in autumn and winter. It reproduces both sexually, through spores produced in reddish-brown capsules, and asexually via single-celled gemmae. N. curvifolia serves as an indicator species in forest ecosystems, particularly in montane spruce-fir forests, where it often forms pure mats on decorticated logs and is associated with the slime mould species Barbeyella minutissima.
Porella platyphylla is a species of liverwort belonging to the family Porellaceae. It has a Holarctic distribution, occurring across Eurasia and North America, where it typically grows on tree bark and rocks in areas with adequate rainfall. The species is most common in regions receiving at least 600 millimetres of annual precipitation. The species forms part of a complex taxonomic group that includes several closely related species and hybrids, with populations showing distinct genetic differences between continents despite their morphological similarity.
Goebeliellaceae is a family of liverworts belonging to the order Porellales. The family consists of only one genus: GoebeliellaSteph..
Frullania wairua, the spirit liverwort or radar bush liverwort, is a species of liverwort in the order Porellales. It is one of 24 species in the large genus Frullania that are native to New Zealand. The species was first described by Matt von Konrat and John E. Braggins in 2005 in the New Zealand Journal of Botany.
Cladoradula is a genus of liverworts in the family Radulaceae. Distinguished by its thick, brown-pigmented stems and distinctive branching pattern, it comprises seven species found primarily in tropical and temperate forest regions worldwide. Originally established as a subgenus of Radula in 1885, it was elevated to genus rank in 2022 following molecular studies that revealed it represents one of the oldest lineages within Radulaceae, having diverged during the late Permian period about 263 million years ago. The genus is characterised by its specialised stem structure, distinctive leaf arrangement, and small protective structures around its reproductive organs. Species in the genus grow on tree bark or shaded rocks from sea level to over 2,000 m (6,600 ft) in elevation.
↑ Castle, H. (1936). "A revision of the genus Radula. Introduction and part I. Subgenus Cladoradula". Annales Bryologici. 9: 13–56.
↑ Jones, Eustace W. (1977). "African Hepatics. XXX: The genus Radula Dumortier". Journal of Bryology. 9 (4): 461–504. doi:10.1179/jbr.1977.9.4.461.
↑ Yamada, K. (1979). "A revision of Asian taxa of Radula, Hepaticae". Journal of the Hattori Botanical Laboratory. 45: 201–322.
↑ Schuster, R.M. (1980). "Phylogenetic studies on Jungermanniidae II. Radulineae (Part I)". Nova Hedwigia. 32: 637–693.
↑ Spruce, R. (1885). "Hepaticae amazonicae et andinae. II". Transactions and Proceedings of the Botanical Society of Edinburgh (in Latin). 15: 309–588 [315].
1 2 3 4 5 6 Renner, Matthew A.M.; Gradstein, S. Robbert; Ilkiu-Borges, Anna Luiza; Oliveira-Da-Silva, Fúvio R.; Promma, Chatchaba (2022). "Molecular and morphological evidence support the recognition of three genera within Radulaceae (Porellales: Marchantiophyta)". Bryophyte Diversity and Evolution. 45 (1): 95–118. doi:10.11646/bde.45.1.7.
1 2 3 Heinrichs, Jochen; Schmidt, Alexander R.; Schäfer-Verwimp, Alfons; Bauerschmidt, Lisa; Neumann, Christian; Gröhn, Carsten; Krings, Michael; Renner, Matthew A.M. (2016). "Revision of the leafy liverwort genus Radula (Porellales, Jungermanniopsida) in Baltic and Bitterfeld amber". Review of Palaeobotany and Palynology. 235: 157–164. doi:10.1016/j.revpalbo.2016.09.004.
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