| Radula | |
|---|---|
 | |
| Radula complanata | |
Scientific classification | |
| Kingdom: | Plantae |
| Division: | Marchantiophyta |
| Class: | Jungermanniopsida |
| Order: | Radulales R.M.Schust. |
| Family: | Radulaceae Müll.Frib. [de][2] |
| Genus: | Radula Dumort.[1] |
| Type species | |
| Radula complanata (L.) Dumort. | |
| Synonyms[1] | |
Radula is agenus ofliverwort in the familyRadulaceae. 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 fivesubgenera with distributions ranging fromtropical totemperate regions. The oldest known fossil species,R. cretacea, found inBurmese amber, dates to theCenomanian age, though molecular evidence suggests the genus originated in theTriassic period, around 228 million years ago.
The genusRadula was historically subject to differenttaxonomic interpretations. In 1936,[3] Hepstead Castle published a broad interpretation of the genus, including all species withperianths on short branches in the subgenusCladoradula, which made the subgenus nearly worldwide in distribution. However, in the late 1970s and early 1980s, several bryologists includingEustace Wilkinson Jones (1977),[4] Kohsaku Yamada (1979),[5] and Rudolf Mathias Schuster (1980)[6] challenged this broad interpretation. These researchers advocated for a return toRichard 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 ofRadula species has been challenging historically due to the genus's high diversity combined with relatively simple morphology. The first infrageneric classification was published byFranz Stephani in 1884, dividing the then-known 92 species into 12sections 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 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 subgenusCladoradula (15 species) orAcroradula (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 ofRadulaceae. Two formersubgenera ofRadula were elevated to genus rank based on their ancient divergence times and distinctmorphological characteristics. The subgenusCladoradula, comprising seven species, was found to have diverged during the latePermian period about 263 million years ago, making it one of the oldestlineages within the family. These species, including the formerR. boryana andR. perrottetii, were transferred to the new genusCladoradula. Similarly, themonospecific subgenusDactyloradula was elevated to genus rank, withR. brunnea becomingDactyloradula brunnea. Both new genera are distinguished fromRadula by having a subepidermis and transverse rather than longitudinal lobule insertion. The remaining five subgenera continue to be recognised withinRadula:[8]
The genus shows considerable morphological and anatomical diversity across these subgenera, including variations inlobule shape,stem anatomy, oil-body morphology,shoot architecture, and reproductive structures.[8]
Radula species are leafyliverworts 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 unequallobes. 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 calledRadula-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: theircapsules 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]
Studies ofRadula 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 calleddioecy), some species have evolved to have both male and female reproductive organs on the same plant (calledmonoecy). This shift to combined sexes appears to have happened multiple times independently in differentRadula species, suggesting it provided evolutionary advantages.[11]
Species that grow on various surfaces (facultativeepiphytes) 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 calledgemmae. 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 lateJurassic period, with most existing species emerging between theCretaceous andEocene periods. While some groups withinRadula 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 theOld World. The current distribution of someRadula groups appears to have been shaped by the ancient breakup of the supercontinentGondwana, 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 someRadula groups appear to be due to their environmental preferences being conserved over evolutionary time, rather than an inability to disperse to new areas.[12]
Several species of Radula are known from fossilized treeresin (amber). The oldest fossil species isRadula cretacea from theCenomanian agedBurmese amber of Myanmar, belonging tosubgenusOdontoradula. Molecular evidence suggests that the genus arose during theTriassic, around 227.8 Ma, and thecrown group began to diversify during theEarly Jurassic, around 176.3 Ma.[13] As of November 2024[update],World Flora Online listsRadula cretacea asunplaced.[14]
Three species have been identified fromBaltic amber andBitterfeld amber deposits:R. baltica,R. oblongifolia, andR. sphaerocarpoides. Baltic amber specimens date to theEocene period (around 35–47 million years ago), while the age of Bitterfeld amber remains debated – it may be either Eocene or lateOligocene (24–25 million years ago) in age.R. oblongifolia andR. 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 andR. tikhomirovae, were reported fromRovno amber, dating back to the late Eocene.[16] The firstRadula fossil ever described wasR. sphaerocarpoides, identified in 1853 from Baltic amber.[15]
Another fossil species,R. steerei, is known fromDominican amber ofMiocene age.Radula is the second most diverse liverwort genus found in Baltic and Bitterfeld amber, afterFrullania 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 otherRadula species that lived primarily on other plant hosts may not have been preserved.[15]
GenusRadula is chemically distinct from other liverworts because it produces unique compounds calledbibenzyls and theirderivatives. These are organic molecules made up of two connectedbenzene rings with various chemical modifications. The most common compounds found inRadula 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 incannabis (cannabinoids) in someRadula species. For example,Radula marginata producesperrottetinene, a compound that haspsychoactive properties similar to THC (tetrahydrocannabinol, the main psychoactive component in cannabis). Research has shown that perrottetinene can cross theblood-brain barrier and cause effects like lowered body temperature and reduced movement in laboratory studies.[17]
Many compounds isolated fromRadula species show variousbiological activities. These includeantimicrobial effects against various bacteria and fungi,cytotoxic (cell-killing) effects against certaincancer 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 manyterpenoids (a class ofaromatic compounds common in plants),Radula species generally contain relatively few, with some exceptions among Portuguese species that are rich in these compounds. ManyRadula species also containα-tocopherol (a form ofvitamin E), which may help protect their other chemical compounds fromoxidation. 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]
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As of November 2024[update], theCatalogue of Life accepts 248 species, 2subspecies, and 12varieties inRadula.[18] SeeList ofRadula species, for the full list. Selected species are as follows.[1]
Data related toRadula at Wikispecies
Media related toRadula (plant) at Wikimedia Commons