Liver le jeune
Published 25 March 2023
Liverwort taxonomy is fraught with challenges. Their small size makes them difficult to study. Their relatively simple morphology makes for high rates of evolutionary convergence. Some significant features can only be observed under specific conditions, such as the structure of oil bodies that can only be studied in living specimens as they rapidly break down after death. Many of these challenges are embodied in one of the most diverse of recognized liverwort genera, Lejeunea.
Lejeunea includes leafy liverwort species found pretty much all over the world. The exact number is uncertain but is at least in the range of 200 to 300 (Lee et al. 2020). A lot of that uncertainty relates to uncertainty over the genus’ own circumscription: over a thousand species included in Lejeunea during the late 1800s were subsequently divided between up to sixty segregate genera, only to have many of those returned to the Lejeunea fold as the features they were based on proved unreliable (Heinrichs et al. 2013). Lejeunea species are mostly epiphytes and some are epiphyllous, growing on the living leaves of trees. Significant features of Lejeunea include leaf lobules with a hyaline papilla on the proximal side of the apical tooth, upright underleaf lobes with underleaves often bifid, a distinctive type of branching in which new branches subtend a lateral leaf, and small, granular or homogenous oil bodies that do not form ocelli (Lee 2013; Heinrichs et al. 2013; ocelli are distinctive dark spots in the thallus found in related genera). Species may be monoicous (with both male and female organs on a single gametophyte) or dioicous (with separate male and female plants). Vegetative propagation is reasonably rare but may be characteristic of particular species.
As already suggested, subdivision of such a diverse genus has not gone smoothly. Recent molecular studes have indicated the existence of two main clades that have been labelled as subgenera Lejeunea and Crossotolejeunea (Lee et al. 2020). However, these clade can only be recognised genetically; no morphological characters have yet been identified to distinguish them. In some cases, specimens thought to belong to a single species on morphological grounds have been placed by molecular data in quite distinct clades of the genus. Conversely, some collections thought distinct have had to be synonymised (Lee et al. 2016). The majority of species appear localised in distribution but there are notable exceptions. For instance, Lee et al. (2016) identified L pulchriflora over a range extending from the African tropics to Fiji.
Molecular data have also been used to infer an Eocene origin for the Lejeunea crown group, seemingly in the Neotropics (Lee et al. 2020). The genus then spread into the Old World in the late Oligocene or early Miocene, perhaps in connection with the late Oligocene warming period. Lee et al. (2020) connected most of the genus’ spread to terrestrial land bridges, rather than long-distance dispersal between independent continents. There are notable counter-examples, though. An African clade was found by Lee et al. to have direct Neotropical connections. Heinrichs et al. (2013) identified connections between the North Atlantic Macaronesian islands and North America, between Gough Island and South America, and between Easter Island and tropical Asia.
A strong correlation also exists between phylogeny and sexual system. The original Lejeunea appears to have been dioicous with monoicy evolving at the base of a number of clades, particularly in the subgenus Lejeunea (Heinrichs et al. 2013). It has been suggested that monoicy in liverworts may be correlated with marginal habitats. Successful fertilisation in liverworts (as with other spore-producing plants) requires the presence of moisture to allow the motile sperm to reach an egg. For monoicous individuals, the greater chance of spore formation that comes from allowing self-fertilisation may compensate for the potential reduced genetic diversity. Needs must, after all.
Heinrichs, J., S. Dong, A. Schäfer-Verwimp, T. Pócs, K. Feldberg, A. Czumaj, A. R. Schmidt, J. Reitner, M. A. M. Renner, J. Hentschel, M. Stech & H. Schneider. 2013. Molecular phylogeny of the leafy liverwort Lejeunea (Porellales): evidence for a Neotropical origin, uneven distribution of sexual systems and insufficient taxonomy. PLoS One 8 (12): e82547.
Lee, G. E. 2013. A systematic revision of the genus Lejeunea Lib. (Marchantiophyta: Lejeuneaceae) in Malaysia. Cryptogamie Bryologie 34 (4): 381–484.
Lee, G. E., J. Bechteler, T. Pócs, A. Schäfer-Verwimp & J. Heinrichs. 2016. Molecular and morphological evidence for an intercontinental range of the liverwort Lejeunea pulchriflora (Marchantiophyta: Lejeuneaceae). Organisms, Diversity & Evolution 16: 13–21.
Lee, G. E., F. L. Condamine, J. Bechteler, O. A. Pérez-Escobar, A. Scheben, A. Schäfer-Verwimp, T. Pócs & J. Heinrichs. 2020. An ancient tropical origin, dispersals via land bridges and Miocene diversification explain the subcosmopolitan disjunctions of the liverwort genus Lejeunea. Scientific Reports 10: 14123.