A quick trip to our local shopping centre, necessitated by a looming draconian COVID lockdown, took me (DC) to one of those characterless carparks. Being a committed bryologist, my eyes averted to the pavement where I spotted the green thalli of Marchantia polymorpha, with its typical splash cups. We knew the species offers insights into early land plant evolution, that it serves as a ‘lab rat’ of systems biology and we had heard of its pharmacological and therapeutic potential.
History and nomenclature
The first indisputable illustrations of M. polymorpha date back to the middle of 15th Century (Bowman, 2016). Later, it featured in Kny’s wall chart (Figure 1) (Kny 1890), and it was probably from these that it made its way into standard botanical text books. Thus, it has become THE quintessential liverwort.
Figure 1 (a) and (b). Marchantia polymorpha with male antheridiophore (a), and (b) female archegoniophore (Kny, 1890) retrieved from https://www.marchantia.org/new-gallery. downloaded 13/01/2021
So, what does this Latin name tell us about M. polymorpha? The genus Marchantia was coined by Jean Marchant, in 1713 in honour of his father Nicholas; both were noted French botanists and mutationists (early non-Darwinian evolutionists). He pointed out that it was not a lichen, because it had ‘seeds’ and ‘stellate flowers’. Despite some dodgy morphology this was a significant advance.
Linnaeus (1753) codified the species name M. polymorpha, implying that it is polymorphic. Indeed, the three subspecies (Table 1) which are recognised in Europe confirm that, though not as Linnaeus had predicted.
Table 1. Summary of differences between the three subspecies of Marchantia polymorpha.
|subsp. polymorpha||subsp. montivagans||subsp. ruderalis|
|Thallus||Mid green, with strong black median line||Yellow green, no median line||Mid green, with interrupted black median line|
|Scale appendage||With undulate margin||Toothed||Toothed|
M. polymorpha is the type or ‘signature’ species for the plant Phylum Marchantiophyta, in common parlance ‘the liverworts’. Our plant does indeed have a thallose vegetative morphology that mimics the lobes of a liver. However, 90% of the Marchantiophyta are leafy, not thallose. Furthermore, the members of the more distantly related Phylum Anthocerotophyta (the Hornworts), are thallose. We therefore need to be careful how we use the term ‘Liverwort’. The informal term ‘bryophyte’ is a little confusing as it encompasses the Liverworts, Hornworts and Mosses; the third Phylum, Bryophyta, is now restricted to the Mosses.
The hornworts, the liverworts and the mosses all share a similar life cycle with alternation of generations. The dominant phase is the haploid gametophyte, while the diploid phase (the sporophyte), which arises from the union of the gametes, is tiny (barely 3mm long). The term ‘alternation of generations’ refers to the fact that both these generations are visually discrete.
Figure 2. The surface of the thallus of Marchantia polymorpha subsp. ruderalis showing splash cups containing the gemmae and the silvery pores to the air chambers. Photo: © Chris Jeffree
The splash cups (Figure 2) house several gemmae (vegetative propagules), identical in genotype to the parent plant. Gemmae are disc-shaped, constricted in the middle with two notches which bear the growing points (Figure 3). They are dispersed by raindrops, which splash them out of the cups onto surrounding ground. This method of reproduction has a striking similarity to that described in the recent bird’s nest fungus blog (https://botsocscot.wordpress.com/2020/12/06/honorary-plant-of-the-week-7th-december-birds-nest-fungus-cyathus-olla/). The gemma cups can be produced on quite young thalli and are more energy effective than are the paraphernalia required for spore generation. Gemmae are a good way to spread the genotype when there is plenty of ground to colonise. The extra cost of sexual reproduction is however worth it in a changing environment as it allows for adaptation to a different set of selection pressures.
Figure 3. A single gemma (0.45mm in diameter). Photo: Des Callaghan. Wikipedia Commons. Licence: CC BY-SA 2.0
Marchantia polymorpha is dioecious, so the male and female sexual structures are produced on separate plants. Both male and female thalli have haploid umbrella-like, stalked reproductive structures. The male structures, or antheridiophores (Figures 1a and 4) are disk-shaped with scalloped edges. Antheridia are borne on their upper surfaces, within which numerous male gametes or motile flagellate spermatozoids are formed. The female structures or archegoniophores (Figures 1b, 5-7) have nine finger-like projections. Under each of these an involucre is suspended which encloses several archegonia, within each of which a single egg is produced. Thus, the gametes that are borne on individual haploid thalli, are not produced meiotically; they are therefore identical to one another.
Spermatozoids are splashed by raindrops onto female plants. Water is essential for fertilisation, as the sperm cells swim through water films to the now open archegonial necks, attracted chemotactically (Brook, 1964). Following fusion (oogamy) the diploid zygote develops into the sporophyte. Figures 7 & 8 show the sporophytes hanging beneath the archegoniophores. Each sporophyte consists of a short seta, with a wedge-shaped foot that absorbs nutrients from the gametophyte. The seta supports a spherical capsule within which are diploid elaters and spore mother cells. The latter divide by meiosis give rise to genetically different haploid spores. The seta then increases in length, breaks through the calyptra, and the capsule dehisces (Figure 8); spore dispersal is aided by the hygroscopic movements of the spring-like elaters (Brook, 1964). The spores swell, become chlorophyllose, and germinate to form filamentous protonemata, from which new gametophytes develop.
Figure 4. Male reproductive structures (antheridiophores) in subsp. ruderalis. Photo: © Chris Jeffree
Figure 5. Female reproductive structures (archegoniophores) in subsp. ruderalis. Photo: © Chris Jeffree
Figure 6. Sporophytes surrounded by the involucres hanging down in subsp. ruderalis. Photo: © Chris Jeffree
Figure 7. Female receptacles with sporophytes hanging down. Marchantia polymorpha subsp. montevagans. Photo: © David G. Long.
Figure 8. Subsp. ruderalis – Archegoniophores with white involucres and yellow dehiscing capsules. Photo: © Chris Jeffree
The sex of the thalli in Marchantia is determined by heteromorphic sex chromosomes. These are reminiscent of the X and Y sex chromosomes as in Homo sapiens (Okada et al., 2001) but in more recent papers, the heteromorphic pair are referred to as ‘U’ & ‘V’. This draws attention to the unique properties of sex chromosomes in haploid systems, which are likely to have originated from mating-type chromosomes of volvocine algae (Coelho et al., 2018).
M. polymorpha is assigned to the complex liverworts in the Order Marchantiales; the term ‘complex’ reflects the layered structure of the thalli. A cross section (Figure 9) shows that the thallus is several cells thick and shows internal differentiation.
Figure 9. Cross section of the thallus of Marchantia polymorpha, showing air pore (po), chloroplasts (ch) filling the photosynthetic cells and some cells with oil bodies (ob). Diagram scanned from Paton, 1999 with the permission of the author.
The photosynthetic dorsal surface of the thallus is divided into diamond-shaped chambers, covered by a single layer of epidermal cells, each chamber communicating to the exterior by a multicellular, barrel-shaped pore. Figure 9 shows a section through one such chamber or ‘room’. The pore serves as a chimney, while the room is furnished with chloroplast-filled filaments. Below the photosynthetic layer are bulk-making parenchyma cells, some with oil bodies. Rhizoids and scales project from the central line of the ventral surface of the thallus. It is these that attach the thallus to the substratum.
Subsp. polymorpha, is a plant of marshes and waterside rocks. Subsp. montivagans (‘mountain wanderer’) occurs in wet, naturally disturbed places and dune slacks, also in montane flushes and streams. The Latin name for the third subspecies, subsp. ruderalis, tells us that it is a ruderal, in other words a weed. Subsp. ruderalis is the most familiar subspecies as it generally occupies dry lowland habitats, on nutrient-rich disturbed gravel, loam, or peaty soil, or between paving stones on pavements or over bricks, thus it is a colonist on ground that has been disturbed by human activity. It can completely cover the soil surface around potted nursery plants and is a common invader of burnt land. Torrey (1932) recorded M. polymorpha in forests denuded of vegetation following wildfires – the liverwort spreading in thick mats over thin soil over an acre or more. M. polymorpha has been found to be metal tolerant (Shaw and Goffinet, 2000), able to withstand lead concentrations up to 400 parts per million (p/m) and zinc concentrations up to 100 p/m (Matthews, 1993).
Mycorrhizal associates and the bacterial biome
A range of mycorrhizal symbiotic fungi are reported from a wide spectrum of liverworts (Rimington et al., 2018). The liverwort host demonstrates cytological changes as the mycorrhizae enter through the rhizoids, especially in the adjacent cell walls within the thallus, thereby restricting the areas that are infected (Ligrone et al., 2007). Marchantia polymorpha subsp. montivagans forms successful Glomeromycotean mycorrhizal associations whereas the other two subspecies do not. Although subsp. ruderalis initially reacts cytologically in the same way to the presence of the mycorrhizal symbiont, it subsequently kills it. Where there are host/mycorrhizal associations, the mycorrhizae benefit from a source of carbohydrates. When sources of minerals, particularly phosphorus, are limiting, the mycorrhizal partner is an effective scavenger from the substrate. When phosphates are not limiting, as on forest fire sites or nutrient-rich soils, a cost/benefit analysis may indicate that, for the host, the loss of photosynthate is simply not worth-while.
It has been suggested that mycorrhizae might have tranferred from the higher plants to the liverworts. However, recent research (Rimington et al., 2018) on a wide range of liverworts and hornworts (mycorrhizae are not associated with the mosses) has demonstrated that the ancient Glomeromycota are at least 10 times more frequent in these groups than they are in the vascular plants. The ancestors of this group of mycorrhizal fungi date back to the appearance of the liverworts. They therefore infer that the symbiosis is contemporaneous with the colonisation of the green plants on dry land.
The association of bacterial biomes with multicellular organisms has become a fashionable subject of research in recent years. Alcaraz et al. (2018) demonstrated a rich microbiome associated with M. polymorpha. Bacteria isolated included a Rhizobium, species of which are known to be the nitrogen fixers associated with root nodules in the legumes.
The model organism of synthetic biology
The unlimited supply of genetically identical propagules, ready for sampling and packaged neatly in cups, makes the species an ideal experimental model for comparative studies, evolutionary biology, molecular genetics and functional genomics. The baby plants (cutely known as gemmalings) grow well in laboratory media and the material is extremely easy to manipulate. Linde et al. (2020), who have now sequenced all three of its subspecies, found evidence of frequent hybridisation and introgression. This is of evolutionary significance particularly in the gametophyte where alleles transferred between genomes are immediately subject to selection. The 280 Mb genome, has approximately 20,000 loci (Marchantia.org., no date). Its use in systems biology is reviewed by Shimamura (2015).
There has been a long history of interest in M. polymorpha as the source of herbal remedies. Aristotle and Theophrastus, referenced it in ancient Greek herbal literature. The plant is supposed to resemble a liver so, applying the ‘doctrine of signatures’, it has been used to treat hepatic diseases both in Europe and China (Shuster, 1966). Jantwal et al., (2019) list a wide range of human pharmacological applications including: treatments for boils and abscesses, and as antipyretic, antibacterial, antifungal, diuretic, vaso-relaxant, muscle-relaxant, cardiotonic, antioxidant, antitrypanosomal and antiviral agents. Marchantins, classed as ethers, have been investigated as potential anticancer drugs: Marchantin A, induces apoptosis in human breast cancer cells (Huang et al., 2010), as does Marchantin M in prostate cancer cells (Jiang, 2013), while Marchantin C acts on lung cancer cells (Zhang et al., 2019). Although most of this research is conducted in China, there is a ready market for alternative anticancer remedies in the West.
M. polymorpha is the species that epitomises the liverworts, but we must realise that it belongs to an advanced group that evolved perhaps 50 million years after the liverworts split off from its sister phyla, the hornworts and the mosses. Its complex anatomy is in some respects reminiscent of the anatomy of leaves in the flowering plants. However, the air pores lack guard cells and cannot regulate the apertures. They are not comparable to stomata because the liverwort thallus is haploid, while in the flowering plants this haploid phase is reduced to obscurity, and the leaves are diploid. So, while air pores and stomata both optimise the opportunity for gaseous exchange and photosynthesis, they are analogous structures which have arisen independently. So where are the stomata in ‘the bryophytes’? According to Harris et al., (2020) the common ancestors of the bryophytes and the vascular plants possessed complex stomata, but these were lost in the liverworts. In the hornworts and mosses, functional stomata occur but in the diploid phase only, controlling water loss during the maturation of the capsules.
The progenitors of M. polymorpha, now extinct, date back to the emergence of green plants from fresh water onto the land that is thought to have occurred around 500 million years ago. This event is almost exactly mirrored in the animal kingdom; perhaps the occurrence of these early green plants made their emergence possible.
The complex structures of the thalli and the sex organs of M. polymorpha were almost certainly not expressed in these progenitors. However, the implications of a dominant haploid phase were shared. It is therefore possible to glean much useful information from our species that throws a light on that momentous colonisation of dry land.
When we were young it was easy to believe that complexity evolves linearly from the ‘primitive’ to the ‘advanced’; later we venerated Darwin’s tree of life metaphor, but what we have now learned about Marchantia lends support to Lewontin’s (2001) view, that evolution is more like an elaborate bit of macrame. A mind that is not curious is a dead mind. So, before you dismiss this weed with ‘familiarity breeds contempt’, think again!
David and Maria Chamberlain
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