Marchantiophyta belongs to bryophytes which is a group of nonvascular plants with approximately twenty-four thousand species. They are placed between algae and pteridophytes. This also includes two other divisions, Bryophyta and Anthocerotophyta. The members usually grow on surfaces that are mostly like wet soil or rock, tree trunks, rivers, and lakes.
Marchantia is a genus of non-vascular land plants commonly included in bryophytes. Technically, they come under the family of Marchantiophyta which again comes under the clade of Embryophyta under the Kingdom Plantae. The organisms falling under the Marchantia classification are commonly known as liverworts.
Marchantia polymorpha is a common species of liverwort that is found on all continents except Antarctica. Marchantia and the related liverworts have been used as a model in biology since the 19th century.
They were instrumental in the discovery of heterochromatin, plant sex chromosomes, and more. For the past ten years, a whole suite of know-hows and protocols have been adapted to work with Marchantia, which has again made this species quite a model system for biological research. In addition, Marchantia is an extant representative of an ancient lineage of land plants that had colonized our planet more than four hundred million years ago.
Studying it has the potential to enlighten us about the origin of the physiological, cellular, and developmental adaptations that have enabled the conquest of the land. Marchantia spends most of its life in a kind of haploid life form. The mature vegetative plant has a thalloid body plan which consists of many differentiated cell types. During the vegetative process, haploid development plants can be asexually propagated through gemmae.
Sexual reproduction is easily induced in them by a change in light quality. The male and female plants differentiate distinguishable sexual organs which produce motile sperm and a retained egg respectively. Fertilization leads to the production of a dependent short-lived multicellular diploid life form in which meiosis produces haploid spores. Spores are long-lived and they can germinate in axenic conditions, re-initiating a new life cycle every single time. The small genome size is partly explained by the absence of genome duplication.
Most regulatory gene families consist of those bare minimal components, which is advantageous for further studies. Marchantia is amenable to homologous recombination with high efficiency. Specific fluorescent proteins and selectable markers have also been engineered.
Transgenic haploid lines can be produced in four to six weeks. Several aspects of Marchantia make this species very suitable for cell biology studies, for some transparent cell types and the availability of different fluorescent markers. As some cells acquire distinct polarity or motility, Marchantia is also a good model for the study of different cytoskeleton-related processes.
The spores are ideal for preserving and storing the genetic material in liquid nitrogen. Asexual reproduction is through clonally produced gemmae and it is commonly used to maintain lines. The cost of culture maintenance is always lower than that of flowering plants and most animal models.
In addition, the easily controllable transition from vegetative to sexual reproduction enables genetic studies. Marchantia is prone to mutagenesis. There is ample genetic diversity for genetic mapping.
The Marchantia genome is relatively small and it contains about an estimated number of twenty thousand protein-coding genes. The genome will soon be available for a diverse range of tissue types. Most importantly, the relatively small genome size is partly explained by the absence of genome duplication.
Most regulatory gene families consist of the bare minimum components. It is quite evident for you now that as they are non-vascular they lack appropriate differentiation into leaves, stems, and true roots. Due to that distinction, Marchantia has a plant body which is called a thallus, a common occurrence in plants lacking any vascular system such as algae and fungi. But the characteristics that make the Marchantia genus distinguishable are the small barrel-shaped pores for gaseous exchanges and cup-shaped structures called gemma cups for reproduction. As you read through, you will learn more about these structures.
There are different characteristics that define various qualities and stages of the life of Marchantia classification. We will look at them one by one.
Structure of Marchantia
Let's start with structural characteristics. Already from above, you know that the plant body of the liverworts is thalloid. Even though it is flat, the thalloid body in this scenario is dorsiventral i.e. it has different upper and lower surfaces. The epidermal layer forms the dorsal or the upper surface while the ventral surface is composed of multicellular purple-colored scales of single-cell thickness and unicellular rhizoids. Apart from that structurally, this thalloid can be divided into two layers - (1) the Photosynthetic part and (2) the Storage part.
Photosynthetic Part: It is the dorsal part i.e. the surface part of the Marchantia thalloid. It consists of the openings of the barrel-shaped pores for gaseous exchange and has a well-defined epidermis. It also contains cup-shaped gemma cups.
Storage Part: It is the lower part of the thalloid. It consists of the chambers present in the pores that are mentioned above.
The given Marchantia diagram depicts its structural qualities.
As mentioned above, there are cup-shaped structures called gemma cups that are present on the thallus of Marchantia which are useful in the process of reproduction. But those structures are for the asexual mode of reproduction. An important feature of Marchantia reproduction is that it can be both asexual as well as sexual.
Asexual Reproduction in Marchantia
The asexual reproduction in the liverworts is carried out by the gamma cups. These gamma cups are present on the upper side of the dorsal side of the thalli. They contain gemmae which can be a single cell or a group of cells that develop into a new individual after detaching from the parent. In this case, when the rainwater fills the gamma cups, it spills out the gemmae, and these gemmae when spread into the nearby region develop into a new independent plant.
This kind of Marchantia reproduction is classified under fragmentation asexual reproduction occurring via the gemmae. You should also know that the asexual reproduction in Marchantia also occurs when older parts of the plant die and the newer ones that survive develop into a new identical organism.
Sexual Reproduction in Marchantia
Even though Marchantia reproduction can occur via asexual and sexual mode, you should know that the Marchantia is sexually dioecious i.e. the male and female reproductive systems are present on different thalli, not one. Thus, they are not bisexual in nature unlike most plants in Kingdom Plantae. Hence, in this mode, the female gametophytes present in a flask-shaped structure known as archegonium in the female plant are fertilized by the sperm from the antheridia on the male plant.
For the facilitation of the fertilization, the archegonia swell after absorbing water and oozes out the neck canal cells and ventral canal cells. These discharged canal cells fuse and form a mucilaginous mass that contains chemical messengers that trigger a chemotic response in the male sperm known as the antherozoids. These antherozoids get attracted towards the archegonia and ultimately one of the fuses with the egg present in the archegonia/archegonium. A diploid zygote is formed by the fusion of the male and female nuclei.
The zygote over a period of time develops into a small sporophyte, which produces small spores. This sporophyte is dependent on the mature or larger gametophyte in whom the fertilization takes place. The spores produced in time become independent male and female liverworts. You should note down that even though the zygote was diploid, these spores are haploid cells and hence, the plants that grow from them are also haploid in nature. This haploid stage is the dominant phase in the life cycle of plants falling under the Marchantia classification.
Life Cycle of Marchantia
From the above, you can easily understand that the life cycle of plants falling under the Marchantia classification is a cycle between the haploid and diploid phases. Hence, Marchantia is haplodiplontic. The free-living phase is the haploid phase wherein there is the production of the male sperm and female ovum. This is followed by the diploid phase where the multicellular sporophyte is developed from the mitotic division of the zygote. During this cycle, the sporophyte as you already know is dependent on the mature gametophyte for nourishment and development. The sporophyte then produces spores which are haploid and formed by meiosis. These haploid spores now develop into new independent individual male and female organisms and the characteristic cycle continues.
Having gone through the short introduction about Marchantia it may interest you to further study the liverworts and their attributes. Understanding this introduction will provide you with a scientific reason, as to why you find the liverworts mostly in the moist and shady places because of the essential requirement of water for its life cycle and various other Marchantia characteristics.