Gametophyte-Dominant Life Cycle (Using Bryophytes as an Example)

Protonema — the Juvenile Stage of the Gametophyte

Protonema of the moss Physcomitrella patens under a microscope

Micrograph of a branched filamentous protonema. At the protonema stage, the young moss gametophyte resembles a filamentous green alga, indicating the evolutionary origin of land plants from algae.

In most mosses (Bryophyta), the spore first develops not into an adult leafy plant but into a special filamentous structure called the protonema (Mauseth, 2017; Stern, 2020). The protonema is a branched green filament composed of a single row of cells, resembling a filamentous green alga (e.g., Spirogyra or Cladophora). However, it can be distinguished by its oblique (rather than transverse) cross-walls and the presence of numerous small chloroplasts in many cells (as opposed to one or two large chloroplasts in algae).

The protonema performs two main functions:

1. Dispersal and anchorage — the filaments grow rapidly across the substrate, colonizing new areas.

2. Bud formation — buds are initiated at specific locations on the protonema (usually on the thicker cells of the caulonema), from which adult leafy plants — gametophores — develop (Naramoto et al., 2022).

In different groups of mosses, the protonema can be more or less long-lived. In many mosses (e.g., Funaria, Physcomitrella), the protonema persists for a long time and can even vegetate on its own, producing new gametophores (Ligrone et al., 2012). In peat mosses (Sphagnopsida), the protonema is plate-like and flat, rather than filamentous, which is considered a more primitive trait (Raven et al., 2017).

In liverworts (Marchantiophyta), the protonema is typically very short, often consisting of only a few cells, and quickly transitions to forming a thallus or leafy shoot (Crandall-Stotler et al., 2009). In hornworts (Anthocerotophyta), the protonema is absent altogether: the spore germinates directly into the thallus (Renzaglia et al., 2009).

Formation of Gametophores

The gametophore is the specialized part of the gametophyte that bears the sexual organs (gametangia — antheridia and archegonia) (Shaw et al., 2011). In mosses, the gametophore is a well-developed leafy plant (caulidium with phyllidia). In liverworts, the gametophore can be either a thallus (e.g., in Marchantia) or also leafy (e.g., in Porella). In hornworts, the gametophore is a simple thallus.

The development of the gametophore from a protonema bud involves the organization of an apical meristem with a single tetrahedral apical cell (in mosses) or a cell of a different shape (in liverworts) (Ligrone et al., 2012). This cell sequentially produces derivatives, which then differentiate into stem and leaf tissues. In mosses, phyllidia are arranged spirally, while in leafy liverworts, they are arranged in three rows (often two upper large rows and one lower reduced row) (Simpson, 2019).

Examples:

• In Polytrichum commune (common haircap moss), gametophores reach 20–40 cm in height, are densely covered with stiff leaves, and form dense turfs (Mauseth, 2017).
• In Marchantia polymorpha, the gametophore is a dorsiventral thallus, 2–4 mm thick, dichotomously branched, with rhizoids on the underside and air chambers on the upper side (Raven et al., 2017).
• In hornworts (e.g., Anthoceros), the gametophore is a simple rosette-shaped plate, often with wavy margins, attached to the soil by rhizoids.

Time to Sexual Maturity and Factors Affecting Development

The ability of the gametophore to produce sexual organs is not immediate. In mosses in temperate climates, it can take from several months to two years from spore germination to the development of a mature gametophore bearing antheridia or archegonia (Raven et al., 2017). In liverworts, this process is often faster (a few weeks). Gametophore development is influenced by:

• Light (photoperiod, intensity).
• Moisture (optimally high).
• Chemical composition of the substrate (e.g., Sphagnum mosses prefer acidic conditions).
• Temperature (many species require stratification or a specific temperature regime).

It is important to note that most bryophyte gametophytes can also reproduce vegetatively (by fragments, brood bodies, gemmae), allowing them to rapidly colonize suitable habitats even when sexual reproduction is infrequent (Ekwealor & Fisher, 2024). However, understanding the complete cycle requires examining sexual reproduction specifically.

Brief Summary of Stage 1

Thus, the first stage of the cycle is the development from a haploid spore, first into a juvenile structure (the protonema, which is well-developed in mosses and poorly developed in liverworts), and then into an adult gametophore bearing future sexual organs. Key points to remember:

• Spore (n) → protonema (n) → gametophore (n) — the sequence of gametophyte development.
• The protonema is particularly characteristic of mosses and is an important adaptation for dispersal.
• Gametophores of bryophytes are diverse in form (leafy in mosses and some liverworts, thallose in hornworts and some liverworts), but are always autotrophic and dominate the cycle.

In the next part (Stage 2), we will examine how sexual organs — antheridia and archegonia — form on the gametophores and how fertilization occurs.

Sexual Organs: Antheridia and Archegonia

In bryophytes, the sexual organs (gametangia) are always multicellular and protected by a layer of sterile (non-gamete-producing) cells (Raven et al., 2017). This is an important evolutionary acquisition that protects gametes from desiccation.

Antheridia (male sexual organs) — typically spherical or elongated sacs on a short stalk. Their wall (sterile "jacket") consists of a single layer of cells. Inside, numerous spermatogenous cells develop, each transforming into one spermatozoid (Mauseth, 2017). Bryophyte spermatozoids are curved, spirally coiled and bear two flagella (undulipodia) at the anterior end (Ligrone et al., 2012; Simpson, 2019). This structure enables motility in an aquatic environment. In most species, antheridia are formed on the same gametophores as archegonia (monoecious species) or on separate male plants (dioecious species).

Archegonia (female sexual organs) — have a characteristic flask (bottle) shape and consist of:

• The venter — the swollen lower part, which contains a single large egg cell (haploid, n).
• The neck — a long canal through which spermatozoids penetrate to the egg cell.
• Neck canal cells and a ventral canal cell, which degenerate and mucilaginize upon archegonium maturation, forming a mucous canal (Raven et al., 2017).

The sterile wall of the archegonium also protects the egg cell from desiccation. In many mosses and liverworts, archegonia are grouped together, sometimes surrounded by modified leaves (perichaetial leaves) or a thallus outgrowth (pseudoperianth).

Chemotaxis and the Mechanism of Spermatozoid Attraction

When the archegonium matures, the neck canal cells and the ventral canal cell degenerate, and their contents are released. This exudate contains sugars (sucrose and others) and specific attractants (e.g., calcium-binding proteins), which create a concentration gradient (Mauseth, 2017). Spermatozoids exhibit chemotaxis — the ability to move towards higher concentrations of these substances, guiding them to the archegonial opening (Stern, 2020). In some species (e.g., mosses of the genus Bryum and liverworts), chemotaxis is highly effective, and spermatozoids can travel distances of several centimeters.

Role of Water and Mechanisms Facilitating Fertilization

Liquid water serves two functions:

1. A medium for spermatozoid swimming.

2. A means of transporting spermatozoids over significant distances (via splash dispersal from raindrops).

In species growing in dense turfs (e.g., Polytrichum), raindrops hitting the antheridial "disks" (specialized expanded tips of male plants) eject clusters of spermatozoids, which then settle on female plants (Ligrone et al., 2012). In the liverwort Marchantia, antheridia are located on stalked antheridiophores, shaped like umbrellas with lobes; rain washes out the spermatozoids, and they flow down the thallus surface towards the archegonia located on female archegoniophores (Mauseth, 2017).

In some mosses, a syndrome of spermatozoid transport by arthropods (phoragamy) has been described. For example, in the moss Bryum argenteum and several other species, microarthropods (springtails, mites) carry spermatozoids on their bodies between plants (Ekwealor & Fisher, 2024; Shaw et al., 2011). The role of flying insects (flies) is also known for mosses of the genus Splachnum, which are attracted by the odor emitted by the sporophytes.

The Fertilization Process

Under sufficient moisture, the antheridium opens (usually at the top), and thousands of motile spermatozoids are released. In water, they swim towards the archegonium, guided by the chemical gradient. One spermatozoid penetrates through the neck of the archegonium into the venter and fuses with the egg cell (Raven et al., 2017). The fusion of gametes (syngamy) results in the formation of a diploid (2n) zygote — the first cell of the next, sporophytic generation.

It is important to emphasize: in bryophytes, unlike algae, the zygote does not leave the archegonium but remains inside its venter. This provides protection and initial nourishment to the developing embryo — so-called matrotrophy (Stern, 2020). The archegonium and surrounding gametophyte tissues form a cap (calyptra) that will protect the young sporophyte.

Consequences of Water Dependence

The dependence of fertilization on water leads to several important ecological and biological consequences:

• Association with moist habitats. Bryophytes are most diverse and active in forests, tundras, bogs, and along water bodies. However, many species survive in arid regions by using short rainy periods for reproduction (Shaw et al., 2011).
• Advantage of dioecy under some conditions and monoecy under others. Dioecy (separate sexes) increases genetic diversity but requires male and female plants to be sufficiently close for successful spermatozoid dispersal (Ekwealor & Fisher, 2024). Monoecious species (hermaphroditic) can self-fertilize, ensuring zygote formation even at low population density.
• Phenological adaptations. In many mosses (e.g., Sphagnum), archegonia mature in late winter or early spring, when meltwater creates ideal conditions for spermatozoid movement.

Brief Summary of Stage 2

Fertilization occurs in the second stage:

• Antheridia formed on the gametophore produce spermatozoids, and archegonia produce the egg cell.
• An obligatory requirement is the presence of liquid water (rain, dew, snowmelt).
• Spermatozoids move chemotactically towards the archegonium, penetrate the venter, and fertilize the egg cell.
• A diploid (2n) zygote is formed, which gives rise to the sporophyte.

This stage is the "bottleneck" in the life of bryophytes, but it is also the one that ensures genetic recombination and offspring diversity. In the next part (Stage 3), we will trace the development of the parasitic sporophyte from the zygote and how spores are formed within it.

General Structure of the Sporogonium

The sporophyte of bryophytes is commonly called the sporogonium. In most mosses and liverworts, it consists of three parts (Simpson, 2019; Stern, 2020):

1. Foot — the basal part, embedded in the gametophyte tissue. The foot cells form haustorium-like outgrowths or transfer cells through which nutrients are absorbed from the gametophyte. At the generation boundary, a placenta — a zone of close contact with gametophyte cells, often featuring developed transfer cells — is formed (Ligrone et al., 2012).

2. Stalk, or seta — an elongated sterile axis that raises the sporangium above the gametophyte. In liverworts, the seta is very delicate and quickly withers; in mosses, it is more robust, often containing primitive conducting cells (hydroids and leptoids). In hornworts, the seta is absent (Renzaglia et al., 2009).

3. Capsule (sporangium) — the swollen terminal part where meiosis occurs and spores are formed.

Important: The sporophyte of bryophytes never branches (unlike the sporophytes of ferns, horsetails, and especially seed plants). It is always solitary and bears only one sporangium (Goffinet et al., 2013). If it appears that there are several capsules in some mosses (e.g., in Sphagnum), this results from the development of multiple sporogonia from different archegonia on the same gametophore, not from branching of a single sporophyte.

Development of the Sporogonium: From Zygote to Mature Capsule

The development of the sporogonium can be divided into several sequential stages.

1. Zygote and the first stage of embryogenesis. The zygote (2n), remaining within the archegonial venter, begins to divide. In mosses and liverworts, the first division of the zygote is transverse (relative to the long axis of the archegonium). In hornworts, it is longitudinal (Raven et al., 2017; Ligrone et al., 2012). A multicellular embryo develops from the zygote.

2. Formation of the foot and seta. In mosses, part of the embryo cells give rise to the foot and seta. Seta growth in mosses occurs via an intercalary meristem — a zone of cell division between the future capsule and the seta (Ligrone et al., 2012). In liverworts, the seta grows by cell elongation (diffuse growth) of already formed cells, rather than via a meristem.

3. Formation of the capsule (sporangium). The upper part of the embryo differentiates into the capsule. Within the capsule, the archesporium — the sporogenous tissue — is established, from which sporocytes (spore mother cells) later form. A multilayered capsule wall develops around the archesporium. In the central part of many mosses and hornworts, a sterile strand — the columella — forms, which helps in the even distribution of spores (Mauseth, 2017). Liverworts lack a columella.

4. Protective structures. The developing sporophyte is surrounded by remnants of the archegonium, which enlarge and form the calyptra — a cap covering the young capsule. The calyptra has a protective function and may also play a role in regulating transpiration and gas exchange of the sporophyte (Raven et al., 2017). As the capsule matures, the calyptra usually falls off.

Features in Different Bryophyte Groups

Male plant of the liverwort Marchantia polymorpha

The photograph shows the gametophyte (thallus) of the liverwort with antheridiophores — umbrella-like stalks on which antheridia develop. Moisture running down the stalk helps spermatozoids disperse towards female plants.

In liverworts (Marchantiophyta), the sporophyte has a very short, quickly withering seta. The capsule is often spherical or ellipsoid and opens by longitudinal slits or valves. Inside, in addition to spores, there are sterile cells — elaters — with hygroscopic spiral thickenings that help shake out the spores when humidity changes (Crandall-Stotler et al., 2009; Simpson, 2019). Example: Marchantia polymorpha.

In mosses (Bryophyta), the sporophyte typically has a long, often rigid seta. The capsule has a more complex structure. In a mature capsule, one can distinguish the urn (the main spore-bearing body) and the operculum (lid). When spores mature, the operculum falls off, revealing the peristome — one or two rows of teeth sensitive to moisture (Goffinet et al., 2013). The peristome plays a key role in the controlled release of spores. Example: Polytrichum commune.

In hornworts (Anthocerotophyta), the seta is absent. The sporophyte is a long, awl-shaped or horn-like outgrowth, growing continuously from the base (indeterminate growth) via a basal intercalary meristem (Renzaglia et al., 2009). The mature sporophyte splits longitudinally from the apex to the base, gradually releasing the spores. Within the sporangium, in addition to spores, there are multicellular pseudoelaters.

Sporogenesis (Meiosis) and Spore Formation

Inside the capsule, the cells of the archesporium (sporogenous tissue) undergo meiosis. Each sporocyte (spore mother cell), being diploid (2n), divides by meiosis, forming four haploid spores (n) (Stern, 2020). In most bryophytes, all spores are of similar size — such species are called isosporous. Only in a few (e.g., some species of Sphagnum, Macromitrium) do spores differ in size (anoisospory, or anisospory), which is considered a step towards heterospory, characteristic of seed plants (Raven et al., 2017).

Spores are encased in a robust wall containing sporopollenin — one of the most resistant biopolymers, allowing spores to withstand adverse conditions (desiccation, temperature fluctuations) and remain viable for many years (Simpson, 2019). On the spore surface of mosses and some liverworts, there is a trilete mark — a three-rayed scar remaining from the spore tetrad.

Sporophyte Nutrition and Substance Transport

The bryophyte sporophyte is incapable of full photosynthesis. Although chloroplasts may be present in the cells of its tissues (especially in the capsule wall), the contribution of its own photosynthetic activity to the overall carbon budget of the sporogonium is very small (usually less than 10–20%) (Ligrone et al., 2012; Goffinet et al., 2013). The bulk of organic substances (sugars, amino acids), as well as water with dissolved minerals, come from the gametophyte.

Transport occurs through the placenta — a zone of close contact between the sporophyte foot and the gametophyte tissues. Here, cells of both generations often have cell wall ingrowths (transfer cells), increasing the absorption surface area (Ligrone et al., 2012). It is believed that substance movement occurs via the apoplast (through the cell walls) and is regulated by active transport.

Function of the Columella, Elaters, and Peristome

Columella (in mosses and hornworts) — a sterile central strand that provides mechanical strength and possibly water transport to the sporophyte (Mauseth, 2017).

Elaters (in liverworts) and pseudoelaters (in hornworts) — sterile cells that twist upon drying, helping to "loosen" the spore mass and expel spores from the capsule (Simpson, 2019).

Peristome of the moss Bryum capillare

Micrograph of the double peristome of the moss Bryum capillare. The outer (exostome) and inner (endostome) teeth, which regulate spore release depending on air humidity, are clearly visible.

Peristome (in mosses) — a crucial adaptation for controlled spore release. The peristome teeth are hygroscopic: in dry weather, they bend outwards, opening the exit for spores; in wet weather, they press against the opening, closing it (Goffinet et al., 2013). This prevents mass spore release during rain when wind dispersal is impossible.

Significance of the Sporophyte in Evolution and Ecology

Although the bryophyte sporophyte appears as an "appendage" to the gametophyte, its role in the life cycle is enormous. Thanks to the sporophyte, the following occur:

• Genetic recombination (via meiosis) and the creation of genetically diverse spores.
• Long-distance dispersal — spores, produced in enormous numbers (one sporogonium can produce up to 50 million spores), are easily carried by wind, water, and animals (Shaw et al., 2011).
• Survival through unfavorable periods — the sporopollenin wall makes spores resistant to desiccation, frost, and UV radiation.

For agricultural science, understanding the structure of the bryophyte sporophyte is useful, for example, for identifying weed moss species by their capsules and peristome (since sporophytes often persist longer than gametophytes). Furthermore, the dispersal of spores of many moss species (e.g., Sphagnum) over vast areas is relevant to peatland reclamation and assessing the introduction of propagules into fields.

Spore Maturation and Preparation for Dispersal

Once meiosis is complete, the spores inside the capsule are tightly packed, often mixed with sterile elements (elaters in liverworts or pseudoelaters in hornworts). For effective dispersal, spores must be:

• dry and light — to be carried by wind;
• resistant to external factors — due to their thick wall containing sporopollenin (Simpson, 2019);
• freed from clumping — here, the hygroscopic movements of elaters or capsule dehiscence mechanisms help.

Simultaneously with spore maturation, dehydration (drying) of the capsule tissues and its seta occurs. In many mosses, the seta retains the ability to twist and untwist with changes in humidity, which also aids in spore ejection (Ligrone et al., 2012).

Mechanisms of Capsule Dehiscence and Spore Release

The three groups of bryophytes have developed different ways of releasing spores, reflecting their long independent evolution.

A. Liverworts (Marchantiophyta). The capsule in most liverworts opens via longitudinal slits (usually 4 slits) or irregularly (Crandall-Stotler et al., 2009). The capsule wall is one- or multi-layered. Inside are elaters — single cells with spiral wall thickenings containing water. Upon drying, the elaters twist and, as they straighten, "sweep" the spores out of the opened capsule (Mauseth, 2017; Simpson, 2019). Spore release is usually simultaneous (the capsule empties quickly).

B. Mosses (Bryophyta). In most "true" mosses (class Bryidae), the capsule has an operculum (lid), which detaches upon maturation. Beneath the operculum is the peristome — one or two rows of teeth formed from remnants of cell walls. The peristome is a key adaptation for controlled spore release (Goffinet et al., 2013). Working principle:

• In dry weather, the peristome teeth bend outwards (or inwards, depending on the type), opening the aperture, and spores are released in batches.
• When humidity increases, the teeth return to their original position, closing the capsule and preventing spore release during rain (Mauseth, 2017).
• In peat mosses (Sphagnopsida), the capsule opens explosively: upon drying, it contracts, internal pressure builds up, and the operculum is shot off with a noise, ejecting a cloud of spores up to 10–15 cm high (Raven et al., 2017).

C. Hornworts (Anthocerotophyta). The sporophyte is a long horn. Dehiscence occurs longitudinally from the apex to the base in two (rarely four) valves. Since sporophyte growth is basal and indeterminate (Renzaglia et al., 2009), new spores mature gradually, so dehiscence and spore release are also spread over time. This mechanism resembles a "time capsule": spores are released in portions over several weeks or months. Inside the capsule, multicellular pseudoelaters also participate in loosening the spores (Raven et al., 2017).

Factors Affecting Dispersal Distance

Bryophyte spores are very small (usually 10–20 µm in diameter) and light. The main dispersal agents are:

1. Wind (anemochory) — the main factor for long-distance transport. Spores can rise in updrafts and be carried hundreds or thousands of kilometers (Shaw et al., 2011). This explains the wide distribution of many species and the relatively low level of endemism in bryophytes compared to flowering plants.

2. Water (hydrochory) — raindrops can knock spores out of capsules; meltwater and rainwater carry spores across the soil surface.

3. Animals (zoochory) — spores can adhere to the bodies of insects, birds, and mammals. For some species (e.g., mosses of the genus Splachnum, growing on dung), transport by flies is the main mechanism (Ekwealor & Fisher, 2024).

Dispersal distance can be vast, but effective dispersal (with successful germination and establishment) occurs over much shorter distances (usually meters, rarely kilometers) due to the need to land in a suitable moist microenvironment (Vanderpoorten & Goffinet, 2009).

Spore Germination and Return to the Gametophyte

When a spore (n) lands in suitable conditions — sufficient moisture, moderate temperature, often light (though some species require darkness) — it germinates. Germination begins with swelling and rupture of the exospore. In mosses, a filamentous chloronema or protonema emerges first (in Sphagnum, a plate-like protonema), which gives rise to the new gametophyte (Stern, 2020). In liverworts and hornworts, the protonema is short or absent, and the spore transitions directly to forming the thallus or the stem with leaves (Raven et al., 2017).

Thus, the cycle is completed: haploid spore → protonema → gametophore (gametophyte) → sexual organs → fertilization → zygote → sporophyte → meiosis → new haploid spores.