Gametophyte-Dominant Life Cycle
All land plants (Embryophyta) have a life cycle in which two multicellular generations regularly alternate: the gametophyte and the sporophyte (Raven et al., 2017; Simpson, 2019). This type of cycle is called haplodiplontic (Stern, 2020), or the alternation of generations. The alternation of nuclear phases (haploid and diploid) and generations varies among different plant groups, allowing for the identification of three main evolutionary types of cycles:
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The gametophyte-dominant cycle;
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The sporophyte-dominant cycle (in spore-bearing vascular plants, e.g., pteridophytes);
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The cycle with a reduced gametophyte (in seed plants).
Bryophytes (Bryophyta sensu lato — liverworts, mosses, and hornworts) serve as a classic example of a cycle where the haploid gametophyte dominates (Mauseth, 2017; Ligrone et al., 2012). Here, "dominance" means that the gametophyte is the long-lived, photosynthetic, and independent plant, while the sporophyte is a short-lived organ dependent on the gametophyte and typically unbranched (Goffinet et al., 2013; Shaw et al., 2011).
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q::What is a gametophyte-dominant life cycle? It is a type of life cycle in land plants where the haploid gametophyte is the dominant (larger and longer-lived) generation, and the diploid sporophyte is attached to and dependent on the gametophyte.[answer-1]
q::Why do mosses and liverworts need water for fertilization? Moss and liverwort spermatozoids are motile, possess two flagella, and can only reach the egg cell in a liquid water environment (dew, rain, meltwater). Water also aids in the dispersal of spermatozoids from the antheridia.[answer-2]
q::How is the sporophyte of bryophytes structured and what does it feed on? The sporophyte (sporogonium) consists of a foot (embedded in the gametophyte tissue), a stalk (seta), and a capsule (sporangium). It receives water and organic substances from the gametophyte through the placenta, a zone of specialized transfer cells.[answer-3]
q::What are the peristome and elaters, and what are their functions? The peristome consists of teeth around the capsule opening in mosses; they are hygroscopic and regulate the gradual release of spores. Elaters are sterile cells within the capsule of liverworts; they twist upon drying and help "sweep" out the spores.[answer-4]
q::What is the agricultural significance of bryophytes? On one hand, mosses (especially Sphagnum) cause waterlogging and acidification of meadows and pastures; on the other hand, Sphagnum forms peat (used as fertilizer, bedding, and a soil component) and acts as a regulator of landscape water regimes.[answer-5] (Section 5.4)
3.5. Scheme of the Complete Life Cycle (Summary)
Schematically, the complete life cycle of bryophytes (using a moss as an example) can be represented as follows:
Meiosis (in capsule) Sporophyte (2n) ——————————————→ Spores (n) ↑ ↓ | Germination Fertilization (protonema) (in archegonium) ↓ | Gametophore (n) Zygote (2n) ←—————————— Gametes (n) ←—— Antheridia and (in venter) (egg cell archegonia spermatozoid) (on gametophore)
Detailed diagrams of the cycles for different groups (liverwort Marchantia, moss Polytrichum, hornwort Anthoceros) are provided in textbooks by Raven et al. (2017), Mauseth (2017), and Simpson (2019).
Let’s reinforce the key points of the cycle that should be remembered: 1. The only diploid stage — the sporophyte (capsule with seta). 2. Meiosis occurs in the sporangium (capsule), producing haploid spores. 3. The only haploid stages — spores, protonema, and the entire gametophyte. 4. Fertilization restores the diploid set and initiates the sporophyte.
In the next section, we will discuss the evolutionary significance of the gametophyte-dominant cycle and its place in agricultural botany.
