Alternation of Generations and Change of Nuclear Phases

The sporophyte is the generation whose cells contain a double (diploid) set of chromosomes (2n). This is the form we most often associate with "a plant" in general: a flowering apple tree, an ear of wheat, a fern in the forest—all these are sporophytes (Mauseth, 2017).

The main function of the sporophyte is to produce, via meiosis (reduction division), specialized cells called spores. Spores may be of one type (in homosporous plants, e.g., most ferns) or of two types: micro- and megaspores (in heterosporous plants, which is a key step toward the seed habit) (Raven et al., 2016).

The sporophyte is generally an "autonomous" being: it has its own roots, stems, and leaves, and it feeds and respires independently. Only at the very beginning of development (at the embryo stage) does it receive nourishment from the maternal gametophyte—hence the name of all higher plants: embryophytes (Embryophyta). However, the degree of independence of the sporophyte varies dramatically among different groups. In mosses, it is a temporary "dependent," while in flowering plants, it is virtually the entire plant.

The gametophyte is the generation that carries a single (haploid) set of chromosomes (n). Its task is entirely different: to produce sex cells—gametes (egg cells and spermatozoids). A crucial difference from the animal world: in plants, gametes are formed not by meiosis, but by mitosis from cells of an already established haploid gametophyte (Bennici, 2008). It is precisely this that makes the gametophyte an independent multicellular phase, not merely a temporary container for gametes.

What the gametophyte looks like depends heavily on the evolutionary level of the plant.

To solidify these complex concepts, imagine…​ building a house.

Thus, the sporophyte and gametophyte are the two poles of a single system, and their relationship (who is bigger, who is smaller, who feeds whom) is the evolutionary "calling card" of each major plant group. In the following sections, we will analyze how exactly nuclear phases and generations changed along the path from algae to flowering plants.

The diploid phase in a plant’s life is the period when all body cells (somatic cells) contain a double set of chromosomes (2n). It begins at fertilization, when two haploid gamete nuclei (n) fuse to form a zygote (2n). The zygote gives rise to the sporophyte. All subsequent vegetative life of the sporophyte—stem growth, leaf development, root formation—proceeds in the diploid phase. Sporophyte cells divide by mitosis, and each daughter cell receives an exact copy of the diploid chromosome set inherited from both parents (Stern, 2020). The diploid phase ends in specialized cells of the sporophyte—the sporocytes (spore mother cells)—where meiosis occurs.

From a genetic and agricultural perspective, the diploid phase has a colossal advantage: it "masks" recessive deleterious mutations. If any damage in a single gene in the haploid gametophyte leads to death or dysfunction, the diploid sporophyte has a second, homologous gene that can compensate for the defect. This is precisely why the evolution of higher plants involved the strengthening and complexification of the sporophyte (Ligrone et al., 2012).

The haploid phase begins with meiosis. As a result of meiosis, four haploid spores (n) are formed from one diploid sporocyte. These spores are the first cells of the new gametophyte. The spores germinate, and the developing gametophyte is now a multicellular haploid body. All its cells, including the future gametangia (antheridia and archegonia), contain a haploid chromosome set. The gametes themselves are formed by mitosis, thus retaining the haploid state (Raven et al., 2016). The haploid phase ends with fertilization.

Haploidy is a powerful driver of natural selection at the cellular level. Any gene in the gametophyte is immediately expressed phenotypically. This allows the gene pool to be "purged" of lethal and deleterious recessive alleles that might otherwise persist indefinitely in the diploid state. Moreover, it is during meiosis (which ends the diploid phase and initiates the haploid phase) that genetic recombination occurs—crossing over and independent chromosome assortment. Without this process, the creation of new genetic diversity, which serves as the raw material for breeding, would be impossible (Bidlack & Jansky, 2021).

The transition between phases occurs only through two strictly defined events, which must never be disrupted (otherwise the cycle would be broken):

Understanding the change of nuclear phases underlies two key technologies in modern crop production:

Thus, fertilization and meiosis are not just abstract biological processes, but the "gates" between generations, directly affecting the productivity, resistance, and genetic plasticity of agricultural crops. In the next section, we will consider what types of life cycles exist in nature and how the main type of higher plants—the sporic type—fits into this picture.

Characteristics: Meiosis occurs immediately after fertilization, in the zygote. The zygote is the only diploid cell in the cycle. All other stages are haploid.

Life form: The multicellular haploid organism (gametophyte) dominates (and is the only one). It produces gametes by mitosis. After gamete fusion, the resulting zygote immediately (or after a short resting period) divides by meiosis, giving rise to new haploid cells that develop into a new gametophyte.

Where found: In many algae (e.g., Chlamydomonas, some green algae and diatoms) and in fungi (Bennici, 2008). This type is considered primitive and ancestral for plant evolution. The sporophyte as a multicellular phase is entirely absent here.

Characteristics: Meiosis occurs directly during gamete formation. All body cells, except the gametes, are diploid.

Life form: The multicellular diploid organism (sporophyte) dominates (and is the only one). In its specialized organs (gametangia), haploid gametes are formed by meiosis. The gametes do not divide by mitosis but immediately fuse, restoring the diploid set. The haploid phase is represented only by gametes.

Where found: In most animals, and among plants, in some brown algae (e.g., Fucus) and diatoms (Stern, 2020). This type is not characteristic of land plants, but knowledge of it is important for contrast: in plants, gametes are formed by mitosis, not meiosis.

Characteristics: Meiosis occurs in the sporangia of the sporophyte and leads to spore formation. Gametes are formed on the gametophyte by mitosis. This is the most complex and, from the standpoint of higher plant evolution, the most important type.

Life form: The cycle includes two multicellular phases: the diploid sporophyte (2n) and the haploid gametophyte (n). Both phases may be independent (as in ferns), or one may be reduced and dependent on the other (as in flowering plants). This type is specifically called alternation of generations (Bidlack & Jansky, 2021).

Key difference: The sporophyte and the gametophyte are two different individuals that succeed each other. Meiosis serves as the "switch" from sporophyte to gametophyte, while fertilization is the reverse switch from gametophyte to sporophyte.

Where found: In all higher plants (embryophytes): from mosses to flowering plants, as well as in many algae (e.g., Ulva, some red and brown algae) (Ligrone et al., 2012; Shaw et al., 2011).

For clarity, the main differences between the three types are summarized in the table below.

It is precisely the sporic type (with intermediate, or sporic, meiosis) that is defining for all land plants. Its key feature is the presence of two multicellular phases potentially capable of prolonged existence, which opened up evolutionary opportunities for the differentiation of their functions. In mosses, the gametophyte dominates; in ferns, the sporophyte dominates; and in seed plants, the sporophyte reaches the highest complexity while the gametophyte is maximally reduced. However, regardless of how the balance changes, the principle of alternation of generations with the switching of ploidy through meiosis and fertilization remains inviolable for all embryophytes (Raven et al., 2016). In the following sections, we will sequentially analyze how this general plan is realized in different plant groups—from mosses to flowering plants.

Main page: Life cycle with gametophyte dominance

Bryophytes (Bryophyta s.l.) are a paraphyletic group including liverworts, mosses, and hornworts. They occupy a basal position among land plants and have retained a number of traits that were likely characteristic of the earliest embryophytes (Ligrone et al., 2012). The main feature of their life cycle is the absolute dominance of the gametophyte (n).

What do we see in nature? The green "carpet" of moss, a cushion, or an individual leafy stem—this is the gametophyte. It photosynthesizes, lives long (often several years), reproduces vegetatively, and ultimately bears the sex organs: antheridia (male) and archegonia (female) (Mauseth, 2017). The moss gametophyte can be relatively complex: in leafy mosses, there is a stem- and leaf-like structure, but these are not the true tissues of vascular plants, but more simply constructed structures (Simpson, 2019).

What does the sporophyte look like? The sporophyte (2n) in mosses is represented by the so-called sporogonium. It is a thin stalk (seta) topped with a capsule (sporangium). It is never independent: the stalk is attached to the gametophyte and receives nutrients from it through a specialized contact zone—the foot. The moss sporophyte has little or no chlorophyll, does not photosynthesize, and is entirely dependent on the maternal gametophyte. Moreover, it is short-lived: after spore maturation and dispersal, it dies (Shaw et al., 2011; Ligrone et al., 2012). In this sense, the sporophyte in mosses can be figuratively called a "parasite" on the body of the gametophyte.

How does fertilization occur? Fertilization requires water. Male gametes—biflagellate or multiflagellate spermatozoids—actively swim to the archegonia through a water film. This strictly ties mosses to moist habitats, although some can withstand drought in a dormant state (Mauseth, 2017).

Key phrase for memorization: "What we call moss is the haploid sexual generation (gametophyte), while its brown capsule on a stalk is the diploid 'free app' (sporophyte) that feeds off the green host."

A detailed examination of the bryophyte life cycle, including protonema development, antheridia and archegonia structure, and spore dispersal mechanisms, will be presented in a separate article of the block.

Main page: Life cycle with sporophyte dominance in spore-bearing plants

Pteridophytes (Monilophyta), including true ferns, horsetails, and whisk ferns, represent the next evolutionary step. Here, for the first time in the evolution of land plants, the dominant generation becomes the sporophyte (2n). The large, dissected plant with roots, stems, and leaves (fronds) is the sporophyte. The gametophyte is reduced to a small but still independent entity.

What do we see in nature? The fern growing in the forest is the sporophyte. It is capable of indeterminate growth, has a developed conducting system (xylem and phloem), allowing it to reach significant sizes and colonize more diverse habitats than mosses (Kumar et al., 2022). On the underside of the sporophyte’s leaves, sporangia are formed, grouped in clusters called sori. Inside the sporangia, diploid sporocytes undergo meiosis, producing many identical (in most homosporous ferns) haploid spores.

What does the gametophyte look like? From the spore, the gametophyte develops—a small, usually heart-shaped, green structure from a few millimeters to 2–3 cm in size, called the prothallus. The prothallus lives independently: it photosynthesizes and has rhizoids for anchorage. On its underside, antheridia and archegonia are formed. Importantly, unlike in mosses, the fern prothallus does not depend on the sporophyte and exists separately (Raven et al., 2016). However, it is very delicate, short-lived, and demanding of moisture.

How does fertilization occur? As in mosses, fern spermatozoids are multiflagellate and require water to reach the egg. Therefore, although the fern sporophyte can grow in relatively dry places, sexual reproduction requires a moist environment. From the fertilized egg, a new sporophyte develops, which initially receives nourishment from the gametophyte but soon roots and begins independent life, while the prothallus dies.

Key phrase for memorization: "The fern is the sporophyte; its leaves produce spores, and the tiny green prothallus is a separate but very small and short-lived haploid generation that can only be seen with a magnifying glass."

The fern cycle demonstrates an important evolutionary transition: the sporophyte becomes the main vegetative body, and the gametophyte is reduced but remains free-living. However, in gymnosperms and angiosperms, we will see an even more radical reduction of the gametophyte and the loss of its independence. This evolutionary trend (strengthening of the sporophyte and weakening of the gametophyte) will be examined in the following subsections.

Main page: Life cycle of gymnosperms

Gymnosperms (Pinophyta, or Gymnospermae) represent the next major milestone in the evolution of the plant kingdom. They not only retained the dominance of the sporophyte (2n) inherited from pteridophytes, but also achieved two groundbreaking evolutionary innovations: the seed and independence of sexual reproduction from liquid water. These achievements became possible through a further, very deep reduction of the gametophyte and a fundamental change in its role.

What do we see in nature? A majestic tree (pine, spruce, cedar) or shrub is the sporophyte (2n). It has well-developed roots, a trunk with a conducting system (xylem composed mainly of tracheids), and needles (modified leaves). On the sporophyte, two types of cones (strobili) are formed: male (microstrobili) and female (megastrobili) (Raven et al., 2016). This is an expression of heterospory: the sporophyte produces two types of spores—microspores (small, numerous) and megaspores (large, few). Each type of spore gives rise to a strictly defined gametophyte.

The male gametophyte: the pollen grain. In the microsporangia of male cones, microsporocytes (2n) undergo meiosis to form microspores (n). Each microspore germinates directly inside the microsporangium, giving rise to the male gametophyte. This gametophyte—the pollen grain—is extremely reduced. In pine, it consists of only four cells: two vegetative (prothallial) cells, one generative cell, and one pollen tube cell (siphonogenic cell). By the time the pollen is shed, it is no longer just a spore, but a microscopic, yet fully formed, male gametophyte (Mauseth, 2017; Simpson, 2019).

The female gametophyte: the haploid endosperm. In the megasporangium (nucellus) of the female cone, one of the megasporocytes (2n) undergoes meiosis, producing four megaspores (n). Three of them degenerate, while one (functional) develops into the female gametophyte. It is multicellular (in pine, several hundred cells) but completely lacks chlorophyll and does not photosynthesize. The entire female gametophyte exists within the nucellus and receives nourishment from the sporophyte. It bears two or three archegonia (in pine), each containing one egg cell (Mauseth, 2017). The remaining cells of the female gametophyte serve a storage function; it is this haploid tissue that is called endosperm in gymnosperms—it has nothing in common with the triploid endosperm of angiosperms!

How does fertilization occur? Water is no longer needed! The pollen (male gametophyte) is carried by the wind (anemophily) and lands on the ovule (unprotected, "naked"—hence the name "gymnosperms"). The pollen grain germinates: the siphonogenic cell forms a pollen tube, which grows through the nucellar tissues toward the archegonium. The generative cell divides, producing two sperm cells—flagella-less male gametes that passively move through the pollen tube. Upon reaching the archegonium, the sperm cells fertilize the egg cell (Ligrone et al., 2012).

Seed formation. After fertilization, the zygote (2n) develops into the embryo of the future sporophyte. The ovule (integument, nucellus) becomes the seed coat, while the haploid female gametophyte (endosperm) becomes the storage tissue for the embryo. Together, all this forms the seed—a complex multicellular structure containing a diploid embryo surrounded by haploid nutritive tissue (derived from the gametophyte) and protected by a sporophytic coat (Raven et al., 2016; Strasburger, 1971).

Key phrase for memorization: "The pollen grain is a packaged male gametophyte that travels through the air, and the 'endosperm' in a pine nut is not 'storage tissue for the embryo as in angiosperms,' but the female haploid generation itself, which feeds the diploid embryo."

The gymnosperm cycle illustrates how reduction and endospory (development of the gametophyte inside the spore wall) freed the fertilization process from dependence on an aquatic environment, while the advent of the seed provided protection and nutrition for the young sporophyte. However, in angiosperms, we will see an even more radical reduction of the gametophyte and the emergence of the unique process of double fertilization.

Main page: Life cycle of angiosperms

Angiosperms (Angiospermae) are the most evolutionarily advanced and diverse group of plants. Their life cycle represents the culmination of the evolutionary trend toward maximal reduction of the gametophyte and its complete "subjugation" to the sporophyte. Key innovations include: the flower (attracting pollinators), the fruit (ensuring seed protection and dispersal), and the unique double fertilization, which results in a triploid endosperm (the seed’s nutritive tissue, genetically distinct from the embryo). These innovations have allowed angiosperms to become dominant in most terrestrial ecosystems (Raven et al., 2016; Simpson, 2019).

What do we see in nature? A flowering plant (herb, shrub, tree) is the sporophyte (2n). It has a complex anatomical structure (true vessels in many groups, an efficient conducting system). The flower bears organs of two types: stamens (microsporangia) and carpels (megasporangia enclosed in the ovary). As in gymnosperms, this is heterospory: the formation of microspores and megaspores (Bidlack & Jansky, 2021).

The male gametophyte (pollen grain). In the anthers (microsporangia), microsporocytes (2n) undergo meiosis, forming microspores (n). The microspore develops into the male gametophyte—the pollen grain. In angiosperms, the male gametophyte is even more reduced than in gymnosperms: it consists of only two cells (at the time of shedding from the anther) or three cells (in some species). These are the vegetative cell (which forms the pollen tube) and the generative cell, which later divides into two sperm cells (Mauseth, 2017). Thus, the male gametophyte of angiosperms is a microscopic structure with no intrinsic nutritive tissues and completely dependent on the sporophyte.

The female gametophyte (embryo sac). In the ovule (inside the ovary), one of the megasporocytes (2n) undergoes meiosis, producing four megaspores (n). Three of them degenerate, while one (functional) undergoes three rounds of mitosis, resulting in the embryo sac—the female gametophyte of angiosperms. The mature embryo sac contains only 7 cells and 8 nuclei (in most species): the egg cell, two synergids (accessory cells near the entrance to the embryo sac), three antipodal cells (at the opposite end), and one central cell with two haploid nuclei (or a single diploid nucleus if they fuse) (Raven et al., 2016; Stern, 2020). The female gametophyte also completely lacks chlorophyll and depends on the sporophyte.

Double fertilization. After pollination (transfer of pollen to the stigma of the pistil), the pollen grain germinates; the vegetative cell forms the pollen tube, which grows through the style and ovary tissues toward the ovule. The generative cell divides, forming two non-motile sperm cells (n). When the tube reaches the embryo sac, two fusions occur:

Thus, the endosperm of angiosperms is a triploid (3n) tissue, genetically distinct from both the embryo and the maternal sporophyte. This is a unique feature of flowering plants that ensures efficient nutrition for the developing embryo (Raven et al., 2016).

Seed and fruit formation. After double fertilization, the ovule develops into the seed (containing the embryo (2n) and the triploid endosperm, covered by the seed coat, derived from the sporophyte). The ovary wall thickens and develops into the fruit—an additional protective and dispersal structure absent in gymnosperms (Mauseth, 2017).

Key phrase for memorization: "The embryo sac is the female gametophyte, hidden in the ovary and consisting of just a few cells, while the endosperm is not the gametophyte (as in pine), but a special triploid tissue resulting from double fertilization."

The angiosperm cycle is the most economical and efficient: gametophytes are reduced to a minimum, fertilization does not require water (pollen tube), and seeds are provided not only with protection but also with a powerful storage tissue (3n endosperm), and often with dispersal adaptations (fruits). This is the evolutionary success of flowering plants.

A comparative characterization of the four main types of alternation of generations (by evolutionary group) is presented in Table 1. The main evolutionary trend is clearly visible: from the dominance and complex organization of the gametophyte in mosses to its maximal reduction in angiosperms. Simultaneously, the sporophyte becomes increasingly larger, more complex, and fully independent, while the mode of fertilization loses its connection to the aqueous environment.

Note to table: In gymnosperms and angiosperms, the female gametophyte (endosperm in gymnosperms, embryo sac in angiosperms) develops within the ovule and is incapable of independent existence. This is an example of endospory (development of the gametophyte inside the spore wall), which is a key adaptation to life on land (Ligrone et al., 2012; Renner & Sokoloff, 2024).

In the subsequent articles of this block, the developmental, morphological, and reproductive features of each of the groups mentioned will be discussed in detail. The present article has provided a general overview of the evolution of alternation of generations and change of nuclear phases—the fundamental basis of plant life cycles.