The Gymnosperm Life Cycle

The gymnosperm sporophyte is a perennial, often large tree or shrub with a well-developed root system, stem, and leaves. It is the dominant generation in the life cycle: the sporophyte determines the lifespan, size, and ecological role of the plant (Mauseth, 2017; Beck, 2010). Reproductive organs — strobili (cones) — form on the sporophyte, where spore formation occurs.

The diploid chromosome complement (2_n_) of the sporophyte is restored at fertilization and maintained in all somatic cells. Unlike ferns, where the young sporophyte is initially nourished by the gametophyte, in gymnosperms the sporophyte receives nourishment from the maternal plant through the ovule from the very beginning (Yakovlev et al., 2018).

All gymnosperms are heterosporous plants. This means they produce two types of spores in different sporangia (Graham et al., 2014; Simpson, 2019):

Heterospory was a necessary evolutionary precursor to the seed and allowed the transition to endosporic gametophyte development — entirely within the spore wall and under the protection of sporophytic tissues (Raven et al., 2013).

The most striking feature of the gymnosperm cycle is the significant reduction of both gametophytes and their complete nutritional dependence on the sporophyte (Odintsova & Rugusova, 2013; Offer et al., 2023).

The male gametophyte develops from the microspore without leaving its wall. At the time of dispersal, it is a pollen grain consisting of a limited number of cells (from 1 to 5 depending on the group). In gymnosperms, unlike ferns, antheridia are absent — the male reproductive organs. Instead, a pollen tube forms from the vegetative cell of the pollen grain, and two sperm cells (or, in cycads and ginkgo, spermatozoids) form from the generative cell (Breygina et al., 2021; Mauseth, 2017).

The female gametophyte develops from a single functional megaspore, remaining within the nucellus of the ovule. It is a multicellular tissue that, in most gymnosperms (except gnetophytes), bears from 2 to several archegonia — female reproductive organs, each containing a large egg cell. This tissue accumulates stored nutrients and is termed haploid endosperm (Raven et al., 2013; Simpson, 2019). In contrast to angiosperms, where the endosperm is triploid and formed by double fertilization, in gymnosperms the endosperm is directly the body of the female gametophyte itself.

Below is a simplified diagram of the alternation of generations and nuclear phases in gymnosperms (based on Raven et al., 2013; Simpson, 2019; Beck, 2010):

Sporophyte (2_n_)

Adult tree (conifer, cycad, or ginkgo), bearing micro- and megastrobili.

Microsporogenesis

In microsporangia, microsporocytes (2_n_) undergo meiosis → 4 haploid microspores (_n_) are produced.

Microgametogenesis

The microspore (_n_) divides mitotically → forms a pollen grain (male gametophyte), containing a vegetative cell and a generative cell (or their derivatives).

Pollination

Pollen is transported by wind (rarely insects) to the micropyle of the ovule.

Megasporogenesis

In the nucellus of the ovule, the megasporocyte (2_n_) undergoes meiosis → forms 4 megaspores (_n_); 3 degenerate, one is functional.

Megagametogenesis

The functional megaspore (_n_) undergoes repeated mitotic divisions → forms a multicellular female gametophyte (haploid endosperm) with archegonia.

Fertilization

A sperm cell (or spermatozoid) from the pollen tube enters the egg cell → a zygote (2_n_) is formed.

Embryo development

The zygote divides mitotically → forms an embryo (young sporophyte) within the seed. The embryo is surrounded by haploid endosperm and a seed coat (from the integument).

Seed germination

The embryo gives rise to the adult sporophyte (2_n_), closing the cycle.

The key difference from pteridophytes: in gymnosperms, the gametophytes never leave the spore and never become free-living (Graham et al., 2014). The evolutionary advantage of this structure is complete independence from an aquatic environment for the sexual process (in siphonogamous forms) and reliable protection and nourishment of the developing embryo by the maternal sporophyte.

The following sections will examine each stage of the cycle in detail, using the most studied group — conifers, particularly Scots pine (Pinus sylvestris) — as an example, with notes on features in other gymnosperm classes.

Microstrobili serve for the production of microspores, from which pollen grains (male gametophytes) subsequently develop. In most gymnosperms, they are clustered or solitary at the tips of branches. In conifers (e.g., pine, spruce, fir), small, often yellow or reddish microstrobili are typical, clustered densely at the base of young shoots (Beck, 2010; Raven et al., 2013). In Scots pine, male cones are 1–2 cm long, oval or cylindrical in shape, and fall off soon after pollen dispersal.

Structure of the microstrobilus (using pine, Pinus sylvestris, as an example) (Mauseth, 2017; Yakovlev et al., 2018):

In cycads (Cycadophyta), microstrobili are large, cone-shaped, solitary at the stem apex; microsporophylls are flattened and bear hundreds of microsporangia in groups resembling fern sori (Offer et al., 2023; Simpson, 2019). In Ginkgo biloba, male strobili resemble catkins — a long axis with numerous spirally arranged microsporophylls, each bearing two sac-like microsporangia (Offer et al., 2023). In gnetophytes (Ephedra, Gnetum), microstrobili are often aggregated into compound strobiloid inflorescences, and microsporangia open by pores; microsporophylls may be partially reduced (Yang et al., 2024; Simpson, 2019).

Megastrobili bear megasporangia, within which megaspores and subsequently the female gametophyte develop. In gymnosperms, ovules are not enclosed in an ovary, meaning they lie exposed ("naked") on the surface of megasporophylls or on specialized outgrowths — seed scales (Raven et al., 2013; Mauseth, 2017). This is the key distinction from angiosperms.

In most conifers, female cones are compound. For example, in pine, the female cone consists of an axis with pairs of scales arranged spirally: an outer, thin, inconspicuous scale — the bract scale (homologous to a leaf) — and an inner, larger, fleshy (in youth) scale — the seed scale. The seed scale represents a highly reduced and flattened axillary shoot (brachyblast), not a simple megasporophyll (Beck, 2010; Mauseth, 2017). This model was confirmed by Florin’s paleobotanical data (Florin, 1951) and modern ontogenetic studies (Yang et al., 2024). On the upper (adaxial) side of the seed scale are two (rarely one or several) ovules (Raven et al., 2013).

Structure of the gymnosperm ovule (using pine as an example) (Yakovlev et al., 2018; Simpson, 2019):

Female strobili vary greatly among gymnosperm groups (Yang et al., 2024):

Microsporangia produce vast quantities of microspores, each of which develops into a pollen grain. In most conifers (pine, spruce, fir), pollen has air sacs — two lateral outgrowths of the exine that facilitate buoyancy in the air and capture by the pollination drop (Breygina et al., 2021; Raven et al., 2013). In pine, at the time of shedding, the pollen grain contains four cells: two rudimentary prothallial cells, a tube cell (vegetative), and a generative cell (Mauseth, 2017). In cycads and ginkgo, pollen lacks air sacs, is bean-shaped, and at dispersal contains 3–4 cells (Offer et al., 2023). In most cypresses and yews, pollen lacks air sacs and sheds the exine upon germination (Odintsova & Rugusova, 2013). In gnetophytes, pollen is longitudinally furrowed (Ephedra) or spherical, without sacs (Yang et al., 2024).

Differences in strobilus and pollen structure reflect adaptations to different pollination modes (wind or insect pollination) and are important systematic characters. The next section will cover micro- and megasporogenesis and the development of male and female gametophytes.

Microsporogenesis occurs in the microsporangia (pollen sacs) of microstrobili. Within the microsporangium, sporogenous tissue differentiates into microsporocytes (microspore mother cells) with a diploid chromosome complement (2_n_) (Raven et al., 2013; Simpson, 2019). Each microsporocyte undergoes meiosis, resulting in a tetrad of four haploid microspores (n). In most gymnosperms, tetrads are tetrahedral or linear (Mauseth, 2017). Microspore formation is supported by the nourishing layer — the tapetum — which provides nourishment and deposits sporopollenin on the exine (Odintsova & Rugusova, 2013; Beck, 2010).

After meiosis, the tetrads dissociate, and the microspores remain in the microsporangium for a time, initiating their first mitotic division (microgametogenesis). The degree of male gametophyte development at the time of pollen dispersal varies among gymnosperm groups (Offer et al., 2023; Yang et al., 2024).

The microspore (a haploid cell enclosed by two walls — exine and intine) undergoes mitotic divisions. The sequence of divisions and final cell number in the pollen grain vary, but the general pattern is similar (Odintsova & Rugusova, 2013; Offer et al., 2023):

Thus, a mature pollen grain at dispersal may contain a variable number of cells (Odintsova & Rugusova, 2013):

The shape and structure of gymnosperm pollen grains are closely linked to the pollination mode. The vast majority (about 98%) of gymnosperms are wind-pollinated (anemophilous) plants (Breygina et al., 2021; Yang et al., 2024). Anemophily is characterized by the following features:

The reduction of the male gametophyte to a few cells, its development within the microspore wall, and the loss of antheridia are among the most important evolutionary acquisitions of gymnosperms (Graham et al., 2014). This allowed:

Cycads and ginkgo retain the ancient trait of forming motile spermatozoids. Their pollen tube does not deliver sperm to the egg but serves only a haustorial function, absorbing nutrients from the nucellus; the released spermatozoids swim independently to the archegonium in the fertilization fluid (Offer et al., 2023; Mauseth, 2017). This characteristic links them to the most primitive seed plants (seed ferns) and confirms their basal position in gymnosperm phylogeny (Yang et al., 2024).

The next section will cover megasporogenesis and female gametophyte development.

In the young ovule, a single large diploid cell — the megasporocyte (megaspore mother cell) — is established within the nucellus. Unlike ferns, gymnosperms typically form only one megasporocyte in the nucellus (Raven et al., 2013; Yakovlev et al., 2018). This cell undergoes meiosis as follows:

Of the four megaspores, three (usually those closest to the micropyle) degenerate. Only one — the functional megaspore (most often the chalazal one, i.e., farthest from the micropyle) — survives and begins to grow (Mauseth, 2017; Raven et al., 2013). In some gnetophytes (Gnetum, Welwitschia), tetrasporic development occurs, where all four megaspores participate in forming the female gametophyte — a unique phenomenon among gymnosperms (Odintsova & Rugusova, 2013; Yang et al., 2024).

The functional megaspore (haploid, n) divides repeatedly by mitosis. Early divisions occur without cell wall formation — producing a coenocytic (free-nuclear) female gametophyte. The nuclei are arranged in a peripheral layer of cytoplasm, with a large central vacuole (Beck, 2010; Odintsova & Rugusova, 2013). The number of free nuclei can reach several thousand (up to 2,000–7,000 in pine).

This is followed by the alveolar stage: anticlinal walls form between the nuclei, growing from the periphery toward the center, creating open alveoli around the central vacuole. After the walls close, a solid multicellular tissue is formed — this is the female gametophyte. In morphological literature, it is often called haploid endosperm (Raven et al., 2013; Mauseth, 2017). In angiosperms, the endosperm is triploid and arises from the central cell after double fertilization, whereas in gymnosperms, the endosperm is directly the body of the female gametophyte (Simpson, 2019).

At the apex (micropylar end) of the female gametophyte, archegonia — female reproductive organs — differentiate from superficial cells (Raven et al., 2013; Offer et al., 2023). Each archegonium consists of the following parts:

The number of archegonia per female gametophyte varies from 1–2 (in many conifers, e.g., pine) to several dozen (in cycads) and even hundreds (in Microcycas) (Odintsova & Rugusova, 2013; Offer et al., 2023). In gnetophytes (Gnetum and Welwitschia), archegonia are absent; the egg cell is not differentiated as a distinct cell, and fertilization occurs within the syncytial tissue of the female gametophyte (Yang et al., 2024; Simpson, 2019).

The female gametophyte in gymnosperms serves a dual function: reproductive (containing egg cells) and storage. During maturation, the cells of the haploid endosperm accumulate proteins, lipids, and carbohydrates, which will be used by the embryo during seed germination (Raven et al., 2013; Beck, 2010). Thus, in gymnosperms, the storage tissue of the seed is haploid and genetically identical to the maternal gametophyte. This is a fundamental difference from angiosperms, where the endosperm is triploid and arises from the fusion of one sperm cell with the central cell (double fertilization).

The prolonged development of the female gametophyte (up to one year in pine) and its complete dependence on the sporophyte are important adaptations that allowed gymnosperms to colonize arid and cold regions with short growing seasons (Torre et al., 2020; Mauseth, 2017).

The next section will cover pollination and pollen tube germination.

The vast majority of gymnosperms (about 98%) are wind-pollinated (anemophilous) plants (Breygina et al., 2021). Anemophily in gymnosperms is associated with the production of enormous amounts of pollen (a single pine microstrobilus can yield millions of pollen grains), the presence of air sacs in many conifers, and the dry, light type of pollen (Raven et al., 2013). In wind-pollinated gymnosperms, female cones are usually positioned higher than male cones on the same tree (in monoecious species) or on separate trees (in dioecious species), promoting cross-pollination (Mauseth, 2017).

Entomophily (insect pollination) is reliably documented for all cycads (Cycadophyta) and gnetophytes (Gnetophyta) (Offer et al., 2023; Yang et al., 2024). In cycads, pollinators include beetles (weevils, leaf beetles, carabids), thrips, and other insects that feed on pollen and microstrobilus exudates and transfer pollen to female cones (Odintsova & Rugusova, 2013; Simpson, 2019). In Ginkgo biloba, wind pollination is considered primary, though its pollen lacks air sacs (Offer et al., 2023). In gnetophytes (especially Ephedra), features of both wind and insect pollination are combined; entomophily is noted in Gnetum and Welwitschia (Yang et al., 2024).

A key adaptation for pollen capture in most gymnosperms is the pollination drop. This is a secretion produced by the nucellus of the ovule and exuded through the micropyle as a droplet of fluid. The drop contains water, sugars (glucose, fructose, sucrose), amino acids, proteins (including arabinogalactan proteins, enzymes such as chitinases and proteases), and trace elements (B, Ca, K, Mg) (Breygina et al., 2021; Odintsova & Rugusova, 2013). The functions of the pollination drop are diverse:

In pine and many other conifers, the pollination drop is well developed. In species with pollen bearing air sacs, the drop captures grains floating on its surface; after evaporation or reabsorption, the pollen is drawn inward (Raven et al., 2013). In some conifers (e.g., larch, Larix), the pollination drop is absent or very short-lived, and pollen is captured directly by the sticky surface of the micropyle (Odintsova & Rugusova, 2013).

In cycads, a pollination drop is also secreted, but its role in attracting insects is even more important than its mechanical function (Offer et al., 2023). In gnetophytes (Ephedra), the pollination drop serves not only for pollen capture but also for feeding insects (the secretion contains sugars) — convergent with the nectar of flowering plants (Nepi et al., 2017).

The pollination process comprises several sequential stages (Breygina et al., 2021; Raven et al., 2013):

The pollen reaches the pollen chamber (if present) or lies directly on the nucellus surface. From this point, the post-pollination phase begins — a prolonged period (from a few weeks to a year or more) until fertilization, during which the female gametophyte completes its development and the pollen tube grows (Mauseth, 2017; Beck, 2010).

The next section will cover fertilization — the final stage of sexual reproduction in gymnosperms.

Two types of male gamete delivery to the egg cell occur in gymnosperms (Breygina et al., 2021; Simpson, 2019):

In conifers (using pine as an example), after pollination and pollen tube germination, the generative cell of the pollen grain divides, producing a stalk cell (sterile) and a spermatogenous cell (Raven et al., 2013; Mauseth, 2017). The spermatogenous cell divides mitotically, yielding two sperm cells — male gametes without flagella. In most conifers, both sperm cells are of similar size and structure, but in some species one sperm is larger and functionally more active (Odintsova & Rugusova, 2013). By this time, the pollen tube, growing slowly through the nucellus, has reached the archegonium.

In cycads and ginkgo, the generative cell undergoes a series of divisions, resulting in two (or more) multiflagellated spermatozoids. In cycads, spermatozoids are extremely large (up to 0.5 mm in diameter in Zamia) and bear from 2,500 to 50,000 flagella (Offer et al., 2023; Simpson, 2019). In ginkgo, spermatozoids are smaller (about 0.1 mm) and have about 1,000 flagella (Norstog et al., 2004, cited in Offer et al., 2023). Spermatozoid formation is completed within the pollen grain, after which the basal part of the male gametophyte ruptures, releasing the motile gametes into the archegonial chamber — a cavity between the nucellus and the female gametophyte.

In gnetophytes (Ephedra, Gnetum, Welwitschia), male gametophyte development resembles that of conifers, but with peculiarities: in Ephedra, the spermatogenous cell produces two unequal sperm cells, one of which fertilizes the egg cell, while the other fuses with the ventral canal nucleus in the archegonium, resembling double fertilization (Friedman, 1990; Yang et al., 2024). In Gnetum and Welwitschia, archegonia are absent, and fertilization occurs within the syncytial tissue of the female gametophyte, with both sperm cells fusing with different female nuclei (though the second fusion product degenerates) (Odintsova & Rugusova, 2013; Simpson, 2019).

In conifers, the pollen tube, having reached the neck of the archegonium, penetrates it and ruptures, releasing two sperm cells. One sperm cell fuses with the egg nucleus, forming a zygote (2_n_). The second sperm cell degenerates. In some species (e.g., Douglas fir), fertilization of a single egg by two sperm cells is observed, but such zygotes are usually non-viable (Mauseth, 2017). In pine, fertilization occurs in late spring – early summer, about one year after pollination (Raven et al., 2013).

In cycads and ginkgo, the spermatozoids, released into the archegonial chamber, actively swim using their flagella toward the archegonium neck. Chemotaxis (likely based on secretions from neck cells or the egg cell) directs their movement. One spermatozoid penetrates the egg cell, and nuclear fusion occurs (Offer et al., 2023). Typically, each archegonium yields one zygote, but several archegonia within a single female gametophyte may be fertilized (see Section 7.4).

Polyembryony — the development of more than one embryo from a single ovule — is common in gymnosperms (Raven et al., 2013; Beck, 2010). Two types are distinguished:

Despite the multiple initiation of embryos, mature seeds typically contain only one embryo; the others degenerate or are suppressed in development. In some species (e.g., some pines), up to 3–4% of seeds may contain two or more normally developed embryos, occasionally leading to multiple seedlings germinating from a single seed (Beck, 2010).

Polyembryony is also common in cycads. For example, in Microcycas, a single female gametophyte may contain up to 200 archegonia, many of which are fertilized (Odintsova & Rugusova, 2013; Offer et al., 2023).

The next section will cover the final stage of the reproductive cycle — seed development.

After fertilization, the zygote (2_n_) begins mitotic divisions. In most gymnosperms, the initial divisions of the zygote are accompanied by the formation of a suspensor — a cellular structure that pushes the developing proembryo deeper into the haploid endosperm (female gametophyte). The suspensor may be unicellular or multicellular, and in some conifers (e.g., pine), it branches, leading to cleavage polyembryony (Beck, 2010; Mauseth, 2017).

Sequence of embryogenesis in pine (Raven et al., 2013; Yakovlev et al., 2018):

In cycads and ginkgo, embryo development also includes a suspensor, but the number of cotyledons is two (in cycads — two cotyledons; in ginkgo — two, but one may be reduced) (Offer et al., 2023). In gnetophytes (Gnetum, Welwitschia), the embryo develops without a typical suspensor, from the syncytial tissue of the female gametophyte (Yang et al., 2024).

Parallel to embryogenesis, changes occur in the ovule integuments (Raven et al., 2013; Mauseth, 2017):

In a mature pine seed, beneath the hard coat is a thin film layer (nucellus remnant), then a massive white or yellowish tissue of haploid endosperm, and inside — the embryo with several cotyledons. In cycad and ginkgo seeds, the endosperm also occupies most of the volume, while the seed coat is differentiated into three layers: an outer fleshy layer (sarcotesta), a middle stony layer (sclerotesta), and an inner membranous layer (endotesta) (Offer et al., 2023; Simpson, 2019).

Seed maturation time varies among groups (Raven et al., 2013; Mauseth, 2017):

A gymnosperm seed, upon encountering favorable conditions (moisture, temperature, aeration), germinates. The germination process includes (Raven et al., 2013; Mauseth, 2017):

Unlike angiosperms, the nutrient reserves in the gymnosperm seed are depleted slowly, as the endosperm is voluminous. Gymnosperm seedlings are often able to survive in shaded and low-humidity conditions longer than many angiosperm seedlings (Torre et al., 2020).

The advent of the seed was one of the most important evolutionary advancements (aromorphoses) in the history of terrestrial plants (Graham et al., 2014). The seed provides (Raven et al., 2013; Simpson, 2019):

In gymnosperms, the seed is the dispersal unit, whereas in pteridophytes, the spore is the dispersal unit. The seed possesses incomparably greater adaptive potential, which underpins the success of seed plants in arid and cold regions.

In the final section, we will summarize and consider the significance of the gymnosperm life cycle for ecology and human economic activities.

Understanding the gymnosperm life cycle has not only theoretical but also significant applied value — for forestry, landscaping, nature conservation, and even the pharmaceutical industry. Gymnosperms (especially conifers) occupy about 39% of the world’s forest area and are the primary source of industrial timber, cellulose, paper, and many chemical products (Torre et al., 2020; Mauseth, 2017). Their reproductive biology determines the success of reforestation, breeding, and biodiversity conservation.