Plant Lifecycle Evolution

Introduction to the Plant Life Cycle

All sexually reproducing organisms have a life cycle that alternates between a haploid and diploid life stage. In many animals, especially vertebrates, the haploid life stage (sperm and egg) is so short-lived that we hardly consider it, as the dominant part of the life cycle is clearly the diploid.

In plants, things are very different. The diploid life stage is called the sporophyte, and in some lineages of plants this is the dominant life stage. However, in some plants, such as mosses, the dominant life stage is actually the haploid life stage, which is called the gametophyte. Only the gametophyte life stage in plants make gametangia - the sexual organs where egg and sperm are produced. Many plant researchers like to joke that sporophytes are asexual - they are not capable of having sex and cannot be male or female. Only the gametophytes can be male or female, and only the gametophytes have sex! Land plants also have a special step in their life cycle that vertebrates do not: the spore. Spores are produced by the sporophyte and give rise the gametophyte, in much the same way that the zygote (i.e. the seed, in seed plants) gives rise to the sporophyte.

There is a nice symmetry to the terms here:
During the haploid life stage: the gametophyte makes gametangia, where gametes are produced.
During the diploid life stage: the sporophyte makes sporangia, where the spores are produced

Evolution of the Land Plant Lifecycle

Land plants, or Embryophytes, are a monophyletic clade that includes flowering plants, conifers, ferns, lycophytes, mosses, hornworts, and liverworts. Fittingly, most of these plants live out of water, although some have returned to a completely aquatic lifestyle.

It is hypothesized (de Vries & Archibald, 2018) that all land plants evolved from a shared charophyte-like algal ancestor. Charophyte algae is photosynthetic and has a multiceulluar, haploid adult life stage that forms two sex organs: the male antheridium and female oogonium. When sperm from the antheridium and an egg from the oogonium fuse, they create a single-celled diploid zygote which immediately undergoes meiosis to create more haploid adults.

[ancestral life cycle diagram]

The name for land plants, Embryophytes, comes from a key change that occurred in the ancestor of all land plants. The diploid zygote began to delay meiosis and became multicellular - in other words, there is now a resting embryo stage in the diploid life phase. Other characteristics that distinguish Embryophytes include multicellular reproductive structures, a cuticle, and thick-walled trilete spores (Judd et al. 2016). These characteristics may have assisted the ancetral Embryophyte to transition to land.

All lands plant life cycles follow this general sequence of steps:

1. the haploid gametophyte life stage undergoes mitosis to form archegonia and antheridia, where eggs and sperm are produced
2. an egg is fertilized by a single sperm to form a diploid zygote sporophyte
3. once mature, the sporophyte creates specialized diploid structures called sporangia, which may be housed within further specialized structures
4. diploid sporocytes, or spore ‘mother cells’, are created by mitosis within the sporangia
5. each sporocyte performs two meiotic divisions to create four haploid spores
6. spores develop into new haploid gametophytes by mitosis.

The meiotic divisions in step 5 are the only opportunity for recombination to occur throughout the life cycle. All subsequent haploid structures that arise from a spore (i.e., gametophyte, archegonia, antheridia, gametes) are genetically identical to the original spore.

Bryophytes

Bryophytes refer to a group that includes mosses, hornworts, and liverworts. The monophyly of this group has been contentious for a long time, but recently Bryophytes have been reported to be monophyletic, with hornworts sister to liverworts and mosses (Su et al. 2021).

Each group has a unique life history and evolutionary ancestry that distinguishes them. However, they are united by a similar life cycle, with a dominant, long-lived haploid life stage and a shorter, ephemeral diploid life stage.

Some Bryophytes are monoicous, which means their gametophyte life stage is capable of producing both female and male gametangia, and therefore both egg and sperm. Others are dioicous: their gametophyte are either male or female, and each individual can only make sperm or eggs, but not both.

The sperm from one gametophyte will fertilize the egg of another and produce a zygote sporophyte, which develops as a superficial structure growing from the gametophyte (in dioicous species, this only occurs on the female gametophytes). Sporangia develop within the sporophyte and release spores when mature. The spores are dispersed into the surrounding environment and develop into new gametophytes. The sporophyte then dies, but the gametophyte parent continues living, and may reproduce again

Pteridophytes

Pteridophytes are a paraphyletic clade that includes lycophytes and ferns, but excludes seed plants. Lycophytes and ferns are quite different from each other, but like Bryophytes they share similarities between their life cycles.

Some Pteridophytes, particularly the Lycophyte family Lycopodiaceae and most ferns, are homosporous. This means that their sporophytes produce one kind of spore, which may grow into a male, female, or hermaphroditic gametophyte. The gametophyte, sometimes also referred to as a prothallus, is a small, flat photosynthetic structure that produces archegonia and/or antheridia. When mature, flagellated sperm are released by the antheridia and require water to swim to an archegonium, where fertilization of an egg creates a new sporophyte zygote. The gametophyte supports the developing sporophyte nutritionally for a time, before it eventually dies away and the sporophyte lives independently

The rest of the Pteridophytes, including the Lycophyte families Isoetaceae and Sellaginaceae, and the fern order Salviniales are heterosporous. Their sporophytes produce two kinds of spores. The smaller microspore gives rise to a male gametophyte, and the larger megaspore gives rise to a female gametophyte. Heterosporous pteridophytes also have endosporic germination: the gametophytes germinate and grow within the spore, and are not free-living. When mature, the spores are released while still containing the highly reduced gametophytes, which quickly undergo fertilization to form a new zygote (Kumar, 2001).

Spermatophytes

The Spermatophytes, or seed plants, are the most diverse lineage of land plants. Most of that diversity belongs to angiosperms, the flowering plants - the rest of spermatophytes are the gymnosperms. As the name implies, a key innovation in the evolution of Spermatophytes was the development of the seed (spérmatos in Greek = seed). The sporophytes and gametophytes of seed plants have become highly modified compared to the rest of land plants, and thus the life cycle is quite different. The spore has essentially been lost in Spermatophytes and instead the releasing a spore to create a gametophyte, the sporophyte life stage in seed plants develops specialized reproductive structures, cones in gymnosperms and flowers in angiosperms, which will give rise to the gametophyte life stages.

Some Spermatophytes are monecious (note: although this word appears very similar to monoicous, it is a different term). Monecious species produce a sporophyte with one kind of flower or cone that can produce both pollen (a new structure in this lineage, which delivers sperm) and eggs.

Other Spermatophytes are dioecious, with two kinds of flowers or cones. Each sporophyte can only make one kind (in gymnosperms, either a pollen cone, which only produces sperm, or an ovulate cone, which one produces eggs; in angiosperms either a carpellate flower, which can only makes eggs or a staminate flower, which can only make sperm).

Many flowering plants use a form of fertilization known as double fertilization. Flowers have a pistil (consisting of a stigma, style, and ovary) and/or stamens (consisting of an anther and filament). Each ovule contained within an ovary produces a single megasporocyte, which will undergo meiosis to produce four recombinant megaspores. Only one megaspore will survive, while the other three degenerate (Boavida and McCormick, 2010). The ovule matures until it forms the megagametophyte, or embryo sac. Microsporocytes are generated within the anther, making it analogous to the microsporangium in other lineages. Each microsporocyte undergoes meiosis to form four recombinant microspores, each of which matures into a single pollen grain, a highly reduced microgametophyte. At maturity, a pollen grain consists of one vegetative cell and two sperm cells, each formed via mitosis of the original microspore. Mature pollen grains are released from the anthers and land on the stigma of a flower. The pollen grain then germinates, growing a pollen tube down the style and into the ovary, where it enters the micropyle and releases its two sperm. One sperm fertilizes the egg to produce a diploid zygote, while the other fertilizes the diploid central cell within the embryo sac to produce the triploid endosperm which will nourish the developing zygote (Mascarenhas 1989); the zygote and endosperm together comprise the seed. The seed is retained on the sporophyte while the embryo develops within, and once mature, is released and will germinate into a new sporophyte. Unlike the life cycles described previously, the reduced gametophytes of seed plants do not give rise to multiple archegonial or antheridial structures. Rather, each gametophyte produces just a single egg or a single sperm (two identical sperm packaged together, actually) that will engage in sexual reproduction

A note on terms

Some of these terms, especially monoicy/dioicy and monecy/dioecy can be very confusing - however they are worth your time to learn because they refer to very specific differences and this makes them incredibly useful. The terms were introduced alongside the lineages that they are most useful for referring to, but it is a fun exercise to test your knowledge by asking which other lineages they encompass. For example, dioicous species have separate female and male gametophytes, which means that all Spermatophytes are dioicous. Likewise, all heterosporous pteridophytes are also dioicous. By contrast, monecy and dioecy only apply to Spermatophytes, because they refer to the specialized reproductive structures of the sporophyte generation, which only exist in gymnosperms (cones) and angiosperms (flowers).

You may see the term dioecious applied to non-spermatophyte lineages. In particular, this term is often used in older literature when describing Bryophytes. The intent is to refer to a dioicous life cycle, however this is an incorrect use of the term and you should automatically translate it to 'dioicous' in your head when you encounter it used in this way.

monoicous vs. dioicous = single vs. two gametophyte sexes
homosporous vs. heterosporous = same vs. different spores
monecious vs. dioecious = single vs. two flower types

Image Downloads

Images are free to download and use in presentations, for teaching purposes, or study materials. When re-using, please credit the illustrator, Elissa Sorojsrisom, and when possible linke back to this resource (https://elissasoroj.github.io/Understory/).

Life Cycle Comparison	Sexual Systems	Simplified Life Cycles
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Additional Resources

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of plants. Macmillan, 2005. An excellent text on Plant Biology, but difficult to find/often expensive

De Vries, Jan, and John M. Archibald. "Plant evolution: landmarks on the path to terrestrial life." New Phytologist 217, no. 4 (2018): 1428-1434. Open-access article covering early-evolution of plants (transition to land). Download