Spore Plants Lecture Notes

Spore Plants

Gametophyte and Sporophyte

• Gametophyte: A haploid (n) gamete-forming phase in the life cycle.
• Sporophyte: A diploid (2n) spore-forming phase in the life cycle.
• Alternation of Generations: Refers to the life cycle pattern involving both haploid and diploid phases.

Defining the Plant Kingdom

What defines members of the Plant Kingdom (mosses, horsetails, ferns, conifers, flowering plants):

1. Shared Green-Algal Ancestor: Plants share a common ancestor with green algae.
2. Alternation of Generations: The life cycle includes both a haploid gametophyte and a diploid sporophyte stage.
3. Complex Reproductive Organisms:
   • Archegonia: Structures that produce and contain eggs.
   • Antheridia: Structures that produce sperm.
   • Sporangia: Structures that produce spores.
4. Development of Multicellular Embryos: Embryos are surrounded by sterile haploid tissues, forming a protective structure. This is why plants are also called "Embryophyta."
5. Nutrient Transport: Embryophyta developed placental tissue ("transfer cell") for nutrient transport between haploid and diploid phases.

• These adaptations provide protection from harsh environments, which is key for survival on land.

Spores: Survival and Dispersal

Spores of green algae:

• Form part of a survival strategy; for example, the zygote of Chlamydomonas can go dormant.
• Minor importance for dispersal.
• Stage in sexual reproduction & gene-recombination.
• Relatively rare.

Spores of plants:

• Vital dispersal stage – frequently found in fossils!
• Small and light for wind transport.
• Stage in sexual reproduction & gene-recombination.

Major Groups of Spore Plants

Two major groups of “spore-plants”:

• Non-vascular Bryophytes.
• Vascular ferns and fern-allies.

Bryophytes comprise 3 phyla/classes:

• Mosses: 14,000 species.

• Liverworts: 8,500 species.

• Hornworts: 100 species.

• The three phyla of Bryophytes evolved independently from ancestral plants and are not closely related, although they share similar life cycles.

• Bryophytes are not thought to have given rise to the vascular plants, i.e., they comprise a distinct lineage of non-vascular plants.

Bryophytes: First Land Plants?

• It is likely that Bryophytes (and especially liverworts) were among the first land plants.
• Poor preservation (little cuticle, no vascular elements) of non-vascular plants biases the fossil record towards early vascular plants (e.g., Rhynia sp, Cooksonia sp.).
• It has been speculated that some Bryophytes may have been present on land much earlier than 470 million years ago (mya).

Moss Reproductive Biology

• Haploid gametophyte dominates life-cycle.
• "Conspicuous" sporophyte.
• Alternation of generations.
• Anisomorphic sporophyte / gametophyte.

Gametangia

Gametophyte has multicellular sex organs (gametangia):

• Male sex organs: antheridia (antheridium sing.).
• Female sex-organs: archegonia (archegonium sing.).

Archegonium Structure

• The archegonium is a bottle-like structure. At the base, there is a cavity containing a single egg.
• Archegonia secrete sugars, proteins, or acids to attract sperm.
• Archegonia of mosses, ferns, and gymnosperms are similar, suggesting a single, common ancestral structure.
• Structure archegonia should be seen in context protection embryo on land.

Antheridia Structure

• Male antheridia are bag-like structures containing multiple sperm cells.
• At maturity, the antheridium bursts open, releasing the sperm which move with flagella.
• An antheridium produces 100’s to 1000’s sperms
• Gametes can only swim a few cm and need water to move from antheridium to archegonium. External water essential for moss reproduction. Hence, mosses are mostly in moist/shaded sites.
• Inbreeding is common, resulting in a relative lack of genetic diversity.

Fertilization and Sporophyte Development

• Fusion of haploid sperm with a haploid egg creates a diploid zygote.
• The zygote develops rapidly into an embryo (mitosis), which is the sporophyte.
• The zygote is nurtured by gametophyte tissues (Embryophyta!).
• The expanding sporophyte grows out of the archegonium, creating a stalk, with a sporangium at its tip.
• The sporophyte is often photosynthetic but otherwise feeds on the gametophyte.

Spore Dispersal and Protonema Formation

• Spores are spread by wind, or rarely insects.
• A spore landing on a suitably damp place germinates & forms filament-like, fine threads, that branch (= protonema).
• Protonema is juvenile gametophyte which develops root-like rhizoids and shoot-like structures that make up the mature gametophyte
• Protonema strikingly resembles filamentous green algae. Some authors consider that this is how “plants” (life on land) originated, and that the whole development of ever more complex gametophytes started from these structures. Spores are around $20 \mu m$ in diameter.

Moss Gametophyte Structure

• Stem-like structures.
• Epidermis with very thin (or no) cuticle allowing water uptake!
• Central cylinder with few or no fibres (no strength).
• Water is mostly transported via the surface, i.e., a fluid film on the outside of the moss.
• In some mosses, primitive transport cells called hydroids and leptoids, analogous to the xylem and phloem of higher plants, are found near the center of the stem. However, mosses lack true xylem/phloem, that is why bryophytes feel soft and pliable.
• Due to convergent evolution (Homologous = due to common ancestry)

Rhizoids and Mycorrhizal Associations

• The stem-like axis is anchored in the soil by rhizoids (root-like structures).
• Each rhizoid consists of a few cells that may also absorb some water.
• Most mosses have mycorrhizal fungi associated with them, which aid with nutrient uptake.
• Lack of vascular tissue precludes efficient water transport to above ground parts of the plant. This explains why there are no tree mosses.

Bryophyte Characteristics

The Bryophytes constitute a separate lineage of plants with:

• Multicellular gametangia [i.e., archegonia and antheridia] and sporangia.
• The zygote is retained within the female archegonium, where it develops into an embryo [true embryophyte].
• The sporophyte is attached to the gametophyte and depends fully/partially on it for nutrition.
• Conspicuous “alternation of generations”.
• Dependency on external water for reproduction [sperm with flagella].
• All cells/tissues directly absorb water/nutrients from the environment [lack of xylem and phloem transport cells].

Early Vascular Plants

• The early stages of land-plant evolution are characterized by the development of primitive, internal conducting systems.
• Rhynia (and related species) were extremely successful in colonizing the land because of their vascular systems.

Rhynia

• Rhynia and related sp. managed to grow upwards and reach “dazzling” heights of up to 10-20 cm as they could transport water to such heights!
• Result: outcompeting of shorter species
• Vegetation of early, short plants is known as “Lilliputian flora”.

Poly-sporangiophytes

• Bryophyta formed just one sporangium per sporophyte stem.
• Early vascular plants evolved branching of the sporophyte stem, which therefore could form multiple sporangia.
• Dichotomous branching (bifurcation).
• These plants are known as “poly-sporangiophytes”, this includes Rhynia and related species, as well as all ferns, Equiseta, Lycophytes, Gymnosperms and Angiosperms

Roots

• Early land plants lacked (true) leaves, roots, and flowers.
• Origins true roots are still “mysterious”.
• Both early vascular and non-vascular plants had rhizoids / root hairs which played a (minor) role in anchorage as well as in water and nutrient uptake.
• Rhizoids are likely to be evolutionary linked to the hold-fast of some algal species.

Root Hairs and Multicellular Roots

• Rhizoids at interface plant and soil. These take the form of root hairs & can be present in both sporophyte and gametophyte
• True multicellular roots in sporophyte.
• The earliest evidence of roots comes from 410 million-year-old clubmosses.
• It has been hypothesized that some apical shoot meristems evolved to grow downwards, thus creating a true, multicellular root.

Ferns and Fern-Allies

• Polypodiophyta (ferns).
• Lycophytes (club mosses).
• Equisetophyta (horsetails).

Origin and Evolution

• Clubmosses, horsetails, and ferns originated in the Devonian ~400 MYA
• The clubmosses and horsetails are called the fern-allies because of the reproductive strategy which is similar to that of ferns
• Clubmosses and horsetails are sometimes named “living fossils”, with most of these species long extinct
• Fern-allies dominated the world (300 million years ago) in the “Carboniferous” (~ 350-290 MYA), when coal deposits were formed.

Polypodiophyta (Ferns)

• About 97% of extant “ferns and fern-allies” are ferns.
• About 12,000 fern species in the world.
• Ferns evolved only “around” 370 – 400 MYA, however large numbers of new species evolved (radiation event) during the last 140 my.
• So, many extant Polypodiophyta/ferns are modern organisms, that evolved “relatively” recently, exploiting a niche among flowering plants.
• Ferns are present in virtually any habitat, although they are mostly present in moist environments.
• Reproduction via swimming sperms. Agal ancestry clearly visible!

Fern Life Cycle

• The sporophyte is the conspicuous/dominant phase of the life cycle of the fern, i.e., what we recognize as a fern, is the sporophyte (2n).
• Prothallus
• Spore
• Meiosis
• Sorus (cluster of sporangia)
• Rhizoids
• Archegonium
• Antheridium
• Mature GAMETOPHYTE (N)
• SPOROPHYTE (2N)
• Sperm
• Fertilization
• sporangium
• Adult sporiphyte
• Rhizome
• Egg
• Embryo
• Leaf of young sporophyte
• Gametophyte

Comparing Sporophytic Phases

Comparing the sporophytic (2n) phase of the life cycle:

• Green algae such as Chlamydomonas sp; single cell, the zygote.
• Green algae such as Ulva sp; isomorphic structure.
• Mosses; parasitic, multicellular, structure comprised of a foot, stem and one sporangium.
• Ferns; large, multicellular, and most conspicuous phase of life cycle. A clear evolutionary trend!

Evolution Trend

• Sporophyte can grow large thanks to vascular transport system
• Why does gametophyte not similarly grow large (the relevant genes are there)? Gametophyte must be close to ground for gametes swimming through water-film

Sori

• Clusters of multiple sporangia called sori.
• Sporophyte has on the lower side of the leaf small, circular, rust-colored patches of powdery looking material. These are clusters of sex-organs (sporangia) called sori (single sorus).
• Spores are key for dispersal; a tree fern will produce more than $1 \times 10^{12}$ spores in its lifetime.

Gametophyte Development

• Spores germinate to form a green heart-shaped gametophyte.
• Gametophytes are small (up to 1 cm), mostly one cell thick, comprising photosynthetic cells and rhizoids.

Sexual Reproduction in Ferns

• Antheridia and archegonia are formed on the gametophyte.
• Sperms swim through surface water from antheridia to archegonia, attracted by sugars.

Sporophyte and Gametophyte Interaction

• Gametophyte requires water for the sperm to be able to move. Therefore, the gametophyte is a ground-hugging organism, while the sporophyte is able to develop in a much larger, less water-dependent structure.
• Distribution of the sporophyte is limited by the gametophyte!!!

Fern Characteristics

• Seedless-plants with developed vascular system.
• Dominant sporophyte (unlike mosses and green algae).
• Multiple sporangia per sporophyte.
• Sperm with flagella depend on water for fertilization.

Killarney Fern

• Sporophyte (Rare) Few fertile spores produced
• Gametophyte (less rare) Reproduces asexually (fragments)
• In this species, the reproductive cycle appears to be broken, with isolated populations of either gametophytes OR sporophytes.

Lycophytes (Clubmosses)

• Seedless Vascular Plants; Lycophytes
• Lycophytes; some 1000 species of clubmosses.
• Clubmosses: ancient lineages going back to the Devonian period.
• In the early Devonian (400 MYA) two separate lineages of Lycopods;
  • small herbaceous, creeping plants very similar to modern Lycopods
  • a lineage of extinct, woody trees including Lepidodendron.

Extant Lycophytes Characteristics

• True vascular plants, with a prominent sporophyte (2n).
• Fern-allies.
• It is thought that the Lycophytes evolved independently from ferns and other fern-allies.
• In terms of sexual reproduction; Lycophytes represents a major step forward in specialized reproductive developments. Heterospory rather than homospory.
• Looks like a moss, but isn’t a moss! Fern-ally with dominant vascular sporophyte
• Even the great Linnaeus got this one wrong, on two counts; 1) vascular, and 2) sporophyte

Life Cycle Clubmoss; heterospory

• Distinct male and female gametophytes
• Separate male and female gametophytes arising from distinct male (micro) and female (mega) spores

Heterospory

• Heterospory: production of spores of different sizes (and different sexes) in distinct sporangia.
• Megaspores are large, but few are produced. The larger size limits dispersal. However, materials stored inside provide nutrition for the female gametophyte (and ultimately the new sporophyte embryo).
• The female gametophyte is becoming dependent on the sporophyte for nutrition (compare with moss where sporophyte depends on gametophyte!!!)
• Microspores are tiny and have better dispersal capabilities. These will develop into small, male gametophytes, consisting of little more than an antheridium that produces sperm.
• The heterosporous condition represents a functional tradeoff between dispersal, spore number, and stored resources.
• Megaspores with a ten-fold larger diameter have 1000 times the volume of microspores, showing this tradeoff.

Clubmoss Reproduction

• Many Lycophytes are heterosporous, megaspores, and microspores are produced in separate sporangia.
• In a megasporangium, 4 megaspores are produced through meiosis, these will dispersed and grow out to form female gametophytes.
• In a microsporangium thousands of microspores are produced through meiosis and these will be dispersed and grow out to form male gametophytes.
• In many Lycophytes the spore-bearing leaves (sporophylls) are grouped in cones or strobili at the shoot tips.

Endosporic Development

• A second advance in reproduction is endosporic development of gametophytes.
• Gametophytes start to develop within the spore wall using stored materials (even before dispersal).
• This shortens the time gametophytes need to reach sexual maturity and thereby reduces their reliance on environmental conditions.

Gametophyte Development

• The microspore develops an antheridium comprised of sterile cells surrounding sperm cells.
• The megaspore develops into multicellular female gametophyte with one or more archegonia.
• Endosporic development means the gametophytes become less nutritionally dependent upon the environment!
• Endosporic development means the gametophytes become nutritionally dependent upon the dominant sporophyte!

Life Cycle Comparison: Homospory vs. Heterospory

• Homospory (Mosses, Ferns): One type of spore gives rise to a bisexual gametophyte.
• Heterospory (Clubmosses): Microspores give rise to male gametophytes, and megaspores give rise to female gametophytes.

Key Developments in Lycophytes

The two “ KEY DEVELOPMENTS” Lycophytes

1. Endospory; the development (cell division) of the gametophyte (n) often begins before spores are released by sporophyte (2n) (i.e. the gametophyte starts to develop within the sporophyte).
2. Heterospory (which has evolved several times in several groups of land plants, from mid Devonian onwards)

• Heterospory and endospory are intermediate stages in the evolution of reproduction; stages that can both be observed in extant plants, as well as in fossils.

Paradox of Heterospory

• There is a paradox about heterospory; because of the limited dispersal of the female megaspore (big & heavy) dispersal of the species is limited (i.e. dispersal is restricted, but outbreeding is enhanced).
• Among living spore plants, heterospory is most common among aquatic plants, where dispersal of the megaspore is easier.

Lepidodendron

• Lepidodendrum, a giant extinct clubmoss
• In Lepidodendron, microsporangia produced microspores which were shed.
• In Lepidodendron, a megasporangia produced a single megaspore which was not shed.
• Development of the female gametophyte (endospory) occurred while the spore was attached to the sporophyte.
• Fertilization also took place while the megaspore was still attached to the tree.
• Following fertilization, a sporophyte embryo developed within the female gametophyte’s tissue, which in turn was still attached to the sporophyte.

Seed Habit Development

• Thus, a simple delay in spore dispersal protects the process of fertilization and embryo development from the hostile environment.
• This is the start of the development of the SEED-habit. From here on, dispersal of fertilized eggs (named seeds!) rather than haploid spores.
• The development of a seed is a major innovation distinguishing the seed plants from seed-less spore plants, a development that will dramatically reduce dependency on (hostile) environmental conditions.
• In the Gymnosperms (spruces, pines etc.) we will see the continued development of the seed to the prime dispersal and stress survival stage in the life cycle of the plant.