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# BIOL 108 Introduction to Biological Diversity
## Topic 16: Gymnosperms
> Lecture A2
> Yan-yin Wang
Introduction to Seed Plants
The origin of seed plants dates back to about 360 million years ago.
Seed : An adaptation of some terrestrial plants, consisting of an embryo packaged along with a store of food within a protective coat.
The domestication of seed plants began around 8,000 years ago, enabling permanent human settlements.
Identifying a conifer .
Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Phylogeny of Embryophyta (top) and # strobili in gymnosperms (bottom).
Qiu, T., Andrus, R., Aravena, M. C., Ascoli, D., Bergeron, Y., Berretti, R., ... & Clark, J. S. (2022). Limits to reproduction and seed size-number trade-offs that shape forest dominance and future recovery. Nature communications , 13 (1), 2381. Shared, Derived Traits of Seed Plants
1. Reduced gametophytes
Gametophytes develop within spore walls that are retained within the parent sporophyte tissues.
From basal to derived, the gametophyte is reduced in size and becomes dependent on sporophyte (sporophyte has more roles):
Non-vascular bryophytes possess independent, dominant gametophytes; a gametophyte-dependent sporophyte.
Seedless vascular plants have dominant sporophyte; an independent gametophyte.
Seed plants have dominant, independent sporophyte; a microscopic, dependent gametophyte.
> Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
> Reduced gametophyte (top) and comparisons of two generations in land plants (bottom).
Shared, Derived Traits of Seed Plants
2. Heterospory
All seed plants are heterosporous.
Megasporangia produce megaspores, which give rise to female gametophytes
Microsporangia produce microspores, which give rise to male gametophytes.
In seed plants, spores are retained within the sporophytes without being dispersed.
> Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Heterospory (top), seed cone (bottom left), and pollen cones (bottom right).
> https://www.pinterest.com/pin/506232814336269087/
> https://www.pinterest.com/pin/506232814336269120/
Shared, Derived Traits of Seed Plants
3. Presence of ovules
An ovule contains a megasporngium (2n), a megaspore (1n), and one or more protective integuments (2n).
Megasporangium produces a megaspore by meiosis, which is not dispersed and is enclosed in protective integument.
Within seed plants:
Gymnosperm ovules have one integument, and female gametophytes consist of thousands of cells.
Angiosperms ovules usually have two integuments, and female gametophytes are reduced to embryo sacs with a few nuclei and cells.
Ovules develop into seeds after fertilisation.
> Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Structure of an ovule (top), ovules in gymnosperm (bottom left) and in angioseprms (bottom right).
> Rudall, P. J. (2021). Evolution and patterning of the ovule in seed plants. Biological Reviews ,96 (3), 943-960.
> Zumajo-Cardona, C., Frangos, S., & Stevenson, D. W. (2021). Seed anatomy and development in Cycads and Ginkgo, keys for understanding the evolution of seeds. Flora ,285 , 151951.
Shared, Derived Traits of Seed Plants
4. Pollen
Microspores develop into male gametophytes within pollen grains.
Microspores, which are not dispersed, develop into pollen grains and produce tiny male gametophytes (1n).
Cell walls of pollen grains are coated with a layer of sporopollenin, which protects pollen against desiccation, UV, and physical damage.
Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Pollen in seed plants (top) and # SEM of pollens (bottom).
Lu, Y., Jin, B., Wang, L., Wang, Y., Wang, D., Jiang, X. X., & Chen, P. (2011). Adaptation of male reproductive structures to wind pollination in gymnosperms: cones and pollen grains. Canadian Journal of Plant Science , 91 (5), 897-906. Shared, Derived Traits of Seed Plants
4. Pollen
Pollen grain is not the same as male gametophyte
Outer layer rich in sporopollenin is produced by sporophyte.
Each pollen grain contains a male gametophyte, which has at least one generative cell and a tube cell that developed into the nuclei and pollen tube, respectively, after pollination.
Male gametophytes are transported to the ovule within the pollen grain.
Sperms are not released in the external environment for fertilisation. Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024).
> Campbell biology (4th Canadian ed.). Pearson.
Pollen (top) and # the development of pollen (bottom).
> Yeung, E. C., Oinam, G. S., Yeung, S. S., & Harry, I. (2011). Anther, pollen and tapetum development in safflower, Carthamus tinctorius L. Sexual plant reproduction ,24 , 307-317.
Shared, Derived Traits of Seed Plants
5. Seeds
A seed develops from a fertilised ovule and is composed of an embryo, nutrient supply, and outer seed coat.
A seed contains the next generation sporophyte surrounded by the tissues of the parent gametophyte.
Zygote is still developed from gametophytes as in basal land plants.
Sizes of seeds vary substantially, which is primarily influenced by nutrient storage derived from gametophyte.
Example: an orchid seed is less than 1 g and a coco de mer seed is weighted up to 18 kg.
> Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Seed (top) and seed # development in Plantago ovata (bottom).
> Phan, J. L., Cowley, J. M., Neumann, K. A., Herliana, L., ODonovan, L. A., & Burton, R. A. (2020). The novel features of Plantago ovata seed mucilage accumulation, storage and release. Scientific reports ,10 (1), 11766.
Pollination
Definition : A necessary reproductive process for fertilisation where pollens are transferred to part of a seed plant containing the ovules.
Pollination is a significant evolutionary shift in fertilisation:
In seed-less plants, single-celled flagellated sperms require water to swim to the archegonium of the female gametophyte.
In seed plants, the entire pollen grain along with the male gametophyte is dispersed. If the pollen germinates, a pollen tube is developed to discharge sperm nuclei into the female gametophytes.
Benefits:
Sporopollenin can protect male gametophyte.
Pollens can survive long-distance transportation by biotic or abiotic agents.
> Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Steps of pollination (top) and # image recognition for pollination (bottom).
> Hiraguri, T., Kimura, T., Endo, K., Ohya, T., Takanashi, T., & Shimizu, H. (2023). Shape classification technology of pollinated tomato flowers for robotic implementation. Scientific Reports ,13 (1), 2159.
Seed Dispersal
Seeds instead of spores are responsible for the dispersal stage in seed plants.
Evolutionary advantages of seeds over spores:
Seeds have protective coats to protect the embryo.
Seeds have energy supply of store food and can geminate in the dark.
Trade-off:
Larger seeds with more stored energy are produced in lower quantity.
Smaller seeds with less stored energy are produced in larger quantity.
Seeds have additional structures (e.g., hooks) for dispersal.
> Traveset, A., Heleno, R., & Nogales, M. (2014). The ecology of seed dispersal. In Seeds: the ecology of regeneration in plant communities (pp. 62-93). Wallingford UK: CABI.
> # Biotic and abiotic agents for seed dispersal.
Phylogeny of Land Plants
Fertilisation becomes independent of water in seed plants.
Phylogeny of Embryophyta and the relationship with water (required!).
Charophytes
Chlorophytes
Liverworts
Mosses
Hornworts
Lycophytes
Monilophytes
Angiosperms
Gymnosperms
> Land plants
Fertilization Dispersal
Water
Water
Water (sperm)
Water
Water
Water (sperm)
Water (sperm)
Water (sperm)
Water (sperm)
Air (pollen)
Air (pollen)
Air (spores)
Air (spores)
Air (spores)
Air (spores)
Air (spores)
Air (seeds)
Air (seeds)
Seed plants @ Neil Harris
Green algae
Non-vascular plants
Vascular plants Origin and Diversification of Seed Plants
Seed plants consist of two clades:
Gymnosperms with about 800 extant species
Angiosperms with more than 250,000 extant species
The first seed plants documented in the fossil record dates to about 360 million years ago (e.g., progymnosperms).
Earliest fossil evidence of extant gymnosperms dates to about 305 million years from the Carboniferous Period.
Angiosperms began to replace gymnosperms in ecological systems in the Cretaceous Period.
Extant conifers are dominant in northern latitudes such as the boreal forests in Canada.
> Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
> Phylogeny of Embryophyta (top), # fossil leaves (bottom left) and stump (bottom right) of the progymnosperm Archaeopteris .
> https://ucmp.berkeley.edu/seedplants/progymnosperms.html
Introduction to Gymnosperms
Gymnosperms: A group of vascular plants with naked seeds
Seeds are exposed on sporophylls that form strobili.
Feature of life cycle in gymnosperms:
Sporophyte (2n) as a mature tree is the dominant generation.
A typical gymnosperm has both pollen and ovulate cones.
Gymnosperms develop seeds from fertilised ovules.
Pollens containing male gametophytes are transferred to ovules before sperms are released.
> Lu, Y., Jin, B., Wang, L., Wang, Y., Wang, D., Jiang, X. X., & Chen, P. (2011). Adaptation of male reproductive structures to wind pollination in gymnosperms: cones and pollen grains. Canadian Journal of Plant Science ,91 (5), 897-906.
# Male cones of gymnosperms. Life Cycle of a Conifer
Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
Is the haploid stage (generation) multicellular? Diversify of Gymnosperms
Gymnosperms contains four phyla with uncertain phylogenetic relationships, but gymnosperms form a monophyletic group.
Four phyla:
Cycadophyta with about 130 species
Gingkophyta with a singular species
Gnetophyta with about 75 species
Coniferophyta with about 600 species
> Liu, G. Q., Lian, L., & Wang, W. (2022). The molecular phylogeny of land plants: progress and future prospects. Diversity ,14 (10), 782.
> # Phylogeny of major land plant taxa.
Phylum Cycadophyta
A division with a long evolutionary history date back to Mesozoic Era but are now limited to small populations in the tropics and subtropics.
Cycads are often mistaken as ferns.
Characteristics:
Cycads are dioecious
Slow growing and long living
Presence of flagellated sperms, which is only seen in Ginkgo biloba among seed plants.
> Clugston, J. A., & Kenicer, G. J. (2022). Sexing cycadsa potential saviour. Nature Plants ,8(4), 326-327.
# Male and female cones of Cycas media in Australia (top) and palm-like leaves (bottom).
> Simpson, M. G. (2019). Plant systematics . Academic press.
Phylum Ginkgophyta
A division with Ginkgo biloba as the only extant species originated from China.
All populations of G. biloba are associated with human habitats, and no wild populations exit.
Characteristics:
G. biloba is dioecious
G. biloba is deciduous
Presence of fan-shaped leaves
Boateng, I. D., Soetanto, D. A., Li, F., Yang, X. M., & Li, Y. Y. (2021). Separation and purification of polyprenols from Ginkgo biloba L. leaves by bulk ionic liquid membrane and optimizing parameters. Industrial Crops and Products , 170 , 113828.
Phylogeny of Embryophyta.
Steinthorsdottir, M., Jardine, P. E., Lomax, B. H., & Sallstedt, T. (2022). Key traits of living fossil Ginkgo biloba are highly variable but not influenced by climateImplications for palaeo-pCO2 reconstructions and climate sensitivity. Global and Planetary Change , 211 , 103786. Phylum Gnetophyta
A small group of gymnosperms recovered from molecular evidence.
Gnetophyta contains about 75 species in three genera: Ephedra , Gnetum , and
Welwitschia .
Gnetophytes can be found in tropical and desert environment.
Characteristics:
Xylem contains vessel elements
Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2024). Campbell biology (4th Canadian ed.). Pearson.
# Diversity of Gnetophyta (top) and the vessel elements in xylem of
Ephedra nevadensis (bottom).
Jacobsen, A. L. (2021). Diversity in conduit and pit structure among extant gymnosperm taxa. American Journal of Botany , 108 (4), 559-570. Phylum Coniferophyta
The largest division of gymnosperms with about 600 largely evergreen species.
The phylum is named after their cones.
Conifer are dominant trees in boreal and alpine regions in Canada.
Coniferophyta includes the longest-living tree, the redwoods Sequoia .
Conifer rely on wind to pollinate, and the rate of reproduction is relatively low.
Characteristics:
Xylem lack vessel elements
> https://www.scientificamerican.com/article/meet-the-giant-sequoia-the-super-tree-built-to-withstand-fire/
# Thes redwood Sequoia (top) and fossil conifer cone (bottom).
> Gee, C. T., Dayvault, R. D., Stockey, R. A., & Tidwell, W. D. (2014). Greater palaeobiodiversity in conifer seed cones in the Upper Jurassic Morrison Formation of Utah, USA. Palaeobiodiversity and Palaeoenvironments ,94 , 363 -375.
Some Reflective Questions (Not an Exhaustive List for Exam!)
What are the synapomorphies of seed plants?
Can we find some character traits shared between seed and seedless plants?
How the life cycle of a typical gymnosperm compared to other land plants?
How the life cycle in gymnosperm fit in the evolutionary history of land plants?
How may we interpret the presence of vessel tissues in the xylem of g netophytes?