|GYMNOSPERMS: Evolution of the Seed
1) describe the basic model of alternation of generations as it applies to gymnosperms and angiosperms, noting similarities and differences with other plant groups
2) identify key structures in the gametophyte and sporophyte generations of select gymnosperms and angiosperms and indicate the ploidy of those structures
3) discuss the evolutionary adaptations in gymnosperms and angiosperms that distinguish them as being more advanced than their bryophyte and/or fern predecessors, including their success and expansion in terrestrial habitats and discuss features that continue to restrict their distribution
4) recognize an unknown specimen as being a gymnosperm or angiosperm based on key characteristics that distinguish it from other plant groups
“Gymnosperm” literally means “naked seed”. This is due to the fact that the seeds of gymnosperms are borne on structures that leave them exposed to the environment. As you will see, the seeds of angiosperms (flowering plants) are contained within protective structures that ultimately develop into fruits. As a group, gymnosperms consist of 4 phyla of seed-bearing plants which together represent multiple evolutionary lineages: Phylum Cycadophyta, Phylum Gingkophyta, Phylum Gnetophyta, and Phylum Coniferophyta. While there is plenty of reason to consider each phylum independently, we will only highlight some of the differences below while emphasizing innovations that distinguish these phyla as a unique group – premier among them being the evolution of the seed.
THE CYCADS: Phylum Cycadophyta
The cycads are quite possibly the closest living representatives of the dominant flora of the Jurassic era when dinosaurs roamed the Earth. Roughly 140 species remain (representing 11 genera) from this previously diverse and abundant plant group, most being restricted to tropical and subtropical habitats.
THE GINKGOS: Phylum Ginkgophyta
Like the cycads, the Ginkgophyta were once a diverse and widespread group, but today is represented by a just a single species: Ginkgo biloba. Ginkgo trees were planted extensively in the temple gardens and formal landscapes of the ancient Orient. Descendents of these plants are now widely cultivated as ornamentals, especially in the US and Europe - there are even several Ginkgos on campus! In fact, G. biloba owes its very existence to its horticultural value as it is no longer found in the wild. Ginkgos are “dioecious”, (“two houses”) meaning the sexes are separate. The male plant is the preferred sex in this case, as fruits borne on the female plant ripen with a rather offensive odor! If not as an ornamental, you are probably most familiar with Ginkgo biloba as the commercially available herbal supplement that is purported to enhance memory by increasing blood flow to the brain . . . could someone please pass some of that Ginkgo my way?!!
THE GNETOPHYTES: Phylum Gnetophyta
Of the gymnosperms, the gnetophytes are the group most closely related to angiosperms and likely represent the most evolutionarily advanced gymnosperm phylum. The presence of vessel elements in the xylem and the structural similarity of the gnetophyte strobilus to the angiosperm flower, suggests that the gnetophytes might represent a “bridge” group linking gymnosperms and angiosperms. (Recall: xylem tissue is composed of 2 cell types - narrow, elongated tracheids (found in both gymnosperms and angiosperms) and shorter, larger-diameter vessel elements (found in angiosperms and gnetophytes)).
The gnetophytes are represented by only 3 genera: Gnetum, Welwitschia, and Ephedra. Despite their evolutionary relatedness, the 3 genera differ dramatically in their appearance. Gnetum is found primarily in moist, tropical regions where they grow as both as trees and vines that use trees as support to reach the upper part of the forest canopy. Welwitschia grows in desert regions in southwestern Africa and can become quite large. While most of the plant body is buried in the sandy substrate, a large bowl-shaped structure extends above the surface which produces only 2 long, strap-like leaves that continue to increase in length throughout the lifetime of the plant. Ephedra is found in desert habitats throughout the world as a highly-branched shrub with jointed stems and very reduced leaves. Ephedra has a rich and controversial history for its uses as a medicinal plant. Ephedrine has long been a popular stimulant that also promotes weight loss, hence was found in many over the counter performance-enhancing and weight loss supplements. In the last few years, ephedrine came under increasing public scrutiny due to several deaths in otherwise healthy adults who were apparently using ephedrine-containing supplements. In November, 2003, the FDA introduced legislation into Congress to ban the sale of ephedrine-containing compounds in the US. In February, 2004, in compliance with the Dietary Supplement Health and Education Act (DSHEA), the FDA issued a final ruling prohibiting the sale of supplements containing ephedrine, stating that it “presents a significant or unreasonable risk of illness or injury when used according to its labeling or under ordinary conditions of use”.
THE CONIFERS: Phylum Coniferophyta
Commonly referred to as “conifers”, this group is without question the most diverse of the gymnosperm phyla representing upwards of 550 species in approximately 50 genera. Conifers include some of the most recognizable plants, such as the pines, spruces, firs, yews and junipers, and represent some of the most ecologically, historically, and economically important plant species in the world. Members of this phylum include both the tallest known plant (giant redwood, Sequoia sempervirens) and the oldest known plant (bristlecone pine, Pinus longaeva). Their economic value to humans is immense with products and applications as diverse as wood for construction, building materials, and heat; pulp and fibers for paper and textiles; taxol (from Pacific Yew) for the treatment of certain types of breast cancer; and gin (from Juniper) for recreational applications – just to name a few!
ALTERNATION OF GENERATIONS IN GYMNOSPERMS
Like the ferns, gymnosperms (as well as angiosperms) have a dominant sporophyte (2n) generation. In gymnosperms and angiosperms, the gametophyte (1n) generation is further reduced from what we observed in the ferns, and in some cases never exists independently from the sporophyte. The basic theme of the alternation of generations remains the same: plant spores germinate into gametophytes which produce gametes that fuse to form zygotes that develop into sporophytes that produce spores. So where’s the seed??
In “homosporous” (“same spore”) plants, a single kind of sporangium contains cells that undergo meiosis to produce spores that gives rise to gametophytes capable of producing both eggs and sperm. Gymnosperms and angiosperms are “heterosporous” (“different spores”) meaning that they produce two different kinds of spores that differ in size. Larger spores, or “megaspores”, produce female gametophytes while the smaller “microspores” produce male gametophytes. (Hint: if it helps you to remember, just think of animals – they produce 2 different kinds of gametes in which the female egg is always much larger (mega) than the male sperm (micro).
So, back to the question of “where’s the seed?”. Seed plants represent the most extreme form of heterospory in which the “seed” is the sum of the female (megasporangiate) tissues plus the embryo that results from fertilization of the egg. But how did we get there? Let’s back up: the megasporangium, which produces megaspores, is typically surrounded by a protective integument. Together, these structures (megasporangium + integument) are called an “ovule”. Inside each megasporangium, a cell undergoes meiosis to produce a megaspore. The megaspore divides by mitosis to form a female gametophyte. Unlike the gametophytes of ferns and bryophytes, the female gametophyte of seed plants is very small and is retained inside the megasporangium. As always, the female gametophyte produces the female gametes, eggs.
On the male side, microsporangia produce microspores (by meiosis) that grow by mitosis to form male gametophytes that produce sperm. In seed plants, the male gametophytes are known as “pollen”. Most, but not all gymnosperms have pollen that is dispersed passively by the wind. Perhaps because of the chance nature of this delivery system, gymnosperms (like other wind-pollinated plants) tend to produce large quantities of pollen to increase the odds that one of the pollen grains will make its way to a receptive ovule.
When a pollen grain successfully reaches an ovule, a “pollen tube” grows out of the pollen grain into the tissues of the ovule to deliver the sperm directly to the egg. Because of the direct delivery of sperm to egg, no water is necessary! At fertilization, chromosome number is restored to the diploid state and the plant is again in the sporophyte generation of its life cycle. The zygote (fertilized egg) grows by mitosis to form an embryo. The ovule is now referred to as a “seed” because it contains a viable embryo.
During its development, the gymnosperm embryo consumes the remaining female gametophyte tissue that is contained within the seed for its nourishment. When conditions are appropriate and the embryo is sufficiently mature, the seed will be dispersed into the environment. The embryo will continue to grow until it is so large that it breaks through the seed coat. A root will emerge first to stabilize the young plant, then leaves will follow shortly after to begin photosynthesizing and producing food to support its rapid growth rate.
A few terms used in the discussion of seed plants are defined below:
Ovule = megasporagium (including all structures contained within, such as the female gametophyte containing the eggs) + integument
Seed = mature ovule after fertilization (thus, contains the embryo)
Pollen = mature male gametophyte containing sperm
Although there is no real difference to the main theme of alternation of generations from the bryophytes and ferns, there are a couple of key distinctions regarding seed plants:
1) there is a clear distinction between male and female structures.
2) the microspores and megaspores are not released into the environment like they were in the mosses and ferns, thus the male and female gametophytes develop within the micro- and megasporangia, respectively.
3) the male gametophyte is released into the environment as pollen. The pollen grain is modified for travel, delivering the sperm directly to an ovule with an egg without the need for water.
4) the female gametophyte is retained on the adult sporophyte throughout.
5) after fertilization of the egg, the ovule develops into a “seed”. The seed is a structure modified for release into the environment where it serves to protect, nourish, and house the developing embryo until it is ready to become self-sufficient.
ANGIOSPERMS: Evolution of the Flower and Fruit
THE ANGIOSPERMS: Phylum Anthophyta
The Angiosperms, or flowering plants, consist of only 1 phylum, the Anthophyta, but represent the most diverse and evolutionarily advanced of any of the plants. You can find angiosperms just about anywhere on Earth in both aquatic and terrestrial habitats. The number of species in this group is estimated between 235,000 to over 260,000. Like gymnosperms, angiosperms produce seeds; but unlike gymnosperms whose seeds are “naked” or exposed, angiosperm seeds are enclosed within a “fruit”. Fruit and flower structures vary enormously due to a broad diversity of functional adaptations, so we reserve these as topics unto themselves. In this lab, you will have the opportunity to explore some of the diversity in the vegetative structures of this group. We will continue to emphasize similarities and differences in the angiosperm life cycle in comparison to other plant phyla with special attention given to the process of double fertilization which is shared by all angiosperms.
ALTERNATION OF GENERATIONS IN ANGIOSPERMS
You will find the overall life cycle of angiosperms to be very similar to gymnosperms with only a few modifications. Like the ferns and gymnosperms, angiosperms have a dominant sporophyte (2n) generation. Of the plants, angiosperms have the most dramatically reduced gametophytes with male (micro) gametophytes represented by a 3-celled structure and female (mega) gametophytes represented by a 7-celled structure. To help understand this complex life cycle, it may be useful to consider each sex separately.
Male: Microsporangia (located in the anthers) have 2n cells that undergo meiosis to produce 1n microspores (1 2n cell divides to produce 4 1n microspores). A single microspore divides once by mitosis to produce 2 cells: the tube cell and the generative cell (remember, each cell is 1n). The tube cell does nothing until pollination has occurred, but the generative cell divides once more to produce 2 x 1n sperm. Together, these 2 simple mitotic divisions represent the entire process of the 1n microspore germinating into the male (micro) gametophyte. The male gametophyte (pollen) is covered by a tough, protective wall which allows it to remain viable in the environment for long periods of time.
Female: Each megasporangium (1 per ovule) has a single 2n cell that undergoes meiosis to produce 4 1n megaspores. Of these 4 megaspores, 3 will die, leaving only 1 1n megaspore that will give rise to the female gametophyte. The end result of some complex cell division results in the formation of a 7-celled female gametophyte with 8 nuclei. (How? One of the cells of the female gametophyte contains 2 nuclei). One of the 7 cells contained within the female gametophyte is the egg.
Pollination and Double Fertilization:
Upon pollination (the transfer of pollen to the stigma of a flower), the male gametophyte grows a pollen tube that burrows into the female tissues of the flower and into the ovule, ultimately delivering 2 nonmotile sperm directly to the female gametophyte. One of the sperm will fuse with the egg to form the zygote. The second sperm fuses with the cell in the female gametophyte which already contains 2 nuclei. All 3 nuclei then fuse to form a triploid (3n) cell called the endosperm. The endosperm continues to divide by mitosis to produce a 3n tissue that serves as nourishment for the developing embryo. This characteristic of double fertilization (fertilization by 2 sperm) is shared by all angiosperms and the gymnosperm genera Ephedra and Gnetum. As with gymnosperms, the ovule is referred to as a “seed” after fertilization has occurred.
1) Examine the array of gymnosperms and angiosperms on display, including prepared slides, live or preserved specimens, various strobili, pollen, ovules, fruits, flowers, and seeds. For each of the specimens provided, can you distinguish roots, stems, leaves, megasporangiate vs. microsporangiate strobili?
2) If available, view prepared slides of cross sections of developing gymnosperm and angiosperm seeds. Draw and label the embryo (or zygote, depending on the developmental phase) and the female gametophyte or endosperm. What is the ploidy of the structures you see?
3) Examine gymnosperm and angiosperm pollen grains under the microscope and sketch them into your notebook. What variations do you see in pollen from gymnosperms compared to angiosperms?
1. a) Is the gymnosperm gametophyte capable of living independently? What is its source of nourishment (food)?
b) Is the gymnosperm sporophyte capable of living independently? What is its source of nourishment (food)?
2. a) Is the angiosperm gametophyte capable of living independently? What is its source of nourishment (food)?
b) Is the angiosperm sporophyte capable of living independently? What is its source of nourishment (food)?
3. The tallest trees in the world are the giant redwoods of California (Sequoia sempervirens) – a gymnosperm. Why don’t ferns grow to the same heights as redwoods? In what ways does stem structure differ between ferns and gymnosperms?
4. What evolutionary adaptations distinguish the gymnosperms as being more "advanced" than ferns and account for their success in a broader diversity of habitats? Are there features of this group that continue to restrict its distribution?
5. What evolutionary adaptations distinguish the angiosperms as being more "advanced" than ferns or gymnosperms and account for their success in a broader diversity of habitats? Are there features of this group that continue to restrict its distribution?