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Alternation of Generations Alternation of Generations GOALS • Be able to draw and explain a simple life cycle exhibiting alternation of generations • Identify the reproductive structures as well as some of the defining characteristics of both seedless and seed plants • Compare and contrast the life cycles of seedless and seed plants Before you begin this module, download the “Alternation of Generations Activity Packet” document located in the module. As you work through this module, follow the instructions to complete the activities in the packet. You can either use your computer’s drawing tools, or you can print the document and complete it by hand. Then upload a picture or scan the document and insert your completed activity into your Class Notebook. I. Introduction One of the defining characteristics of the Kingdom Plantae is a life cycle that exhibits an alternation of generations (Figure 1). This cycle is characterized by two heteromorphic generations: A haploid (n), multicellular gametophyte and a diploid (2n), multicellular sporophyte. These generations alternate by means of two major events: Meiosis (cellular division that reduces diploid cells into haploid cells) and fertilization (the fusion of two haploid gametes to form a diploid zygote). Within specialized structures in the sporophyte, meiosis occurs producing spores. This initiates the gametophytic generation. Spores are released into the environment and eventually undergo mitosis (cellular division of a diploid cell that produces a replicate diploid cell) to become multicellular, haploid gametophytes. Gametophytes contain specialized structures that give rise to haploid gametes. Fusion of the gametes produces a diploid zygote which initiates the sporophytic generation. Similar to the spore, the zygote undergoes mitotic divisions which eventually becomes the multicellular sporophyte. Figure 1. Simple life cycle exhibiting alternation of generations. The gametophyte and the sporophyte are heteromorphic, or different in appearance. Two separate events occur to transition between the haploid and diploid generations. https://commons.wikimedia.org/wiki/ File:Alternation_of_generations.svg (Links to an external site.) Watch the following video for an introduction to alternation of generations: Alternation of Generations Video (Links to an external site.) https://commons.wikimedia.org/wiki/File:Alternation_of_generations.svg https://commons.wikimedia.org/wiki/File:Alternation_of_generations.svg https://commons.wikimedia.org/wiki/File:Alternation_of_generations.svg https://www.youtube.com/watch?v=WNkUpdZmM0I%20 Activity 1. Download the “Alternation of Generations Activity Packet” document. Using the video you just watched and any other internet search, define the terms in the packet. Then, fill in the blanks in the figure that follows to begin building a life cycle. II. Seedless, Non-vascular plants Seedless, nonvascular plants are commonly referred to as the bryophytes. However, they include three phyla, one of which is Bryophyta (mosses) (Figure 2). Bryophytes are typically small terrestrial plants found in moist conditions, although some species can withstand dry habitats. In the bryophytes, the dominant generation (the generation that occupies the largest portion of the life cycle) is the gametophyte. The sporophyte is at least partially, if not completely, dependent upon the gametophyte throughout its lifetime. Most sporophytes are only short lived and consist of a foot attached to the gametophyte, a seta, and a capsule or sporangium from which spores are released. Figure 2. Common moss, of the phylum Bryophyta. Typically, mosses are observed in the gametophyte form. This is the soft, green, “cushiony” growth pattern you see attached to rocks, or growing on soil under trees or in other shaded areas. The sporophyte, develops up out of the gametophyte for reproduction. Activity 2. Identify the structures on the moss sporophyte, then answer the questions that follow. Moss gametophytes contain distinct male and female reproductive structures; the egg is produced in an archegonium and the sperm are produced in the antheridium. These are often on separate male and female gametophytes. The sperm are flagellated and must have water present so that they may swim to the egg. After fertilization, the zygote and the developing embryo are retained within the archegonium. Figure 3. Microscopic cross sections of moss archegonia (A) and antheridia (B). Notice the eggs located within the vase-like structures of the archegonia and the sack-like structures containing sperm in the antheridia. Sperm is carried through water from the antheridia to the archegonia, where it travels down the canal to fertilize the egg to produce a diploid zygote. Activity 3. Correctly label the figure, then answer the questions that follow in the activity packet. Activity 4. Putting it all together. Correctly label the life cycle of a non-vascular plant. III. Seedless, Vascular Plants Most seedless vascular plants are terrestrial, and size is quite variable. Development of true leaves, stems, and roots varies in each phylum. In these plants, as in the seed plants, the dominant generation is the sporophyte. As opposed to most bryophytes, the gametophytes of seedless vascular plants are usually free-living but short-lived and very reduced in size. Vascular plants are either homosporous (producing only one type of spore) or heterosporous (producing two morphologically different spores) (Figure 4). Most early seedless vascular plants, including ferns, horsetails, and including most Lycopodium sp. are homosporous. These plants will undergo a life cycle wherein sporophytes produce bisexual spores which have the potential to develop into gametophytes that contain both antheridia and archegonia on a single organism. Heterospory is present in a few of the lycophytes (such as Selaginella) and ferns, but is the dominant reproduction method for seed plants. Two distinct types of spores are produced in two different sporangia. The often smaller (although not always), male structures are called microsporangia which produce microspores; the female structures are called megasporangia and produce megaspores. In this life cycle the gametophytes are called mega- and microgametophytes, are reduced in size, and retained within the spore wall. Figure 4. Examples of homosporous and heterosporous plants. In homosporous plants, only one type of spore is produced that is bisexual. In heterosporous plants, two distinct spores are produced: female megaspores are produced within megasporangia, and male microspores are produces within the microsporangia. Activity 5. Answer the questions in the activity packet. Most true ferns are homosporous. Upon germination, each spore gives rise to a small heart- shaped bisexual gametophyte called a prothallus. The prothallus is anchored by rhizoids and produces antheridia and archegonia on its lower surface. The sperm are flagellated and hence require water to swim to the egg within the archegonium. Following fertilization, the young sporophyte is dependent upon the gametophyte only briefly. It soon becomes independent and the gametophyte dies. Activity 6. Correctly label the fern prothallus, then answer the questions that follow. Activity 7. Life Cycle of Seedless Vascular Plants. Follow the link below to watch an animation on the life cycle of a fern. Complete the final fill-in-the blank activity and copy your answers to your activity sheet. Fern Life Cycle https://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/interactivemedia/activities/load.html?29&D%20 Alternation of Generations Activity Packet. Activity 1. Using what you learned from the video and any reliable internet source, define the terms below. Heteromorphic: Haploid: Diploid: Gametophyte: Sporophyte: Meiosis: Mitosis: Spore: Gamete: Zygote: Basic life cycle exhibiting alternation of generations. Fill in A and B with the appropriate heteromorphic generations, and fill in 1 and 2 with the two events that occur to transition between the haploid and diploid generations. Later we will add structures and developmental stages between the generations and the events. Activity 2. Identify the structures of a moss sporophyte. A. B. C. Q2.1. Are spores typically haploid or diploid? Q2.2. What event or process results in the production of spores? Q2.3. From which structure are the spores released? Q2.4. What do you think happens after the spores are released? Activity 3. Correctly label the figures below with the terms: antheridia, archegonia, egg, sperm. Be sure to answer the questions that follow. Q3.1. If the gametophyte is haploid AND the gametes (eggs and sperm) are haploid, what type of cellular division must the plant undergo to produce the eggs and sperm? Q3.2. What event results in the production of a zygote? Q3.3. What does the zygote eventually become? Activity 4. Putting it all together. Correctly label each stage, event, and structure in the life cycle of a non-vascular plant with the following terms (Hint: the major events that transition between the two heteromorphic stages are in yellow): gametophyte, sporophyte, meiosis, fertilization, spore, sperm, egg, foot, seta, stalk, antheridia, archegonia, zygote. Activity 5. Fill in the blanks below, based on what you read and observed in the online module. Q5.1. Plants that only produce one type of spore are called __________________. Q5.2. In heterosporous plants, the male sporangia are called _________________. Q5.3. Megaspores are produced in structures called ________________. Q5.4. Heterospory has evolved multiple times amongst unrelated land plant groups. What advantages do you think heterospory could impart on a species? Q5.5. Based on what you’ve learned so far, how do you think alternation of generations evolved? What benefits does it offer? What drawbacks? Activity 6. Correctly label the antheridia, archegonia, and rhizoids on the image of the fern prothallus below, then answer the questions that follow. Q6.1. How might the locations of the antheridia and archegonia on the prothallus help promote fertilization. Remember, sperm, require water to travel to the egg. Q6.2. Based on the structures on the prothallus, is this organism heterosporous or homosporous? Activity 7. Life Cycle of Seedless Vascular Plants. Follow the link in the module to watch an animation on the life cycle of a fern. Complete the final fill-in-the blank activity and copy your answers below.