sign in sign up
<<
Home , Archegonium, Bract, Dioecy, Flower, Flowering plant, Gametophyte

Workshop on Alternation of Generations by Dana Krempels Introduction For students new to the study of Plantae, the cycle of --in which a diploid generation alternates with a haploid generation--can be difficult to understand. The purpose of this workshop is to allow the student to better relate to the phenomenon of Alternation of Generations by (1) examining the details of and structure and , and (2) creating an analog to this type of life history. In today's workshop, your goals will be to 1. Understand the alternation of haploid and diploid individuals in the plant life cycle. 2. Understand the terminology used to describe parts of the life cycle, and recognize what each life cycle stage looks like in the major plant taxa. 3. Acquire a more "personal" understanding of how the alternation of generations works by designing an imaginary animal that goes through this type of life cycle. I. Alternation of Generations in Plants: Processes and Terminology The painful part comes first: knowing the general course of events, and what each life cycle stage and structure is called. A. An Overview of Alternation of Generations The plant life cycle, unlike that of , consists of alternating generations of individual that are haploid (the gametophyte) and diploid (the sporophyte). Specialized diploid cells in the sporophyte undergo to produce haploid (hence the name "sporophyte"). Each grows mitotically to become the new gametophyte, which then produces (hence, the name "gametophyte") which fuse to form a . This grows into the sporophyte, and the cycle continues, as shown in the diagram below. In the diagram 1. Indicate the (n for haploid, 2n for diploid) of each life cycle stage or structure. 2. Over each of the five arrows, indicate which of the following processes is taking place: meiosis fertilization B. Terminology Part of learning a new subject is learning its vocabulary and terminology. So here we go! 1. In the sporophyte, what is the name of the structure within which diploid cells undergo meiosis to become haploid spores? Describe what this structure looks like and where it is found in: a. a (or other ) It is the caplike structure at the end of the simple, stalked gametophyte. b. a (or other seedless tracheophyte) Found on the undersides of the , in clusters called sori (singular = ) c. a (or other ) Found on the underside of male pine cone , or the upper side of pine cone bracts. d. a (any of the angiosperms) Found inside the within the carpel. 2. If the spore-producing structures you just named are found on a specialized to bear those structures, that leaf is called a Describe what this structure looks like in: a. a moss (or other bryophyte) Does not have true , so has no sporophylls. b. a fern (or other seedless tracheophyte) Fern with dusty sporangium clusters (sori) c. a pine (or other gymnosperm) Thin scale riding on the of either a male or a female pine cone d. a flowering plant (any of the angiosperms) male: female: carpel (sometimes incorrectly called the pistil) 3. A spore that develops into a female gametophyte is called a Describe where this is found in: a. a liverwort Inside the sporophyte, found under the female gametophyte's archegoniophore. b. a fern have bisexual spores; they are found within the sporangium. c. a pine Inside the female pine cone sporangium in the ovule (which, once fertilized, will become the ) d. a flowering plant Inside the carpel/pistil of the 4. A spore that develops into a male gametophyte is called a Describe where this is found in: a. a liverwort Inside the sporophyte, found under the female gametophyte's archegoniophore. b. a fern Ferns have bisexual spores; they are found within the sporangium. c. a pine Inside the male pine cone sporangium (on the upper side of the pine cone bracts) d. a flowering plant Inside the anther 5. A sporophyll that bears is called a MEGASPOROPHYLL Describe what this structure looks like in: a. a liverwort or moss No true leaves, so they lack sporophylls b. a fern Spores are bisexual; no megasporophylls are found in most . c. a pine The scale-like structure riding on the female pine cone's woody bract. d. a flowering plant The carpel (one to many carpels make(s) up the pistil 6. A sporophyll that bears is called a MICROSPOROPHYLL Describe what this structure looks like in: a. a liverwort or moss No true leaves, so they lack sporophylls b. a fern Spores are bisexual; no megasporophylls are found in most species. c. a pine The scale-like structure riding on the male pine cone's woody bract. d. a flowering plant The stamen 8. Describe the appearance of the mature male gametophyte in a. a liverwort or moss This is the typical liverwort plant one usually sees; flattened on the ; It will eventually bear antheridia on stalklike antheridiophores. b. a fern Small, fingernail-sized plant shaped like a little valentine; bisexual c. a pine grain d. a flowering plant pollen grain 9. Describe the appearance of the mature female gametophyte in a. a liverwort or moss This is the typical liverwort plant one usually sees; flattened thallus on the soil; It will eventually bear archegonia on stalklike archegoniophores. b. a fern Small, fingernail-sized plant shaped like a little green valentine; bisexual c. a pine A nondescript blob of undifferentiated cells except for an ; found inside the ovule (which will eventually become the seed) d. a flowering plant A coenocytic mass of cells containing eight nuclei, each of which has a specific function. (See diagram of the mature flowering plant's nuclei) 10. In the female gametophyte, what is the name of the structure within which haploid cells develop into ova? ARCHEGONIUM Describe what this structure looks like in: a. a liverwort or moss differentiated mass of cells found on the underside of the archegoniophore in liverworts, and the upper surface of the female moss gametophyte. b. a fern Physically resembles liverwort archegonium (an ovum surrounded by haploid forming a "neck," with a micropyle through which swim. Found at the base of the valentine-shaped gametophyte's "notch." c. a pine Found inside the undifferentiated mass of cells making up the female gametophyte, and consists of haploid tissue surrounding the ovum. d. a flowering plant Female gametophyte of flowering plants lacks an archegonium. (Derived with respect to other plants) 11. In the male gametophyte, what is the name of the structure within which haploid cells develop into sperm? Describe what this structure looks like in: a. a liverwort or moss differentiated mass of cells found on the upper surface of the antheridiophore in liverworts, and the upper surface of the male moss gametophyte. b. a fern Physically resembles liverwort antheridium. Found at the base of the valentine- shaped gametophyte's ) c. a pine Pollen never develop an antheridium. (Derived with respect to other plants) d. a flowering plant Pollen never develop an antheridium. (Derived with respect to other plants) 12. What does it mean if a plant is dioecious? It means that there are individual plants of each , male and female. Male plants produce only male reproductive structures, and only female reproductive structures. For example, some dioecious flowering plants include and . 13. What does it mean if a plant is monoecious? A single individual plant has both male and female reproductive parts. Bisexual. II. Creating an Animal Analogy Understandably, it's sometimes difficult for animals like us to easily relate to the plant life cycle, as ours is so dissimilar. In an attempt to make the Alternation of Generations a little bit more "real" to us, let us engage in a bit of fantasy. A. Creating an Imaginary Animal Species that undergoes Alternation of Generations Don't get your hopes up too high. We are merely going to mimic the life cycle stages as one typically sees them in a SEEDLESS TRACHEOPHYTE. The sporophyte produces spores in sporangia on sporophylls, then releases the spores to the environment. A spore germinates into a free-living gametophyte that produces gametes in gametangia (analogous to testes and ovaries). Fertilization occurs when sperm travel from the male gametophyte to the female, enter her and fertilize her ovum to produce a zygote. The zygote grows into the new sporophyte, obliterating the female gametophyte. The male withers and dies shortly after the sperm are released. Keep this cycle in mind when you design your animal model. 1. As a group, create an animal (it can be an existing animal, or something similar to a species with which you are already familiar) that is diploid. This animal will be your sporophyte generation, and you should decide in advance whether it will be dioecious or monoecious. Does this animal have gonads (ovaries or testes)? Explain. Briefly describe your animal sporophyte. For the sake of grossness, I will choose a . You can do something else. It's going to be monoecious, and produce two kinds of spores. This animal does not have ovaries or testes. 2. Next, choose an area on the animal where specialized diploid cells will undergo meiosis to produce spores. Remember that this should be an external area, since the spores will be released to the environment. Also remember to create the areas on your animal as appropriate to or monoecy, whichever you have chosen your animal to be. At a certain point in the , it develops grotesque little (the "sporangia") all along its arms (the "sporophylls"). Inside these bulbs, diploid cells are undergoing meiosis. Inside small bulbs, male spores (microspores) are formed, which will grow into male . Inside larger bulbs, female spores (megaspores) are formed, which will grow into female gametophytes. 3. Release the spores! What happens to the spores that land in an area appropriate to ? Describe the resulting (the gametophyte generation of this species), and again note whether it is monoecious or dioecious, since this will be important in the next few steps. Be sure to note this animal's ploidy, and whether it has gonads (testes and/or ovaries). Female spores landing in nice, moist areas grow mitotically into little, haploid animals that look like Smurfette (tm). Male spores landing on a similar substrate grow into little, haploid animals that look like the rest of the Smurfs. 4. If your gametophyte is bisexual, describe how and in what anatomical locations it will produce sperm and ova. How will sperm reach the ova? Describe the process. Mine is dioecious, so skip this part for the example. You might do this, though. 5. If your gametophyte animal is male, describe how and in what anatomical location it will produce sperm. How will these gametes reach the female gametophyte's ova? Describe the process. Oh, use your imagination. Those Smurfs were wearing pants, after all. (You'll never be able to watch that show again, you know.) They are anatomically correct little mini- humanoids, and they do things just the way you'd expect them to. Except for one thing. Like plant gametophytes, there is no intromission (look it up). Instead, the little Smurfs spill their seed upon the wet ground during a good rainstorm, and the sperm swim their. Once the male gametophyte releases his sperm, he shrivels up and dies. 6. If your gametophyte animal is female, describe how it will produce ova, and where. Will these gametes be released into the environment, or will they remain inside the female? If they remain inside the female, describe where they will be found. Once again, this is a mini-humanoid. So use your imagination, and pretend that although Smurfette is haploid and will produce her ova via mitosis, that the "ovaries" she has are in the expected place. As is the rest of the equipment. 7. Describe fertilization between the male and female gametophytes of your species (if the species is monoecious) or how fertilization takes place in your individual bisexual gametophyte (if the species is dioecious). What is the result of fertilization? Where would you find it if you were to dissect your gametophyte animal? Sperm swim into the appropriate area, and fertilization takes place inside the female. If you were to dissect her, you'd find the zygote inside the little Smurfette uterus/archegonium. (This is getting really disgusting.) 8. What will now happen to the fertilized ovum (zygote)? Describe how it grows, and where. What happens to the gametophytes once fertilization is complete? The zygote then proceeds to grow mitotically into the new sporophyte. But sadly, like a fern, this gametophyte doesn't give birth. Instead, as the zygote grows, little Smurfette is stretched and smashed out of existence. 9. Describe the mature result of growth of the zygote. What will be the next step in this life cycle? The new sporophyte will again grow mitotically until mature, and then produce more male and female spores, which will grow into Smurfs and Smurfettes. B. Thought Questions 1. Do any other organisms alternate haploid and diploid conditions? If so, how--and at what point in the life cycle--does this occur in each group you have named? All sexually reproducing organisms alternate haploid and diploid conditions. Animals just don't have separate haploid and diploid individual organisms. 2. What is the significance of alternating haploid and diploid conditions in any organism? It allows for the recombination of , with all the evolutionary baggage that loads onto the scenario. (Discuss the importance to organisms of sexual . Why do you suppose sex exists? Wouldn't it be less risky to just reproduce asexually? What are the risks and benefits of asexual versus ?) 3. How is the alternation of haploid and diploid conditions in plants different from that seen in most other types of organisms? Only plants (and some macroalgae) have separate haploid and diploid individual organisms. 4. Are there any other organisms that have an alternation of generations similar to that seen in plants? If so, what are they? Many macroalgae in the Phaeophyta and Rhodophyta exhibit alternation of generations, as do some of the more derived .

5. If you answered "yes" to the questions above, how is the alternation of generations in plants different from that seen in the organism(s) you named in the previous question? Macroalgae do not have HETEROMORPHIC alternation of generations: sporophyte and gametophyte are physically indistinguishable. Their alternation of generations is ISOMORPHIC. 6. How does the alternation of generations cycle differ among plant taxa? How does the cycle differ between the taxa considered to be more primitive, and those considered more derived? In , the gametophyte is the long-lived generation, and the sporophyte is ephemeral, living only a season. In the Tracheophytes, the situation is reversed. In the more derived plant taxa the gametophyte is more and more reduced, with the most derived gametophytes even lacking antheridia or archegonia. 7. Do you believe that the alternation of generations seen in plants and a few other taxa are synapomorphic, or convergent? How would you test your hypothesis? A cladistic analysis of the taxa being considered would yield data to support or refute the contention that alternation of generations is symplesiomorphy among diverse taxa. The most parsimonious would indicate whether alternation of generations evolved once or is more likely to have evolved several times and been secondarily lost several times. 8. What do you suppose might be the evolutionary significance of a heteromorphic alternation of generations? The sporophyte and gametophyte won't compete for resources. Also, the smaller gametophyte generation in the more derived plants is more motile, allowing for farther dispersal from the parent sporophyte, and greater chance of outbreeding. This increases vigor, and could confer a selective advantage.