Workshop on Alternation of Generations

The scale-like structure riding on the female pine cone's woody bract.d. a flowering plantThe carpel (one to many carpels make(s) up the pistil6. A sporophyll that bears microspores is called a MICROSPOROPHYLLDescribe what this structure looks like in:a. a liverwort or mossNo true leaves, so they lack sporophyllsb. a fernSpores are bisexual; no megasporophylls are found in most species.c. a pineThe scale-like structure riding on the male pine cone's woody bract.d. a flowering plantThe stamen8. Describe the appearance of the mature male gametophyte ina. a liverwort or mossThis is the typical liverwort plant one usually sees; flattened thallus on the soil; Itwill eventually bear antheridia on stalklike antheridiophores.b. a fernSmall, fingernail-sized plant shaped like a little green valentine; bisexualc. a pinepollen graind. a flowering plantpollen grain9. Describe the appearance of the mature female gametophyte ina. a liverwort or mossThis is the typical liverwort plant one usually sees; flattened thallus on the soil; Itwill eventually bear archegonia on stalklike archegoniophores.b. a fernSmall, fingernail-sized plant shaped like a little green valentine; bisexualc. a pineA nondescript blob of undifferentiated cells except for an archegonium; found insidethe ovule (which will eventually become the seed)d. a flowering plantA coenocytic mass of cells containing eight nuclei, each of which has a specificfunction. (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 developinto ova? ARCHEGONIUMDescribe what this structure looks like in:a. a liverwort or mossdifferentiated mass of cells found on the underside of the archegoniophore inliverworts, and the upper surface of the female moss gametophyte.b. a fernPhysically resembles liverwort archegonium (an ovum surrounded by haploid tissueforming a "neck," with a micropyle through which sperm swim. Found at the base ofthe valentine-shaped gametophyte's "notch."c. a pineFound inside the undifferentiated mass of cells making up the female gametophyte,and consists of haploid tissue surrounding the ovum.d. a flowering plantFemale gametophyte of flowering plants lacks an archegonium. (Derived with respectto other plants)11. In the male gametophyte, what is the name of the structure within which haploid cells developinto sperm? ANTHERIDIUMDescribe what this structure looks like in:

Inside small bulbs, male spores (microspores) are formed, which will grow into malegametophytes.Inside larger bulbs, female spores (megaspores) are formed, which will grow intofemale gametophytes.3. Release the spores! What happens to the spores that land in an area appropriate to germination?Describe the resulting organism (the gametophyte generation of this species), and again notewhether it is monoecious or dioecious, since this will be important in the next few steps. Besure 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 animalsthat 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 producesperm 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 producesperm. 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 neverbe able to watch that show again, you know.) They are anatomically correct little minihumanoids,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 thesegametes be released into the environment, or will they remain inside the female? If they remaininside the female, describe where they will be found.Once again, this is a mini-humanoid. So use your imagination, and pretend thatalthough 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 speciesis monoecious) or how fertilization takes place in your individual bisexual gametophyte (if thespecies is dioecious). What is the result of fertilization? Where would you find it if you wereto 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 Smurfetteuterus/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 afern, this gametophyte doesn't give birth. Instead, as the zygote grows, little Smurfette isstretched 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 moremale 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 pointin 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 genes, with all the evolutionary baggage that loadsonto the scenario. (Discuss the importance to organisms of sexual reproduction. Why doyou suppose sex exists? Wouldn't it be less risky to just reproduce asexually? What arethe risks and benefits of asexual versus sexual reproduction?)3. How is the alternation of haploid and diploid conditions in plants different from that seen inmost other types of organisms?Only plants (and some macroalgae) have separate haploid and diploid individualorganisms.4. Are there any other organisms that have an alternation of generations similar to that seen inplants? If so, what are they?Many macroalgae in the Phaeophyta and Rhodophyta exhibit alternation ofgenerations, as do some of the more derived Chlorophyta.5. If you answered "yes" to the questions above, how is the alternation of generations in plantsdifferent from that seen in the organism(s) you named in the previous question?Macroalgae do not have HETEROMORPHIC alternation of generations: sporophyteand gametophyte are physically indistinguishable. Their alternation of generations isISOMORPHIC.6. How does the alternation of generations cycle differ among plant taxa? How does the cyclediffer between the taxa considered to be more primitive, and those considered more derived?In Bryophytes, the gametophyte is the long-lived generation, and the sporophyte isephemeral, living only a season. In the Tracheophytes, the situation is reversed. In themore derived plant taxa the gametophyte is more and more reduced, with the most derivedgametophytes even lacking antheridia or archegonia.7. Do you believe that the alternation of generations seen in plants and a few other taxa aresynapomorphic, or convergent? How would you test your hypothesis?A cladistic analysis of the taxa being considered would yield data to support or refutethe contention that alternation of generations is symplesiomorphy among diverse taxa.The most parsimonious tree would indicate whether alternation of generations evolvedonce or is more likely to have evolved several times and been secondarily lost severaltimes.8. What do you suppose might be the evolutionary significance of a heteromorphic alternation ofgenerations?The sporophyte and gametophyte won't compete for resources. Also, the smallergametophyte generation in the more derived plants is more motile, allowing for fartherdispersal from the parent sporophyte, and greater chance of outbreeding. This increaseshybrid vigor, and could confer a selective advantage.