This is a take-home exam given before Thanksgiving break, it must be handed in on the Monday after Thanksgiving. All answers must be limited to the space provided on the PRINTED side of each page; answers continuing on the backs of pages or on attached pages will not be scored...brevity and concise answers are required. Late papers receive the late penalty listed on the syllabus. This exam covers eukaryotic algae, bryophytes, and lower-vascular plants (ferns and allies including Selaginella). In this coverage, the plant kingdom evolves toward an assemblage of vascularized seed-bearing plants. The questions on this exam should reflect your understanding of this evolutionary progression.
Common Threads. Assuming for a moment that these organisms constitute a clade, what are the synapomorphies among these plants? (8 points)
|photosynthetic pigments||chlorophyll a,b|
|main storage carbohydrate||starch|
|cell wall polysaccharide||cellulose + pectin etc.|
|ribosomes||80S cytoplasmic, 70S chloroplast and mitochondrion|
|DNA organization||histone bound, linear, multiple chromosomes|
|nuclear envelope||present except in mitosis|
Evolution of life history. Among these groups there are three distinctly different life-histories: haplontic, diplontic, and diplohaplontic. Briefly describe each one. (6 points)
|Haplontic||dominant multicellular haploid|
diploid is single cell
|Diplontic||dominant multicellular diploid|
haploid is single cell
|Diplohaplontic||both haploid and diploid are multicellular|
|Zygotic (haplontic) is the plesiomorphic history. Monerans are haploid as the outgroup. Aquatic organisms of the ancient environment were shielded from UV mutagen so it was sufficient to be haploid through much of the life history. With the move to land, exposure to UV in the atmosphere gave higher mutation rates, so it was adaptive to shield lethal mutant alleles as recessives in heterozygous diploid genotypes. The elaboration of the diploid phase of the life history leads to greater genetic diversity for more-rapid colonization of diverse habitats and better ability to withstand a changing environment. The progression of adaptations to terrestrial life among these organisms correlates with transition to the sporic (diplohaplontic) life history among these organisms. This progression is probably leading ultimately to the gametic (diplontic) life history as the haploid is further reduced toward a single cell; it is not there yet in higher plants however.|
Evolution of gametophytes. Trace the evolution through these organisms of the gametophyte from most primitive of the algae through those in Selaginella. While describing the gametophyte plant body in each group, be sure to indicate apomorphies that appear along this path of evolution both within and among the groups. (41 points divided as shown below)
|female: isogametangium to oogonium|
male: isogametangium to antheridium
cell wall is the only protective layer
PSN in chambers
both with sterile jacket of cells
vascular tissues (hydroid/leptoid)
thallus + rhizoid as liverwort
|Selaginella||mega: endosporic thallus|
micro: none, no thallus!
male: microspore wall is antheridium
no sterile jacket!
Evolution of gametes. Among these groups there are three distinctly different gamete conditions: isogametes, anisogametes, oogametes. Briefly describe each one. (6 points)
Isogametes same size
Anisogametes different size
Oogametes large sessile egg
small motile sperm
Isogamous is plesiomorphic as conjugating monerans (the outgroup) have isomorphic cells. Motile gametes are needed in an aquatic environment where life originated to keep these photosynthetic organisms in the photic zone. Anisogamy is transitional to the apomorphic oogamy. Having sessile gametes avoids the need for water in the life history for the transition to terrestrial life. Having the sessile gamete attached to the sporophyte provides continuous nourishment from sporophyte to sporophyte as the gametophyte is reduced ultimately into a single cell.
Evolution of sporophytes. Repeat your tracing of the evolutionary pathway as you did for gametophytes, but now with respect to sporophyte and sporangia. (52 points divided as shown below)
|Green Algae||single cell to thallus|
|cell wall only (no sterile jacket)|
dependent on gametophyte
true xylem and phloem
|synangium sterile jacket|
aggregated into sori
shed with endosporic microgametophyte
Evolution of spores. Among these groups there are distinctly different spore (meiotic product) conditions. Briefly describe the progression from algae to Selaginella. Write a short paragraph explaining what you believe is the plesiomorphic and which is the most-apomorphic spore condition among these groups. Defend your answer carefully and as thoroughly as possible in the space below. Consider the transition to terrestrial life in your answer. (10 points)
|The zoospore, a motile meiotic product, would be the plesiomorphic condition in aquatic algae. The cell wall was not invested with cutin or sporopollenin. With transition to land, the spore became immotile and its wall was cutinized or invested with sporopollenin in bryophytes and ferns; this avoided desiccation in the terrestrial environment. The spore became sexually dimorphic in size and color as heterospory evolved for some ferns and Selaginella, and gender expression was moved into the sporophyte. The spore hosted mitotic divisions resulting in endosporic gametophytes. Ultimately this happened inside the spore wall prior to dispersal so that the microspore became a pollen grain. The megaspore was held in the sporangium bringing on a requirement for pollination; a fundamentally terrestrial process. This permits the gametophyte generation to be completely hosted by the sporophyte in terrestrial plants. Syngamy is hosted within the spore wall in these organisms so that ultimately (in gymnosperms) free water is no longer necessary for syngamy.|
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