Biology 332
Fall 2003

Exam III

Name_________________________

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. This exam covers eukaryotic algae, bryophytes, lower-vascular plants (ferns and allies including Selaginella), and gymnosperms. 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?

photosynthetic pigments

chlorophylls, xanthophylls, carotenoids

8-
main storage carbohydrate

starch

cell wall polysaccharide

cellulose

ribosomes

70S plus 80S

DNA organization

histone bound in linear chromosomes

nuclear envelope

present, double with pores

nuclear division

mitosis

cytokinesis

cell plate

Evolution of life history. Among these groups there are three distinctly different life-histories: haplontic (zygotic), diplontic (gametic), and diplohaplontic (sporic). Briefly describe each one.

Haplontic
(zygotic)

zygote becomes sporocyte

example:

Chlamydomonas, Volvox, etc.

6-
Diplontic
(gametic)

spores become gametes

example:

Chlamydomonas, Codium

Diplohaplontic
(sporic)

multicellular sporophyte and gametophyte

example:

moss, ferns, pines, etc.

Now write a short paragraph explaining what you believe is the plesiomorphic life history and which is the most-apomorphic history among these groups. Defend your answer as thoroughly as possible in the space below. Consider the transition to terrestrial life in your answer.

Since bacteria are haploid and are the archetype of eukaryotic cells, the zygotic life history must have evolved first and so is the plesiomorphic of those here.

The sporic life history is transitional but observed in a very wide range of plants. The elaborate haploid plant body is gradually reduced through plant evolution.

The ultimate reduction of the haploid would be to the gametic life history, where the spore immediately becomes the gamete; so this is the most apomorphic.

The reduction of the haploid phase down to just 4 cells in pine reduces the selection against recesssive alleles to just those involved with those four cells. This increases the allelic (genetic) diversity in a population, and thus is adaptive.

4-

Evolution of male gametophytes. Trace the evolution through these organisms of the male gametophyte from most primitive of the algae through those in Selaginella. While describing the male gametophyte plant body in each group, be sure to indicate apomorphies that appear along this path of evolution both within and among the groups.

  Thallus Gametangia

15-
Green Algae

unicells
colonies
filaments
parenchymatous

cell wall antheridium

Moss

"leaf"
"stem"
rhizoid

stalk
sterile jacket

Selaginella

microspore wall=none

spermatogenic tissue=none

Pine

microspore wall=none

tube cell
(generative cell)
2 prothallial cells


 

Evolution of gametes. Among these groups there are three distinctly different gamete conditions: isogametes, anisogametes, oogametes. Briefly describe each one.

Isogametes

male/female gametes motile indestinguishable

6-
Anisogametes

male/female gametes motile, female larger

Oogametes

male small motile, female large sessile

Now write a short paragraph explaining what you believe is the plesiomorphic and which is the most-apomorphic gamete 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.

Bacterial conjugation is primitive base upon which isogametes evolved as most-plesiomorphic of these three.

Anisogamy is transitional.

Oogamy is apomorphic observed in all higher plant taxa, correlating with all other traits in the evolutionary pathway.

The move away from dependence on external water and loss of motility are the final stages of this evolutionary trend.

4-

As the final step in the evolution of the male gamete, what was the apomorphic change between the lower plants and pine?

The male gamete:

lost its motility and its centriole

1-

Evolution of seeds. We have observed the evolution of the female gametophyte to the point of forming a seed. What are the three basic components of a seed?

seed coat

storage tissue

embryo

3-

Now describe what comes closest to each of these three components in the two cases below:

Selaginella

microspore wall

megagametophyte

embryo

3-

Pine

integument, nucellus

megagametophyte

embryo

3-


 

Evolution of sporophytes. Repeat your tracing of the evolutionary pathway as you did for gametophytes, but now with respect to sporophyte and sporangia.

  Sporophyte Body Sporangium 32-
Green Algae

vegetative cells
holdfast

cell wall of
zoosporangium/zygote
if any!

Liverwort

foot
stalk
capsule

sterile jacket

Anthoceros

foot
stalk
intercalary meristem
capsule

sterile jacket

Psilotum

enation
stem
rhizome
rhizoid

stalk
sterile jacket

Lycopodium

strobilus
sporophyll
microphyll
stem
rhizome
root

stalk
sterile jacket

Fern

megaphyll
rhizome
root

stalk
annulus
lip cells


 

Evolution of spores. Among these groups there are distinctly different spore (meiotic product) conditions. Briefly describe the progression from algae to pine. 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.

In algae, the meiotic products are often motile zoospores. These may become gametes or grow into a gametophyte. This is what we find in the most primitive of plant-like organisms and so we consider this plesiomorphic.

As plants made the transition to land motility of spores is given up.

Protection from desiccation develops with sporopollenin

Spores are wind dispersed

Starting with Selaginella, the male and female gender identity is found in the sporophyte as microspores (male) and megaspores (female).

The gametophyte becomes endosporic as dependency on water for syngamy is lost

The gametophyte is reduced to minimize inhibition of genetic variation of haploid generation.

7-