WEEK OF FEB 16 1.Begin discussion of journey from aquatic to terrestrial habitats by looking at nonvascular and vascular aquatic plants 2.Changes in the.

WEEK OF FEB 16

1. Begin discussion of journey from aquatic to terrestrial habitats by looking at nonvascular and vascular aquatic plants

2. Changes in the lecture and lab schedule - discuss algae and bryophytes earlier

3. Date of exams - WEDNESDAY FEB 3

4. Lab review session prior to the exam on MON MARCH 1

5. Lab notebooks due today (e-mail Carol your topics so she can be on the lookout for resources for you)

Outline

1. Alternation of generations (haploid and diploid phases)

2. Green algae as ancestors of terrestrial plants

3. Possible first kinds of terrestrial4. Bryophytes - terrestrial

“amphibian” plants

Evolutionary Trends

1. Green algae that show trend from unicellular to multicellular with modest differentiation

2. Alternation of generations

3. Earliest land plants - small with a cuticle to minimize transpiration relative to maximize CO2 uptake

4. Bryophytes – 1. Poikilohydric = in equilibrium with

water status of local environment2. Flattened small body size also in

equilibrium with protective cuticle

5. Early vascular tissues and supported upright plant bodies

Alternation of Generations: Background

1. Move from water to land presents two problems

1. Gametes must be dispersed in nonaquatic environment

2. Embryos must be protected from drying out

2. Solutions:1. Produce gametes in “gamatangia” -

prevents drying out, “internal” fertilization of egg

2. Retain developing zygote within the protective “shell” of the female gamatangia

3. All plants and some algae

Alternation of Generations:

Evolutionary trends

1. Multicellular haploid photosynthetic (PSN) organisms before diploid PSN organisms

2. Earliest sporophyte (2n) generation highly reduced - meiotic cell division produced gametes to restore haploid condition

3. Emergence of dominant diploid phase - WHY???

Alternation of Generations: Dominance of Diploid

Sporophytes

1. Selective pressure for diverse gene pool

2. Long haploid phase in a life cycle means intense selection - all genes are expressed

3. Diploidy allows recessive genes to remain in population (genetic bank account for changing environment)

4. How to transition from dominant gametophyte to dominant sporophyte?

1. One cell --> multicellular sporophyte2. Multicellular --> one cell

gametophyte

Alternation of Generations: the Cycle

1. More than one free-living stage of the organism.

2. Most plants have two recognizable phases - the sporophyte and the gametophyte.

3. The sporophyte phase of the life cycle produces spores by MEIOSIS. 

4. The gametophyte phase produces gametes by MITOSIS. 

5. Two variations:1. Isomorphic - sporophyte and

gametophyte same size2. Heteromorphic - sporophyte and

gametophyte different size size

Ancestors of Green Plants - the Green

Algae

1. Body plan:  No specialization into root, stem, leaves with vascular tissue.

1. photosynthetic thallus 2. attachment - hair-like rhizoids 

2. No Embryo: 1. in most algae, sperm and eggs fuse in

the open water 2. zygote develops into a new plant without

any protection 

• Reproductive structures: – The gametes are produced within a

single cell– No jacket of sterile cells protecting the

gametes. 

Chlorophyta - the green algae

1. Several groups of algae - best represented in marine habitats1. Chlorophytes (green algae)2. Phaeophytes (brown algae)3. Rhodophytes (red algae)

2. Chlorophytes 1. closest to the higher plants in many

ways 2. considered their ancestors due to

presence of:1. photosynthetic pigments -

chlorophylls a & b, ß-carotene2. cell wall - cellulose-rich3. reserves - starch

3. diverse group - the full spectrum of morphological possibilities - from unicells to macroalgae.

Body Plans of Chlorophytes

1. UNICELLS - These algae are single cells, with or without flagella. (ex. Chlamydomonas)

2. COLONIES - single cells which typically exists as clumps; . is no division of labor and each cell can survive on its own (ex. Volvox).

3. SIPHONOUS ALGAE - actually giant unicells. 1. coenocytic - undergo repeated

nuclear division without the accompanying formation of cell walls.

2. tubular structure with the multinuclear cytoplasm lining the thallus (ex. Caulerpa)

Body Plans of Chlorophytes,

continued

1. FILAMENTS - result from cell division in one plane (ex. Spirogyra)

1. Can have branched filaments. 2. heterotrichous form - Specialization

to have basal, prostrate filaments for attachment and erect branches for photosynthesis

2. PARENCHYMATOUS & PSEUDOPARENCHYMATOUS ALGAE

1. cells look like those of higher plants (ex. Ulva)

2. in cross-section appear to be parenchymatous but really made up of interwoven filaments 

3. May see some differentiation

The Transition to Terrestrial Life

• The first plants were algae and these still thrive in a range of aquatic habitats today (not primitive - simple compared to more complex groups; highly evolved and well adapted to the niche they occupy.)

• Aquatic environment is predictable (~stable). Why venture onto land? Selection pressure may have been competition!

• Selective pressures on pioneer land plants   – Desiccation - the plants would dry out   – Water for reproduction - even if there was sufficient

water for survival they would need free water for fusion of gametes 

– Support - buoyancy supports and spreads the algal thallus. These plants would now be plastered on the mud 

– Water for spore dispersal - to colonize new terrestrial habitats spores would have to be released in air not water 

Evolutionary Trends in the Transition to

Land

1. ~ 400 million years ago freshwater, green, filamentous algae invaded the land. T

1. probably isomorphic alternation of generations

2. probably heterotrichous.

2. Selection favors individuals more able to withstand periods without submergence (e.g. at pond margins, on wet mud)

3. Gametophytes need water for reproduction1. basal part of the gametophyte developed with

loss of the upper portion. 2. sterile jacket of cells evolved to protect the

developing gametes during periods of exposure.

4. Sporophytes - spore dispersal was originally in water.

1. Spores need to be dispersed in air. 2. upper spore-bearing part of the plant would

need to be held above water

5. The generations began to diverge.