Transcript Slide 1

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
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3.
4.
Alternation of generations (haploid
and diploid phases)
Green algae as ancestors of
terrestrial plants
Possible first kinds of terrestrial
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.
2.
5.
Poikilohydric = in equilibrium with
water status of local environment
Flattened small body size also in
equilibrium with protective cuticle
Early vascular tissues and
supported upright plant bodies
Alternation of Generations:
Background
1.
Move from water to land presents
two problems
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2.
2.
Solutions:
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2.
3.
Gametes must be dispersed in
nonaquatic environment
Embryos must be protected from
drying out
Produce gametes in “gamatangia” prevents drying out, “internal”
fertilization of egg
Retain developing zygote within the
protective “shell” of the female
gamatangia
All plants and some algae
Alternation of Generations:
Evolutionary trends
1.
2.
3.
Multicellular haploid
photosynthetic (PSN) organisms
before diploid PSN organisms
Earliest sporophyte (2n) generation
highly reduced - meiotic cell
division produced gametes to
restore haploid condition
Emergence of dominant diploid
phase - WHY???
Alternation of Generations:
Dominance of Diploid
Sporophytes
1.
2.
3.
4.
Selective pressure for diverse
gene pool
Long haploid phase in a life cycle
means intense selection - all genes
are expressed
Diploidy allows recessive genes to
remain in population (genetic bank
account for changing environment)
How to transition from dominant
gametophyte to dominant
sporophyte?
1.
2.
One cell --> multicellular sporophyte
Multicellular --> one cell gametophyte
Alternation of Generations:
the Cycle
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2.
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5.
More than one free-living stage of the
organism.
Most plants have two recognizable
phases - the sporophyte and the
gametophyte.
The sporophyte phase of the life cycle
produces spores by MEIOSIS.
The gametophyte phase produces
gametes by MITOSIS.
Two variations:
1.
2.
Isomorphic - sporophyte and gametophyte
same size
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.
2.
2.
No Embryo:
1.
2.
•
photosynthetic thallus
attachment - hair-like rhizoids
in most algae, sperm and eggs fuse in the
open water
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 habitats
1. 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-carotene
2. cell wall - cellulose-rich
3. 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.
2.
2.
Can have branched filaments.
heterotrichous form - Specialization to
have basal, prostrate filaments for
attachment and erect branches for
photosynthesis
PARENCHYMATOUS &
PSEUDOPARENCHYMATOUS
ALGAE
1.
2.
3.
cells look like those of higher plants (ex.
Ulva)
in cross-section appear to be
parenchymatous but really made up of
interwoven filaments
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
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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 reproduction
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4.
Sporophytes - spore dispersal was originally in
water.
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2.
5.
basal part of the gametophyte developed with loss of
the upper portion.
sterile jacket of cells evolved to protect the developing
gametes during periods of exposure.
Spores need to be dispersed in air.
upper spore-bearing part of the plant would need to be
held above water
The generations began to diverge.