Transcript Plant and Fungal Biology 6 September 2007

Evolutionary origins of plants: algae
 “Algae”
is a historical term. It describes a
number of groups of organisms that are
plant-like in that they contain chloroplasts
and carry out photosynthesis but are
outside the lineage of plants.
 “Algae”
is a paraphyletic group i.e. does
not consist of an ancestor and all of the
ancestor’s descendents.
Algae
 Algae



have a widespread occurrence
Aquatic: marine, freshwater
Terrestrial: deserts, soils, trees, rocks, etc
Some are symbiotic
• e.g. lichen is a symbiotic alliance between a
fungus and an alga.
• e.g. Green Algae (zooxanthellae) live within reef
building corals.
Growth forms of algae
 Algae
take on a variety of forms both
microscopic and macroscopic
 Unicellular
 Colonies
 Filaments
 Multicellular thallus
Ecological Importance of algae

Are very important primary producers
especially in marine ecosystems.

Play major roles in global cycling of C, N,
and O2.

Their photosynthetic activity forms the
basis of complex communities.
Major groups of algae
 Red
algae: Rhodophyceae
 Brown algae: Phaeophyceae
 Green algae: Chlorophyta
Rhodophyceae (red algae)

Fossil record: 1.5-2 BYA

~ 5,500 species
Mostly marine, few
freshwater

Live attached to surfaces
(rocks, shells, other algae)

Many are reef-building algae
(corallines: CaCO3
accumulates in cell walls)
Rhodophyceae (red algae)

Body forms: Unicellular,
simple filaments or
complex filamentous
aggregations

Chlorophyll a

Cell walls: cellulose,
some with CaCO3
Phaeophyceae (brown algae)

Largest and most complex
algae.

All are multicellular and most
are marine.

Body form: Thallus (plant-like
but lacks true roots, stems and
leaves).

Thallus includes holdfast, stipe
and leaflike blades.

Include the largest seaweeds
such as the kelps.

Cell walls contain cellulose.

Chlorophyall a and c.
Kelp Forest, New Zealand – Ian Skipworth
Chlorophyta (green algae)

Fossil record: 1.5-2 BYA

~ 8,000 species (500 genera)

Marine, freshwater, terrestrial.

Attached or planktonic.

Chlorophyll a and b.
Many species form symbiotic
relationships with other
organisms.
Unicellular, filaments, colonies,
also thallus body form.


Chlorophyta (green algae)

Cell walls: absent,
cellulose, or
modifications
 Land plants are derived
from green algae.
 Many taxonomists
believe green algae
(and red algae) should
be included among the
Plantae.
http://www.botany.hawaii.edu/BOT201/
Endosymbiosis

Symbiotic organisms are those that have a close
mutually dependent relationship with another
organism.

An endosymbiont is a cell that lives within
another cell.

The forerunners of modern eukaryotic cells are
believed to have been symbiotic associations of
prokaryotic cells.
Endosymbiosis
 The
role of endosymbiosis in evolution
was developed most extensively by Lynn
Margulis of the University of
Massachusetts.
Endosymbiosis
 The
proposed ancestors of mitochondria
most likely were aerobic heterotrophic
bacteria.
Mitochondria

Mitochondria
contain own DNA
(circular plasmids
like bacteria)
 Likely originated as
aerobic bacteria,
then engulfed
~1.5 Billion Years ago
Chloroplasts

Chloroplasts are believed to be descendents of
photosynthetic prokaryotes (most likely
cyanobacteria) that became endosymbionts
within larger cells.

About 1.5 mya thus the first “plants” evolved
from the engulfing of a photosynthetic
prokaryote by an aerobic eukaryote.
Ploidy Level

How many sets of chromosomes?

Diploid (2n) - “typical”


Haploid (1n) - ex. sex cells


2 forms of each gene (alleles)
Single copies; product of meiosis
Polyploid (4n, 8n, 6n…) - anything more than
2n
Animals:
One animal, one generation
 Single
generation
is diploid (2n)



Produces shortlived haploid (n)
sex cells
2n = 46
n = ??
Paul Decelles
Plants:
One plant, two generations

One generation is
diploid (2n)


Produces n spores
that grow into…
One generation is
haploid (1n)

Produces n
gametes - later
fuse and make a
2n individual
Paul Decelles
Plants:
“One plant is always two”


One generation is
diploid (2n)
 Produces n
spores
One generation is
haploid (1n)
 Produces 1n
gametes - later
fuse and make a
2n individual
SPOROPHYTE
“spore [producing] plant”
GAMETOPHYTE
“gamete [producing] plant”
Alternation
of
Generations
Alternation of
generations

Usually the diploid sporophyte is dominant and
most obvious
 Examples:
• A fern
• A pine tree
• A Venus fly-trap
Alternation of generations

…the exception: Bryophytes
 Gametophyte is dominant
 The “green plant” that you see is the
gametophyte
Alternation of generations

…and the moss sporophyte is dependent
 Sprophyte is parasitic on the gametophyte
Sporophyte
Gametophyte
Alternation of
generations


Sporophyte
Two generations
nearly always
physically linked
PARASITISM is
common
Gametophyte
Moss
Parasitic generations
Plant group
Parasitic
generation
Dominant
generation
Bryophytes
Sporophyte
Gametophyte
Gymnosperms
Gametophyte
Sporophyte
Angiosperms
Gametophyte
Sporophyte
Gymnosperms and
Angiosperms

Parasitic reduction of the gametophyte

Two genders:
 Female gametophyte
• Housed in the sporophyte ovule
 Male gametophyte
• Housed in the pollen grain
And ferns?


Each generation is
independent
Except for a short
time when the
sporophyte
originates from the
prothallus
Fern
Sporophyte
Gametophyte
Gametophyte