Evolutionary origins of plants: algaeEvolutionary origins of plants: algae

““Algae” is a historical term. It describes a Algae” is a historical term. It describes a number of groups of organisms that are number of groups of organisms that are plant-like in that they contain chloroplasts plant-like in that they contain chloroplasts and carry out photosynthesis but are and carry out photosynthesis but are outside the lineage of plants.outside the lineage of plants.

““Algae” is a paraphyletic group i.e. does Algae” is a paraphyletic group i.e. does not consist of an ancestor and all of the not consist of an ancestor and all of the ancestor’s descendents. ancestor’s descendents.

AlgaeAlgae

Algae have a widespread occurrenceAlgae have a widespread occurrence Aquatic: marine, freshwaterAquatic: marine, freshwater Terrestrial: deserts, soils, trees, rocks, etcTerrestrial: deserts, soils, trees, rocks, etc Some are symbiotic Some are symbiotic

• e.g. lichen is a symbiotic alliance between a e.g. lichen is a symbiotic alliance between a fungus and an alga.fungus and an alga.

• e.g. Green Algae (zooxanthellae) live within e.g. Green Algae (zooxanthellae) live within reef building corals.reef building corals.

Growth forms of algaeGrowth forms of algae

Algae take on a variety of forms both Algae take on a variety of forms both microscopic and macroscopicmicroscopic and macroscopic

UnicellularUnicellular ColoniesColonies FilamentsFilaments Multicellular thallusMulticellular thallus

Ecological Importance of algaeEcological Importance of algae

Are very important primary producers Are very important primary producers especially in marine ecosystems.especially in marine ecosystems.

Play major roles in global cycling of C, N, Play major roles in global cycling of C, N, and Oand O2.2.

Their photosynthetic activity forms the Their photosynthetic activity forms the basis of complex communities.basis of complex communities.

Major groups of algaeMajor groups of algae

Red algae: RhodophyceaeRed algae: Rhodophyceae Brown algae: PhaeophyceaeBrown algae: Phaeophyceae Green algae: ChlorophytaGreen algae: Chlorophyta

Rhodophyceae (red algae)Rhodophyceae (red algae) Fossil record: 1.5-2 BYAFossil record: 1.5-2 BYA

~ 5,500 species ~ 5,500 species

Mostly marine, few Mostly marine, few

freshwaterfreshwater

Live attached to surfaces Live attached to surfaces

(rocks, shells, other algae)(rocks, shells, other algae)

Many are reef-building algae Many are reef-building algae

(corallines: CaCO(corallines: CaCO33

accumulates in cell walls)accumulates in cell walls)

Rhodophyceae (red algae)Rhodophyceae (red algae)

Body forms: Unicellular, Body forms: Unicellular, simple filaments or simple filaments or complex filamentous complex filamentous aggregationsaggregations

Chlorophyll aChlorophyll a Cell walls: cellulose, Cell walls: cellulose,

some with CaCOsome with CaCO33

Phaeophyceae (brown algae)Phaeophyceae (brown algae) Largest and most complex algae.Largest and most complex algae.

All are multicellular and most are All are multicellular and most are

marine. marine. Body form: Thallus (plant-like but Body form: Thallus (plant-like but

lacks true roots, stems and lacks true roots, stems and leaves).leaves).

Thallus includes holdfast, stipe Thallus includes holdfast, stipe and leaflike blades.and leaflike blades.

Include the largest seaweeds Include the largest seaweeds

such as the kelps.such as the kelps.

Cell walls contain cellulose. Cell walls contain cellulose.

Chlorophyall a and c.Chlorophyall a and c.

Kelp Forest, New Zealand – Ian Skipworth

Chlorophyta (green algae)Chlorophyta (green algae)

Fossil record: 1.5-2 BYAFossil record: 1.5-2 BYA

~ 8,000 species (500 genera)~ 8,000 species (500 genera)

Marine, freshwater, terrestrial.Marine, freshwater, terrestrial.

Attached or planktonic.Attached or planktonic.

Chlorophyll a and b.Chlorophyll a and b. Many species form symbiotic Many species form symbiotic

relationships with other relationships with other organisms.organisms.

Unicellular, filaments, colonies, Unicellular, filaments, colonies, also thallus body form. also thallus body form.

Chlorophyta (green algae)Chlorophyta (green algae) Cell walls: absent, Cell walls: absent,

cellulose, or cellulose, or modificationsmodifications

Land plants are derived Land plants are derived from green algae. from green algae.

Many taxonomists Many taxonomists believe green algae believe green algae (and red algae) should (and red algae) should be included among the be included among the Plantae.Plantae.

http://www.botany.hawaii.edu/BOT201/

EndosymbiosisEndosymbiosis

Symbiotic organisms are those that have a close Symbiotic organisms are those that have a close mutually dependent relationship with another mutually dependent relationship with another organism.organism.

An endosymbiont is a cell that lives within An endosymbiont is a cell that lives within another cell. another cell.

The forerunners of modern eukaryotic cells are The forerunners of modern eukaryotic cells are believed to have been symbiotic associations of believed to have been symbiotic associations of prokaryotic cells. prokaryotic cells.

EndosymbiosisEndosymbiosis

The role of endosymbiosis in evolution The role of endosymbiosis in evolution was developed most extensively by Lynn was developed most extensively by Lynn Margulis of the University of Margulis of the University of Massachusetts.Massachusetts.

EndosymbiosisEndosymbiosis

The proposed ancestors of mitochondria The proposed ancestors of mitochondria most likely were aerobic heterotrophic most likely were aerobic heterotrophic bacteria.bacteria.

Mitochondria Mitochondria

Mitochondria Mitochondria contain own DNA contain own DNA (circular plasmids (circular plasmids like bacteria)like bacteria)

Likely originated as Likely originated as aerobic bacteria, aerobic bacteria, then engulfedthen engulfed

~1.5 Billion Years ago

ChloroplastsChloroplasts

Chloroplasts are believed to be descendents of Chloroplasts are believed to be descendents of photosynthetic prokaryotes (most likely photosynthetic prokaryotes (most likely cyanobacteria) that became endosymbionts cyanobacteria) that became endosymbionts within larger cells.within larger cells.

About 1.5 mya thus the first “plants” evolved About 1.5 mya thus the first “plants” evolved from the engulfing of a photosynthetic from the engulfing of a photosynthetic prokaryote by an aerobic eukaryote.prokaryote by an aerobic eukaryote.

Ploidy LevelPloidy Level How many sets of chromosomes?How many sets of chromosomes?

Diploid (2n) - “typical”Diploid (2n) - “typical” 2 forms of each gene (alleles)2 forms of each gene (alleles)

Haploid (1n) - ex. sex cellsHaploid (1n) - ex. sex cells Single copies; product of meiosisSingle copies; product of meiosis

Polyploid (4n, 8n, 6n…) - anything more than Polyploid (4n, 8n, 6n…) - anything more than 2n2n

Animals: Animals: One animal, one generationOne animal, one generation

Single generation Single generation is diploid (2n)is diploid (2n) Produces short-Produces short-

lived haploid (n) lived haploid (n) sex cellssex cells

2n = 462n = 46 n = ??n = ??

Paul Decelles

Plants: Plants: One plant, two generationsOne plant, two generations

One generation is One generation is diploid (2n)diploid (2n) Produces n spores Produces n spores

that grow into…that grow into…

One generation is One generation is haploid (1n)haploid (1n) Produces n Produces n

gametes - later gametes - later fuse and make a 2n fuse and make a 2n individualindividual Paul Decelles

Plants: Plants: “One plant is always two”“One plant is always two”

One generation is One generation is diploid (2n)diploid (2n) Produces n Produces n

sporesspores

One generation is One generation is haploid (1n)haploid (1n) Produces 1n Produces 1n

gametes - later gametes - later fuse and make a fuse and make a 2n individual2n individual

SPOROPHYTE“spore [producing] plant”

GAMETOPHYTE“gamete [producing] plant”

Alternation Alternation of of

GenerationsGenerations

Alternation of Alternation of generationsgenerations Usually the diploid sporophyte is dominant and Usually the diploid sporophyte is dominant and

most obviousmost obvious Examples: Examples:

• A fernA fern• A pine treeA pine tree• A Venus fly-trapA Venus fly-trap

Alternation of generationsAlternation of generations ……the exception: Bryophytesthe exception: Bryophytes

Gametophyte is dominantGametophyte is dominant The “green plant” that you see is the The “green plant” that you see is the

gametophytegametophyte

Alternation of generationsAlternation of generations ……and the moss sporophyte is dependentand the moss sporophyte is dependent

Sprophyte is parasitic on the gametophyteSprophyte is parasitic on the gametophyte

Sporophyte

Gametophyte

Alternation of Alternation of generationsgenerations

Two generations Two generations nearly always nearly always physically linkedphysically linked

PARASITISM is PARASITISM is commoncommon

Gametophyte

Moss

Sporophyte

Parasitic generationsParasitic generations

Plant group Parasitic generation

Dominant generation

Bryophytes Sporophyte Gametophyte

Gymnosperms Gametophyte Sporophyte

Angiosperms Gametophyte Sporophyte

Gymnosperms and Gymnosperms and AngiospermsAngiosperms

Parasitic reduction of the gametophyteParasitic reduction of the gametophyte

Two genders:Two genders: Female gametophyteFemale gametophyte

• Housed in the sporophyte ovuleHoused in the sporophyte ovule Male gametophyteMale gametophyte

• Housed in the pollen grainHoused in the pollen grain

And ferns?And ferns? Each generation is Each generation is

independentindependent Except for a short Except for a short

time when the time when the sporophyte sporophyte originates from the originates from the prothallusprothallus

Fern

Gametophyte

Sporophyte

Gametophyte