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Evolutionary origins of plants: algae

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1 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.

2 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.

3 Growth forms of algae Algae take on a variety of forms both microscopic and macroscopic Unicellular Colonies Filaments Multicellular thallus

4 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. These organisms are ecologically important because they are primary producers, taking up CO2 and producing O2 during photosynthesis, They form the basis of complex communities, and contribute to the global cycling of oxygen, carbon and nitrogen. Algae are economically important and we use them for various food sources, (a well known example is sushi), supplements, biochemical and biotechnological applications, in fact one recent application is using green algae to produce biodiesel. Certain species are used as environmental indicators to track temperature fluctuations and nutrient levels, particularly in aquatic systems. ** All of these applications and used of algae are reliant on accurate understanding of species and their relationships.***

5 Major groups of algae Red algae: Rhodophyceae
Brown algae: Phaeophyceae Green algae: Chlorophyta

6 Rhodophyceae (red algae)
Fossil record: 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)

7 Rhodophyceae (red algae)
Body forms: Unicellular, simple filaments or complex filamentous aggregations Chlorophyll a Cell walls: cellulose, some with CaCO3

8 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.

9 Kelp Forest, New Zealand – Ian Skipworth

10 Chlorophyta (green algae)
Fossil record: 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.

11 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.

12 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.

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

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

15 Mitochondria Mitochondria contain own DNA (circular plasmids like bacteria) Likely originated as aerobic bacteria, then engulfed ~1.5 Billion Years ago

16 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.


18 Ploidy Level How many sets of chromosomes? Diploid (2n) - “typical”
2 forms of each gene (alleles) Haploid (1n) - ex. sex cells Single copies; product of meiosis Polyploid (4n, 8n, 6n…) - anything more than 2n

19 Animals: One animal, one generation
Single generation is diploid (2n) Produces short-lived haploid (n) sex cells 2n = 46 n = ?? Paul Decelles

20 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

21 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”

22 Alternation of Generations

23 Alternation of generations
Usually the diploid sporophyte is dominant and most obvious Examples: A fern A pine tree A Venus fly-trap

24 Alternation of generations
…the exception: Bryophytes Gametophyte is dominant The “green plant” that you see is the gametophyte

25 Alternation of generations
…and the moss sporophyte is dependent Sprophyte is parasitic on the gametophyte Sporophyte Gametophyte

26 Alternation of generations
Sporophyte Two generations nearly always physically linked PARASITISM is common Gametophyte Moss

27 Parasitic generations
Plant group Parasitic generation Dominant generation Bryophytes Sporophyte Gametophyte Gymnosperms Gametophyte Sporophyte Angiosperms Gametophyte Sporophyte

28 Gymnosperms and Angiosperms
Parasitic reduction of the gametophyte Two genders: Female gametophyte Housed in the sporophyte ovule Male gametophyte Housed in the pollen grain

29 And ferns? Fern Each generation is independent
Except for a short time when the sporophyte originates from the prothallus Sporophyte Gametophyte Gametophyte Fern

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