Presentation on theme: "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."— Presentation transcript:
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 Aquatic: marine, freshwater Terrestrial: deserts, soils, trees, rocks, etc Terrestrial: deserts, soils, trees, rocks, etc Some are symbiotic Some are symbiotic e.g. lichen is a symbiotic alliance between a fungus and an alga.e.g. lichen is a symbiotic alliance between a fungus and an alga. e.g. Green Algae (zooxanthellae) live within reef building corals.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 O 2. 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: BYA ~ 5,500 species Mostly marine, few freshwater Live attached to surfaces (rocks, shells, other algae) Many are reef-building algae (corallines: CaCO 3 accumulates in cell walls)
Rhodophyceae (red algae) Body forms: Unicellular, simple filaments or complex filamentous aggregations Chlorophyll a Cell walls: cellulose, some with CaCO 3
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: 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.
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” 2 forms of each gene (alleles) 2 forms of each gene (alleles) Haploid (1n) - ex. sex cells Single copies; product of meiosis Single copies; product of meiosis Polyploid (4n, 8n, 6n…) - anything more than 2n
Animals: One animal, one generation Single generation is diploid (2n) Produces short- lived haploid (n) sex cells Produces short- lived haploid (n) sex cells 2n = 46 2n = 46 n = ?? n = ?? Paul Decelles
Plants: One plant, two generations One generation is diploid (2n) Produces n spores that grow into… Produces n spores that grow into… One generation is haploid (1n) Produces n gametes - later fuse and make a 2n individual 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 Produces n spores One generation is haploid (1n) Produces 1n gametes - later fuse and make a 2n individual 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: Examples: A fernA fern A pine treeA pine tree A Venus fly-trapA Venus fly-trap
Alternation of generations …the exception: Bryophytes Gametophyte is dominant Gametophyte is dominant The “green plant” that you see is the gametophyte The “green plant” that you see is the gametophyte
Alternation of generations …and the moss sporophyte is dependent Sprophyte is parasitic on the gametophyte Sprophyte is parasitic on the gametophyte Sporophyte Gametophyte
Alternation of generations Two generations nearly always physically linked PARASITISM is common Gametophyte Moss Sporophyte
Gymnosperms and Angiosperms Parasitic reduction of the gametophyte Two genders: Female gametophyte Female gametophyte Housed in the sporophyte ovuleHoused in the sporophyte ovule Male gametophyte Male gametophyte Housed in the pollen grainHoused in the pollen grain
And ferns? Each generation is independent Except for a short time when the sporophyte originates from the prothallus Fern Gametophyte Sporophyte Gametophyte