Plant Evolution Flashcards
Chlorophytes and Streptophytes: Early Separation and Adaptations
- Genetic evidence suggests an early physical separation between the ancestors of chlorophytes and streptophytes.
- This separation resulted in the two groups evolving in distinct environments, leading to unique adaptations.
Freshwater Adaptation in Streptophytes
- Hypothesis: Streptophytes were separated into freshwater environments before the Precambrian explosion.
- Being in freshwater is a drastically different environment due to osmotic balance.
- Only organisms with the right variations survived the transition to freshwater.
- Key Point: Adaptations for freshwater survival could be repurposed for terrestrial life.
Distinguishing Chlorophytes and Streptophytes
- Focus on synapomorphies to differentiate extant chlorophytes and streptophytes.
Ancestral Green Flatulence (AGF)
- The common unicellular ancestor of both chlorophytes and streptophytes is termed AGF.
Multicellularity
- Multicellularity evolved independently in chlorophytes and streptophytes.
- Basic requirement for multicellularity: cells must adhere to each other.
- Chlorophytes and streptophytes use completely different mechanisms for cell adhesion.
Cell Adhesion Structures
- Multicellular chlorophytes use phycoplast
- Multicellular streptophytes use phragmoplast
Cytokinesis Differences
- During cytokinesis, cell division is incomplete, leaving pores.
- In chlorophytes, microtubules assemble in the same direction as the plane of division, leaving a single pore.
- Materials can pass through this pore, facilitating communication between cells.
- In streptophytes, microtubules go at a 90-degree angle, which is significant, suggesting it evolved completely independently.
- Instead of one pore, a series of pores are created.
- The endoplasmic reticulum (ER) of one cell extends through the pore into the adjoining cell, forming plasmodesmata.
Plasmodesmata
- Plasmadesmata connects all plant cells and enable transfer of ribosomes, mRNA, and proteins between plant cells.
- Plasmadesmata, resulting from phragmoplasts, are only found in streptophytes.
Synapomorphies Summary
- Streptophytes: Phragmoplast leading to plasmodesmata.
- Chlorophytes: Phycoplast leading to pore formation.
- Multicellularity arose independently in both clades with distinct mechanisms.
Chlorophyte Examples: Chlamydomonas and Ulva
- Examples of Chlorophytes: Chlamydomonas, Ulva, and Bulwarks.
Chlamydomonas
- Exclusively unicellular organism with two flagella (bichon characteristic).
Life Cycle
- Haploid plant cells reproduce asexually through mitosis when conditions are favorable.
- Under nutrient-limiting conditions (e.g., nitrogen depletion), cells differentiate as gametes.
- Gametes are isogamous, indicated by plus and minus signs (+ and -), meaning they are the same size.
- Fusion of gametes occurs in the water column to form a diploid zygote.
- The zygote immediately undergoes meiosis to produce haploid cells again.
- Key points: Always unicellular, gamete fusion happens loose in environment.
Ulva (Sea Lettuce)
- Multicellular organism with alternation of generations.
Terminology
- Gametophyte: Haploid multicellular organism.
- Sporophyte: Diploid multicellular organism.
Roles
- Sporophyte produces spores (via meiosis).
- Gametophyte produces gametes (via mitosis).
Life Cycle
- A multicellular sporophyte plant contains sporangia where meiosis occurs to produce haploid spores.
- Spores are released and swim around since it is an aquatic plant.
- Spores grow (germination) via mitosis into a haploid gametophyte plant.
- Gametophyte plant has gametangia where gametes are produced by mitosis.
- Gametes (+ and -) are released into the environment.
- Fusion of gametes to form a diploid zygote happens out in the water column.
- Zygote grows mitotically, giving rise to the sporophyte.
- Key Points: Gametes fuse loosely in the water column.
Streptophytes and the Significance of Egg Retention
- Focus shifts to streptophytes.
- Key difference: One gamete (egg) is retained in the tissues of the parent plant.
- Anisogamous gametes: eggs and sperm of different sizes.
- Sperm must come to the egg for fusion and zygote formation within the parent plant's tissues.
- This is a major innovation.
Chara (Stonewort) Life Cycle
- Example: Chara (stonewort), which does alternation of generations.
Process
- Multicellular haploid plant produces sperm (released) and retains the egg.
- Sperm swims to the egg for fusion to form the zygote.
- Zygote does meiosis to produce haploid spores which grow into a new plant.
Important Synapomorphy
- Egg is retained in parent tissues.
Sporopollenin
- The gametangia, which is the structure that houses the egg and zygote, has a cell wall with sporopollenin, this sporollenin is also in the walls of the spores themselves.
- Sporopollenin is a very stable chemical.
- Resistant qualities: resistant to dehydration, chemical perturbation, and UV degradation.
- Benefit for plants: survives pond dry-ups, protecting spores until conditions are right.
- Found in walls of plant spores and pollen grains.
- Forms fossils really well.
Life Cycle Types
- Zygotic Life Cycle: The stonewort shows a zygotic life cycle.
- Only diploid cell in the cycle is the zygote.
- Gametic Life Cycle: (e.g., animal life cycle)
- Only haploid cells are the gametes.
- Alternation of Generations: This has already been heavily studied in previous lectures.
Transition to Alternation of Generations
- In a zygotic life cycle, the zygote undergoes meiosis immediately.
- For alternation of generations, the zygote undergoes mitosis to form a sporophyte.
Embryophytes
- Streptophyte plants with plasmodesmata, egg retention, and alternation of generations are embryophytes.
- Embryophytes: Land plants.
- The ancestor of all land plants probably looked similar to stoneworts but evolved to have alternation of generations.
Adaptations of Embryophytes
- The rest of the unit focuses on how embryophytes adapted to dry environments.
Essential Concepts
- Understand that alternation of generations looks different from Ulva's.
- In chlorophytes, gametes fuse loose in the environment.
- A protected embryosporophyte is only possible with egg retention.
Archegonia and Antheridium
- Gametangia that produces an egg is called archegonia (female gametangia).
- Gametangia that makes sperm are the antheridium.
- In embryophytes, the archegonia produces and retains the egg. The antheridium makes and releases lots of sperm into the environment.
Derived Conditions of Embryophytes
- Zygote does not do meiosis. It does mitosis to make a multicellular embryonic sporophyte plant which gets nourished and retained in the gametophyte tissues.
- Embryophyte is a term used to emphasize this key feature.
- Informal grouping: vascular and nonvascular plants.
- Vascular plants are a clade. Nonvascular plants are not.
- Clade name for vascular plants: tracheophyte.
Nonvascular Plants
- All are very small and require a lot of water.
- Cannot support a body with much mass: they will collapse under their own mass.
Lignin
- Lack lignin, preventing them from getting very tall.
- Lignin is part of the cell wall of xylem tissue in vascular plants. Lignin provides tensile strength to cellulose in plant cells to hold up a heavy mass.
Dominant Generation
- Dominant generation: the plant you see most often in nature.
- These small nonvascular plants have a dominant gametophyte. Gametophyte is the greener and longer lived plant.
- The sporophyte is really, really small.
- Sporophytes are permanently attached to the gametophyte parent and dependent on it for nutrition and support.
Symbiotic Associations
- Symbiotic associations with cyanobacteria.
- Key role of cyanobacteria: nitrogen fixation.
