Lecture 14 – Multicellular Life3 Important Questions:- Why are unicellular organisms unable to reach large sizes?o Physiochemical Constraints- What features of ancestral eukaryotes predisposed modern lineages to evolve large, multicellular bodies?o Evolutionary origins - How do multicellular organisms get around these constraints?o “Design” principlesSingle-celled eukaryotes can be pretty big:- Ex: Valonia ventricosa o A “bubble alga”o Also called “sailor’s eyeballs”o A Chlorophyte. or green algao One plasma membrane; ~5cm diameterPhysiochemical Constraints- Imagine a Valonia-like cello Surface Area = 4r2o Volume (& mass) = 4/3 r3 - Question: As the cell gets bigger, what happens?o Answer: As size increases and surface/volume decreases, cells begin to suffer from: Inefficient gas exchange & nutrient uptake Increased distance from periphery to nucleus-diffusion limits - (Valonia cheats here it’s multicnucleate) Inability of external (cell wall, plasma membrane) and internal (cytoskeleton) supports to scale up- (Cells supported by water, like Valonia, can get bigger than those on land?)o Volume will increase faster than the SA as the cell gets bigger!- Result: Anoxia, starvation, slow response to stimuli, collapse/rupture- It is clearly evident that multicellular organisms had to find solutions to these problems!Evolutionary Origins Part 1 – Intermediate forms and what phylogenies tell us- Intermediate Formso Maybe it started simply? o Small steps?o Choanoflagellates similar to sponges relationship to eukaryotes!- What phylogenies tell us?o Extant unicellular sister groups to 3 big multicellular clades: Animals : choanoflagellates Land plants : green algae; closest relative to land plants is charaphyceae Fungi : still unclear!Evolutionary Origins Part 2 – The Lynch Hypothesis- Animals, plants, and fungi have tiny populations compared with prokaryotes and protists. - In these populations, chance outweighs minor fitness differences in determining fate of new mutations (a processcalled “genetic drift”).- Immediate result: selfish DNA (transposons, etc.) proliferate as long as they don’t inactivate important genes.Genome gets huge!- Overall…o Population size varies based on the type of organismso Population size decreases, the genome gets bigger and your ability to detect “junk” DNA decreases(transposons)- A genome full of selfish DNA is bad, right?o Maybe not! Repetitive DNA stimulates gene duplications (unequal crossing over) Introns allow alternative splicing and evolution of chimaeric genes Big intergenic spaces allow more complex types of regulatory circuitso Big Idea! Given longer periods of time, accumulated “junk” DNA eventually allowed features of animal and plantgenomes that we know matter for the development of multiple cell types to evolve!Multicellular Solutions to Physiochemical Constraints1. Evolution of Efficient Gas Exchange and Nutrient Uptake - Branched or villous architecture - Increases surface areao Whole organismo Key organs (gills/lungs, roots or branched organs)- Vasculature - Special cells and organs that move gas and fluido Passive (insect trachea, plant vascular elements)o Active (heart and vessels)- Carriers - Molecules that transport important solutes between cellso Ex. Oxygen (hemoglobin), lipids (sterol carriers), metal ions2. Increased Distance from periphery to nucleus – Diffusion limits - Be Multi-nucleate – pack multiple nuclei into that big cell, and problem goes awayo Problem: How does such a thing divide? Solution 1 (growth): decouple nuclear division and cytokinesis Solution 2 (for sex): produce a unicellular phase for reproductiono Ex. Fungi Cells of mycelium interconnected. Meiotic products (spores)uninucleate- Be Multicellular – link separate cells together by adhesiono Problem: How to reproduce? Solution 1 (asexual): Budding, fission, etc. Solution 2 (sexual): produce a single-cell spore or gameteo Problem: Who gets to reproduce? Solution: tight control over mitotic and meiotic potential- How this evolved in one of the biggest mysteries of evolution.Failure to do it = cancer3. Inability of External and Internal Supports to Scale Up - Stay Squishyo Tough cuticle (extracellular surface layer) often presento Hydrostatic skeleton allows controlled shape changes; coordinated muscle contractions result inlocomotion- Get rigid – evolve a skeletono Exoskeleton Mineralized (corals, molluscs) Organic (arthropods)o Endoskeleton Mineralized (echinoderms, sponges, bony fishes); Organic (cartilaginous fishes, woody plants) Lecture 14 – Study Questions1) What do phylogenies tell us about the origins of multicellular organisms? (Think about why we are interested in organisms like Volvox and choanoflagellates.)2) Under the Lynch Hypothesis, how/why does a reduction in population size lead to an increase in genome size? (Hint: What evolutionary force becomes particularly relevant in small populations?) How does this increase in genome size allow for the evolutionary innovations seen in multicellular organisms?3) What are three major physiological constraints faced by unicellular organisms? Describe adaptations that allow organisms to overcome these
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