EUKARYA Figure 8-8 Dinoflagellates Diplomonads Euglenids Foraminifera Red algae* Apicomplexa Land plants Brown algae Diatoms Ciliates Slime molds* Animals Fungi* Oomycetes Green algae* BACTERIA ARCHAEA Red branches have at least some multicellular representatives. Groups marked with a * also have unicellular species
3 important questions: • Why are unicellular organisms unable to reach large sizes? – physiochemical constraints • What features of ancestral eukaryotes predisposed modern lineages to evolve large, multicellular bodies? – evolutionary origins • How do multicelluar organisms get around these constraints? – “design” principles
single-celled eukaryotes can be pretty big: Ex: Valonia ventricosa, a “bubble alga,” also called “sailor’s eyeballs” (a Chlorophyte, or green alga): Up to 5 cm diameter, one plasma membrane!
Physiochemical Constraints Imagine a Valonia-like cell • surface area = 4r2 • volume (& mass) = 4/3 r3 Q: As cell gets bigger, what happens?
A: 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 multinucleate). • 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?) • Result: anoxia, starvation, slow response to stimuli, collapse/rupture Oh well, guess I’ll have to stay small, then. Sigh…
evolutionary origins, part 1: intermediate forms and what phylogenies tell us ? Maybe it started simply? Chlamydomonas a green algal protist Volvox (two cell types)
Choanoflagellates are sessile protists; some are colonial. Figure 32-11a Colony Choanoflagellate cell Food particles Water current
what phylogenies tell us: extant unicellular sister groups to three big multicellular clades: animals: choanoflagellates land plants: green algae. closest relative to land plants = family charaphyceae (freshwater multicellular alga). fungi: still unclear Deepest diverging group, the chytrids, are aquatic. descendants of fungi that didn’t colonize land?
evolutionary origins, part 2: the Lynch Hypothesis Michael Lynch Indiana Univ. M Lynch, J S Conery Science 2003;302:1401-1404
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 process called “genetic drift”). Immediate result: selfish DNA (transposons,etc.) proliferate as long as they don’t inactivate important genes. Genome gets huge: count M Lynch, J S Conery Science 2003;302:1401-1404
A genome full of selfish DNA is bad, right? • 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 circuits • big idea: Given longer periods of time, accumulated “junk” DNA eventually allowed features of animal and plant genomes that we know matter for the development of multiple cell types to evolve. WOW!
multicellular solutions to physicochemical constraints • evolution of efficient gas exchange & nutrient uptake branched or villous architecture: increases surface area • whole organism • key organs (e.g. gills/lungs, roots or branched organs) 32.2 31.5 Multicellular fungi have web- like bodies called mycelia Feather worm (annelid) tentacles filter debris. Plant roots are highly branched to maximize contact with soil.
multicellular solutions to physicochemical constraints • evolution of efficient gas exchange & nutrient uptake vasculature: special cells and organs that move gas and fluid • passive (e.g. insect trachea, plant vascular elements) • active (heart + vessels) carriers: molecules that transport important solutes between cells. Ex: oxygen (hemoglobin), lipids (sterol carriers), metal ions Fig. 36.17 http://en.wikipedia.org/wiki/File:Circulatory_System_en.svg
multicellular solutions to physicochemical constraints • 2. increased distance from periphery to nucleus--diffusion limits • be multi-nucleate: pack multiple nuclei into that big cell, and problem goes away. • Problem: How does such a thing divide? • Solution 1 (growth): decouple nuclear division and cytokinesis • Solution 2 (for sex): produce a uninuclear phase for reproduction • Ex: fungi: • cells of mycelium • interconnected. • meiotic products • (spores) • uninucleate. Fig. 31.7
Primary cell walls Middle lamella multicellular solutions to physicochemical constraints • 2. increased distance from periphery to nucleus--diffusion limits • --OR-- • be multicellular: Link separate cells together by adhesion villi increase absortive surface plant cell walls are cemented together by the middle lamella This cell contains vesicles filled with mucus 1 2 3 4 Figure 8-9 Figure 8-7 Section through small intestine. Each number represents a distinct cell.
Problem: How to reproduce? • Solution 1 (asexual): Budding, fission, etc. • Solution 2 (sexual): Produce a single-cell spore or gamete. • Problem: Who gets to reproduce? • Solution: tight control over mitotic and meiotic potential [How this evolved is one of the biggest mysteries of evolution. Failure to do it = cancer]
multicellular solutions to physicochemical constraints 3. inability of external and internal supports to scale up. • stay squishy • tough cuticle (extracellular surface layer) often present • hydrostatic skeleton allows controlled shape changes Primary cell walls Middle lamella
Hydrostatic skeleton of a nematode Body wall (in tension— creates pressure in fluid) Fluid-filled pseudocoelom (under pressure—creates tension in body wall) Gut Figure 32-7 Muscles (cause shape change) Coordinated muscle contractions result in locomotion. Muscles relaxed Muscles contracted Muscles contracted When the muscles on one side contract, the fluid-filled chamber changes shape and the animal bends. Muscles relaxed
get rigid--evolve a skeleton • exoskeleton • mineralized (corals, molluscs) • organic (arthropods) • endoskeleton • mineralized (echinoderms, sponges, bony fishes) • organic (cartilaginous fishes, woody plants)