multicellularity

complex multicellular organisms can have upwards a trillion more cells

complex multicellularity

has occurred upwards (evolved) to half a dozen times

prokaryotic organisms

unicellular, however live in colonies (filaments)

bacteria

does not develop macroscopic bodies with functionally differentiated tissues

simple multicellular organisms

36 of the 119 major groups of eukaryotes are multicellular (filaments, hollow balls, or sheets of little-differentiated cells)

the 83 unicellular organisms properties

eat other microorganisms or ingest small organic particles, live suspended in water columns, or are parasites within other organisms

properties or simple multicellularity organisms

• adhesion molecules enables to stick, but little communication
• most cells retain full range of functions including reproduction
• every cell is in contact with external environment

coenocyte organisms

multinucleate cell which can result from multiple nuclear divisions without accompanied cytokinesis

six groups have evolved coenocytic organization

nuclei evolved many times, not divided into individual cells, and results in very large cells with many nuclei

pros and cons of coenocyte multicellularity

• helps organisms avoid predator feeding, can help maintain position in surface/water
• most don't reproduce but instead support the ones that do (point of life is to procreate), some cells "cheat" and use resources for themselves (cancer)

complex multicellularity

land plants, animals, red/brown algae, fungi

common features

• Highly developed mechanisms for cell adhesion
• Cell communication
• Tissue/Organ differentiation
• 3-D structure that enables some cells to have direct contact with the external environment. Requires complex mechanisms for communicating external info to interior where genes will be activated or repressed in response.

evolution occurred 6 times

1. Animals
2. Green algae to Vascular plants
3. Red Algae
4. Brown Algae
5. Fungi (1x)
6. Fungi (2x)

diffusion (important mechanism to transport substances in multicellular organisms)

random motion of molecules with net movement from areas of high to low concentration (entropy shiet)

bulk transport (important mechanism to transport substances in multicellular organisms)

once an organism gets large enough diffusion doesn’t cut it for the movement of material; Bulk Transport is any means that are used to move material at rates higher than that of diffusion!

diffusion limitations

1. Effective only over small distance: this limits the size and shape of bacterial cells
2. Constrains how eukaryotic organisms function

sponges

circumvent this by placing metabolically active cells in close contact with their external environment, hence their size

jellyfish

have thin layers of metabolically active tissue above inactive material

bulk transport in humans

we use a high ration of surface area to volume in lung tissue in order to bypass diffusion restrictions. We pump blood through the circulatory system to oxygenate tissues!

bulk transport in plants

vascular tissues transport water from roots to photosynthetic sources, wherein other vascular tissues transport sugar from the leaves to other parts

bulk transport in fungi

rely upon osmosis to pump material (hyphae)

bulk transport in animals

pumping blood through the circulatory system to oxygenate tissues that are great distances from the lungs, during digestion, and during hormone signaling

main components required for complex multicellularity (functional key)

1. Cells must stick together (cell adhesion)
2. Cells must communicate
3. Cells must participate in a network of genetic interactions for cell division/differentiation (genetic programs the guide growth and development)

cell adhesion in animals

cadherins, integrins, and addn. Transmembrane proteins provide the molecular mechanisms for adhesion

cell adhesion in plants

use adhesion molecules called pectins (where we get jelly from)

choanoflagellates

• are the closest protistan (free-living or colonial single-celled eukaryotes) relatives to animals.
• Their genome contains cadherins and integrin proteins they use to clump together and protect themselves
• although unicellular, simple multicellular structures can be induced by molecular signals in a number of species

cell adhesion in choanoflagellates

signals caused by their pray (bacteria) , form a multicellular structure, support hypothesis that they facilitate predation

communication

molecular signals found in animals, plants, and some protistan relatives

gap junctions

protein channels allows ions and signaling molecules to move from one cell into another, help cells communicate with neighbor cells and targeted (specific) cells

plasmodesmata

intracellular strands of cytoplasm that extend to neigh cell and allow same type of cell communication, permit signaling molecules to pass between cells (important step in evolution of complex multicellularity)

complex multicellularity involves the

genetic programming of cells so that they can differentiate in space (3D organism)

genes are turned off or on depending

molecular signals the cell receives

number of gene families play a role in life cycle differentiation

Plant multicellularity

plant cell wall (cellulose) -> structural and mechanical support, enables trees to grow, also for plant rigidity (developmental consequences)

• Growth and development: cell division, expansion, and differentiation
• developmental/mechanical consequence: plant growth confined to meristem (the actively growing primarily undifferentiated cells at the tips of stems and roots)
• Necessity to evolve mechanisms to transport water/nutrients from soil without moving parts/ATP expenditure
• Adjust to environmental conditions via adjusting meristem activity(growth rate)
• Unable to flee predators, developed mechanical structures/poisons to keep from being eaten

Animal Multicellularity

no cell wall, can move with each other, sperm + egg -> fertilized egg -> w/mitosis -> blastula (ball of undifferentiated cells) -> blastula migration, cell reorganization -> gastrula (hollow ball, folds inward at one location to form layered structure)

• Gastrula formation: cells in direct contact with each other, creates molecular signaling patterns and gene regulation (begins growth and tissue specification)
• Animal cells CANNOT move during development -> cell division/tissue differentiation occurs throughout whole body
• No cells walls, organs w/moving parts (muscles) to power active transport of food/fluids and movement

Evolution of Complex Multicellularity

natural selection favors increase/diversification of genes regulating growth/development (more complex over time)

Fossils do not preserve molecular features, but can show phylogenetic pattern

Complex multicellularity from 575-555 mya, oldest being rangeomorphs (found in newfoundland) showing no evidence of head or tail, no limbs, no opening as mouth , appeared to be very simple organisms that obtained both carbon and oxygen by diffusion

Same time, preserved tracks showing movement, a bit later, animals get more complex(identifiable heads, tails, etc.)

Oxygen enrichment vs. first appearance of complex animals

more oxygen = greater size permitted, greater size leads to evolution of bulk transport and signaling = even larger size allowance

Complex multicell and land plants

origin about 465 mya (much later than marine animals and algae)

Two challenges led to later evolution:

1. Photosynthesis had to be carried out in tissues surrounded by air rather than water
2. Process required nutrients/water from soil, transported throughout plant, opposed to simple diffusion via nearby water

400 mya, plants evolved specialized tissue for bulk transport, plant biomass increased, created new opportunities for multicell organisms, specifically fungi

Regulatory Genes (played a role in multicellular complex evolution)

• Molecular signal induces expression of gene -> protein product induces expression or repression of another gene
• Complex interplay of these genes results in patterned butterfly wings or large pincers in crabs

Distalless

regulatory genes account for coloration, mutation in genes creates variation within species, Distalless responsible for “eye” pattern on wing (linked to protein GFP), expression of gene traced to GFP visualization (affects number and size of eyes)

Evo-Devo (evolutionary-developmental biology)

new field, growing understanding of how developmental genes underpin evolutionary change, illuminates long-suspected relationship between individual development and patterns of evolutionary relatedness among different species