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September 14, 2012 Evolution Friday
The evolution of the cell, especially the development from prokaryote to eukaryote, is an important topic as it demonstrates core differences between living organisms. Archaea, Bacteria, and Eukarya are dramatically different from each other, but even within these domains, there are vast differences in cell types. When studying the development of cellular differences and structures, and thus studying the evolution of cells, biologists often look toward existent organisms that do not fit the typical cellular model. Anomalous structures give us insight into possible evolutionary steps.
For example, the genus Symbiodinium is an algae of the phyllum Dinoflagellata. These algae are known to live inside the cells of cnidarians (corals and jellyfish). Within the cell, they algae will deliver photosynthetic products to the cnidarians. Sound familar? How does this cell "know" to become symbiotic? Are there signals between the two cells? Does this mimic the entry of a Cyanobacteria into a cell where it became the chloroplast?
Then there is the bacterial phyllum Planctomycetes which researchers suspect is the "missing-link" between prokaryotic and eukaryotic cells, i.e., the development of a nucleus. Beyond membrane alterations, this is a bacteria that reproduces by budding (the daughter cell has a reduced cytoplasm). Other bacteria reproduce by binary fission, which results in two daughter cells with equal cytoplasm.
The bacterium Bdellovibrio is of interest in that it is a predator of other bacteria. This organism hunts other bacteria, enters their cell wall, then devours them. This is a very unusual characteristic. Not only in the recognition of prey, but in the entire life cycle. This is a complexity that we do not see in other bacterial groups.
Daily Challenge
Review the following diagram showing proposed steps in the evolution of the eukaryotic cell.
In your own words, discuss the origin of the eukaryotic cell. Using the examples above, and any others you find, how can we provide evidence of these evolutionary steps or stages. Some claim that the Archaea are the ancestors of the Eukarya, but if so, why are there so many differences, espeically when it comes to phospholipids? How easy would it be to change the phospholipids a cell uses?
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