Sorry for brining up an old thread but...
Adz795 wrote:Organisms with a defined, heritable body plan. That's perfect for ruling out prokaryotes and it is indeed met by all eukaryotes. Great definition!
There are plenty of unicellular Eukaryotes.
There are also multicullular Eukaryotes that don't have a defined shape.
Triochoplax being the best example, it has some general organization, but not a defined shape (though it does have defined sides).http://en.wikipedia.org/wiki/Trichoplax
No two trees are the same shape, they do share a similar fractal organization, and specific structures (xylem, phloem, leaves, etc) are highly organized with tightly constrained shapes.
Meanwhile, there is some definition to the shape of cyanobacteria, and organization regarding where the heterocysts form.
But there are better examples:
Myxospoes will travel in swarms, and when conditions get bad, form macroscopic "fruiting bodies"/spores, a multicellular structure with an organized shape, for disseminating the more resistant spores.http://en.wikipedia.org/wiki/Myxococcus_xanthushttp://en.wikipedia.org/wiki/Myxobacteria
But best of all, are these: magnetotactic multicellular prokaryotehttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC2869151/http://ijs.sgmjournals.org/content/57/6/1318.full
They too have a defined shape (a spiral ball with flagella on the outside and an acellular interior compartment), a constrained cell number.... and unlike the myxococcus or the cyanobacteria -> the individual cells cannot survive outside the organism.
1 myxococcus can divide and make more of itself (though outside the swarm, in the "wild" its unlikely it will be able to acquire enough food given the diffusion of the digestive enzymes it secretes - they really do benefit greatly from travelling in a swarm).
1 cyanobacteria can do that
1 Magnetoglobus -> dead
The MMP is a large prokaryotic microorganism that ranges from about 3 to 12 μm in diameter. It is best described as an aggregation of about 10 to 40 Gram-negative, genetically similar cells that swim only as an intact unit and not as individual cells . Cells that separate from the intact unit die quickly according to viability studies. The surface of the cell exposed to the surrounding environment is covered with numerous flagella. Most described MMPs are spherical, although some are ovoid in morphology, and all appear to possess a central, acellular compartment. The MMP divides as aggregates without an individual cell stage.
Phylogenetic analysis and phenotypic characterization were used to assign a multicellular magnetotactic prokaryote the name ‘Candidatus Magnetoglobus multicellularis’. ‘Candidatus Magnetoglobus multicellularis' lives in a large hypersaline coastal lagoon from Brazil and has properties that are unique among prokaryotes. It consists of a compact assembly or aggregate of flagellated bacterial cells, highly organized in a sphere, that swim in either helical or straight trajectories. The life cycle of ‘Candidatus Magnetoglobus multicellularis' is completely multicellular, in which one aggregate grows by enlarging the size of its cells and approximately doubling the volume of the whole organism. Cells then divide synchronously, maintaining the spherical arrangement; finally the cells separate into two identical aggregates.