And Now Begins Our Tour of The Kingdoms.
We've already met the THREE DOMAINS

Domain Bacteria - the "true bacteria"

Domain Archaea - the archaebacteria

Domain Eukarya - the eukaryotes

The evolutionary relationships have been determined via comparison of DNA, mRNA and rRNA sequences.

Signature sequences (base sequences at specific locations along the rRNA) are comparable and relatively taxon specific, yielding powerful evidence for recency of common descent of Archaea (specifically, the thermophiles) and Eukarya.

We've already seen that prokaryotes display a wide array of metabolic pathways, with a variety of elements/compounds used as oxidizing agents and terminal electron acceptors.

In EUKARYOTES, only one of two pathways is seen:

• aerobic metabolism (oxygen as terminal electron acceptor)

• fermentation
  

This suggests that prokaryotes may have evolved several times, whereas eukaryotes are more likely to share a common ancestor.

There are two models for eukaryotic origin:

• THE AUTOGENOUS MODEL
  Eukaryotes arose when the outer membranes of ancestral prokaryotes underwent extensive inpocketing and pinching off to form complex internal network of membranes.
  • Evidence: some extant species of cyanobacteria (e.g., Gloeocapsa) have complex internal membrane systems and resemble simple chloroplasts.

• THE ENDOSYMBIONT MODEL
  Proposed by Lynn Margulis (University of Massachusetts), this model holds that small, energy-transducing prokaryotes took up residence (either by being ingested as prey, or as internal symbionts) inside larger prokaryotes, where they survived just fine. Eventually, host and symbiont became inextricably linked in a symbiotic relationship.
  • Evidence:

    • such symbioses exist today (e.g. Chaos, which resembles a very large amoeba, but has symbiotic, energy-transducing bacteria instead of mitochondria)

    • there are extant species of cyanobacteria and heterotrophic bacteria that strongly resemble chloroplasts and mitochondria, respectively.

    • the enzymes embedded in mitochondrial and chloroplast internal membrantes are more similar to those of prokaryotes than they are to other enzymes found in eukaryotes

    • ribosome enzymes in the mitochondria and chloroplasts are more like those of prokaryotes than those of eukaryotes

    • mitochondria and chloroplasts have their own genome, separate from and largely independent of the nuclear genome.

    • mtDNA and cpDNA are circular, and though there may be multiple copies, all are genetically identical within a given cell (hence, these organelles are--like bacteria--essentially haploid
    • mtDNA and cpDNA circular chromosomes have no associated histones or RNA

    • cytochromes and other transport proteins used in mitochondria and chloroplasts are made in situ, without cooperation from nuclear genome enzyme products.

    • mitochondria and chloroplasts reproduce via binary fission very similar to that seen in prokaryotes.

    • nucleus, mitochondria and chloroplasts all have a double membrane system

Here's an overview of the two models:

Until fairly recently, the "protists" were all included in a single form taxon, Kingdom Protista.

Now that more sophisticated methods for determining similarities and differences among cells are in use (DNA, RNA analysis; cell ultrastructural analysis, etc.), the polyphyly of that "Kingdom" is painfully obvious.

The Earliest Eukaryotes were Protists

The oldest known eukaryote fossils (2.1 billion years old, found in pre-Cambrian fossil beds in Michigan) are called "acritarchs"

acrit = "confused" (Gr)

arch = "beginning" (Gr.)

What do "protists" have in common? Other than their unicellularity, few characters link them as a large group. Rather, they are now being divided into the candidate kingdoms we see above.

Protists may be

• unicellular
• colonial
• colonial with a division of labor among cells

• planktonic (free-floating in a marine or freshwater water column)
• terrestrial

• photoautotrophic
• chemoheterotrophic, including
  • predatory
  • parasitic
  • commensal
  • mutualistic
  • detritivorous (feeding on dead, organic matter and turning it into smaller organic molecules, but NOT decomposing it)

Let's have a look at some of the protists we know today, and the most educated suspicions about their evolutionary relationships. For a really excellent overview of current protist biology and systematics, visit this comprehensive site U.C. Berkeley. (Go, Bears.)

This may be a polyphyletic garbage can akin to the old "Kingdom Protista," as it contains a variety of protists whose evolutionary affinities are not clear. A broad, non-committal overview of the most primitive protists can be seen HERE. A parasitic species Giardia lamblia serves as a good example of the primitive characters shown by many of the organisms in this assemblage.

• it is flagellated (though flagellum has typical eukaryote morphology: two central microtubules surrounded by nine paired microtubules)

• pathogenic (transmitted via contact with infected water; causes severe diarrhea and intestinal cramps in a variety of mammals and birds). If you're interested in learning more about giardiasis, and in seeing pictures of this evolutionary wonder, click HERE.

• lacks mitochondria or any type of plastids

• has a very simple cytoskeleton

• contains two haploid nuclei

• Is Giardia's anatomy a holdover of its ancestor's autogeny? Or perhaps endosymbiosis? Or maybe both?