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Eukarya

What Makes a Eukaryote?

All eukarotic cells have:
  • a cytoskeleton, consisting of tubulin-based microtubules and actin-based microfilaments

  • flagella (or their shortened versions, cilia) constructed of an axoneme of 9 peripheral microtubular doublets and 2 central microtubules

  • An endomembrane system consisting of

  • a primary genome contained on multiple, linear chromosomes within a membrane-bounded nucleus

    (NOTE: the prokaryotic genome above is from a lysed cell; the DNA doesn't actually look like that in a live bacterium. This is just a more direct way of visualizing the prokaryotic DNA strand.)

  • the ability to undergo asexual reproduction via mitosis.

  • mitochondria, energy-transducing organelles bounded by two membranes

  • unique 80S ribosomes, each consisting of four molecules of RNA complexed with many proteins. Functional whole consists of a 40S small and a 60S large subunit



    The Origin of Eukaryotes

    Two processes likely contributed to eukaryotic origin:

    Evidence:

    Sequential Endosymbiosis

    After the first eukaryotic cells had formed, further endosymbioses occurred to produce various eukaryotic lineages.
  • Primary endosymbiosis - a larger cell engulfs a smaller cell, which then takes up residence to the benefit of both cells
    (as described above).

  • Secondary endosymbiosis - the product of primary endosymbiosis is engulfed by a larger cell, and then takes up residence to the benefit of both cells.

    Secondary endosymbiosis has given rise to a vast array of eukaryotic lineages.

    The typical cladogram we saw earlier could be modified to account for this combining of ancestral lineages:

    Transfer of genes from one individual to another in this manner (i.e., not from parent to offspring, which is vertical gene transfer) is known as HORIZONTAL (= LATERAL) GENE TRANSFER.


    Protists: the Tiny Beasts


    "No more pleasant sight has met my eye than this,
    of so many thousands of living creatures in
    one small drop of water."
    - Anton van Leeuwenhoek

    The majority of eukaryotic diversity lies with the single-celled organisms known as protists.

    Protists range in size from the mighty, shelled, multinucleate Syringammina fragilissima (more than 20mm!) to the tiny Ostreococcus tauri (1 μm).

    Other than unicellularity, few characters link the protists.

  • Most are unicellular, but some may be aggregate, colonial, or colonial with a cellular division of labor.

    This is illustrated by the diversity and progression of complexity in green algae in the "Volvocine Line of Evolution".

  • Protists live in both terrestrial and aquatic habitats (though they generally need moisture when in their active life cycle stages.)

  • Various species of protists may be
  • Protists may be

    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.)


    Protist Diversity

    See evolving phylogenies at the Tree of Life Project.

    Currently, the main protist clades are

    (It may still turn out that some of these groups are not monophyletic.
    Stay tuned for further research...)



    Excavates


    This group is named for a groove that appears to be "excavated" on on side of the cell.
    Its members tend to exhibit primitive characters, and it may be that this group is basal to all others.

    Giardia lamblia: A Living Fossil
    A parasitic diplomonad, Giardia lamblia, presents an interesting showcase of primitive characters exhibited by these "basal" protists.

    (Watch only the first two minutes; the rest is an advertisement for a Giardia detection test.)

    Other Important Excavates

    Trichomonas vaginalis is another opportunistic pathogen (originally first identified in the human female vagina, hence its name) that shares many primitive characters with Giardia.

    It is the most common protist pathogen of humans in industrialized countries.

    Trichonympha and Personympha are two basal Excavates that make life possible for termites.
    They are termite intestinal endosymbionts, able digest cellulose, which termites cannot.

    Euglena is a ubiquitous mixotroph that some of us remember from after Hurricane Andrew, when it turned contaminated swimming pools solid emerald green.

    Kinetoplastid Excavates can cause serious parasitic diseases.

    Trypanosoma spp. are the causative agents of such deadly diseases as Chaga's Disease, leishmaniasis and "Sleeping Sickness", more accurately known as African Trypanosomiasis.)


    Phylogenetic Boo Boos: The Flagellum

    Along with many other flagellum-bearing protists, euglenoids and kinetoplastids were once lumped in the now-defunct, polyphyletic taxon "Mastigophora" (mastig = "whip"; phor = "to bear").

    This should serve as a reminder about that symplesiomorphies are not informative when one is trying to construct monophyletic taxa.

    The flagellum is a very ancient, widespread eukaryotic structure. Its presence provides no information useful for classifying anything with a flagellum into a less inclusive taxon.

    Side note: The flagellum of the euglenozoans has a unique feature: a spiral or crystalline rod inside the protein filaments. Its function is unknown, but this unique feature of the flagellum is derived. Its presence can be used as a synapomorphy that links the Euglenoids into a single monophyletic taxon.

    Also unique to euglenoids are disk-shaped cristae in the mitochondria.

    These two very basic characters suggest monophyly of the euglenoids, kinetoplastids, and a few other small groups of euglenozoans with these synapomorphies.



    Chromalveolata

    This diverse group includes both some of the most important photoautotrophs in the biosphere (diatoms; macroalgae such as "brown algae"), and economically important pathogens. It is currently divided into two clades

    Alveolata
    These are linked by the presence of alveoli under the plasma membrane, which is highly complex in function and anatomy.

    Important Alveolates include


    Stramenopila
    Their name comes from the Latin stramen ("straw") and pilos ("hair")). The taxon gets its name from its fuzzy flagellum, which is often paired with a smooth one. Flagellated cells occur in all members of this taxon, though in some highly derived groups, they occur only during reproductive cycles and function as gametes. This group includes the Also nestled within this clade are the haptophytes, unicellular algae that produce plated shells (coccoliths) that presumably protect them from predators. The calcium carbonate "skeletons" of a famous haptophyte, Emiliana huxleyi are the primary component of the White Cliffs of Dover

    Rhizaria
    Many of these aquatic Chromalveolate amoebas secrete ornate shells, through which highly derived, threadlike pseudopods emerge. The pseudopods facilitate movement, may be involved in prey capture, and also provide buoyancy via greatly increased surface area.
    They are some of the most beautiful protists.

    Foraminifera

    Radiolaria



    Archaeplastida

    This clade includes the Red Algae (Rhodophyta) and the Green Plants.

    Rhodophyta
    The Rhodophyta form a monophyletic clade united by the synapomorphic appearance of pigments known as

    all contained in specialized light-collecting systems known as phycobilisomes.
    These are found elsewhere only in the cyanobacteria, strongly suggesting that a secondary endosymbiosis involving cyanobacteria gave rise to the Rhodophytes.

    These pigments absorb blue wavelengths, transmitting their energy to the photosynthetic cycle.
    Thus, Rhodophytes can live in relatively low light conditions where other algae cannot

    Rhodophytes They are a diverse and beautiful group, and are economically important as the source of
  • agar
  • sushi wrap (Porphyra sp.)

    (We will visit the Green Plants later. For now, know they are sister taxon to the Rhodophytes.)



    Unikonta

    This clade includes several groups of protists as well as the Fungi and Animalia.

    Amoebozoa
    Yes, it's true. We are closely related to amoebas (most specifically, to entamoebas).

    Don't get this one up your nose. It will eat your brain.

    Slime Molds
    We also share a relatively recent common ancestor with slime molds, whose phylogenetic relationships are still being determined. Most of these are believed to be Amoebozoans.

    Choanoflagellates
    The closest protist relatives to Animalia are unikont protists known as choanoflagellates.

    As you will see, they are very similar to specialized cells found in sponges, the most primitive animals.