Eukarya: Membrane-bounded nucleus and more Recall the Phylogeny of the Three Domains

Bacteria - the "true" bacteria
Archaea - the archaeans
Eukarya - the eukaryotes

What Makes a Eukaryote? Unifying Features

  • The 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 endoplasmic reticulum, Golgi bodies, vacuoles, lysosomes, peroxisomes, and the nuclear envelope
  • Primary genome of each cell consists of multiple, linear chromosomes contained within a membrane-bound nucleus.
  • Chromosomes segregate during somatic growth (or unicellular asexual reproduction) by the process of mitosis.
  • Mitochondria: energy-transducing organelles bounded by two membranes
  • Unique 80S ribosomes: each consistis of four molecules of RNA complexed with many proteins. Functional whole consists of a 40S small and a 60S large subunit

    Other eukaryote characteristis, not present in all eukaryotes. (These may have evolved independently, several times):

    Two processes can be combined to explain eukaryotic origin:

  • Autogeny

  • Endosymbiosis

    An overview...

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

    Recently, viruses have been implicated in the evolution of the eukaryote genome (and phenotype).

    Single-celled and colonial eukaryotes were once included in a single form taxon, "Kingdom Protista", now known to be paraphyletic. A more recent understanding of eukaryote relationships can be seen here:

    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

    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.
  • They live in both terrestrial and aquatic habitats (though they generally need moisture when in their active life cycle stages.)
  • Some are extremely complex--the most complex living cells.
  • May be
  • 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.)

    A Tour of the Protists See evolving phylogenies at the Tree of Life Project.

    The main protist clades (at the moment) are...

    This group is named for a groove that appears to be "excavated" on on side of the cell.

  • A parasitic diplomonad, Giardia lamblia, presents an interesting showcase of primitive characters exhibited by "basal" protists.

  • Trichomonas vaginalis is another opportunistic pathogen (found in the human female vagina) that shares many primitive characters with Giardia.
  • Trichonympha and Personympha are two parabasalids that make life possible for termites (endosymbionts in the intestine, they digest cellulose, which termites cannot)
  • Euglena is a ubiquitous mixotroph that some of us remember from after Hurricane Andrew.
  • kinetoplastids (such as Trypanosoma spp., the causative agents of such deadly diseases as Chaga's Disease, leishmaniasis and "Sleeping Sickness", more accurately known as African Trypanosomiasis.)

    Along with many other flagellum-bearing protists, euglenoids and kinetoplastids were once lumped in the now-defunct, polyphyletic taxon "Phylum 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 (and its shorter cousin, the cilium is a very ancient, widespread eukaryotic structure. It gives little information useful for classifying anything with a flagellum into a less inclusive taxon.

    Side note: However, the flagellum of the euglenozoans has a unique feature: a spiral or crystalline rod inside the protein filaments. It's function is unknown, but this unique feature of the flagellum is derived, and so its presence has been used as a synapomorphy that links the Euglenoids into a single, putatively monophyletic taxon.

    Also unique to euglenoids are disk-shaped cristae in the mitochondria. The two very basic characters suggest monophyly of the euglenoids, kinetoplastids, and a few other small groups of euglenozoans.

    This diverse group includes not only some of the most important photoautotrophs in the biosphere (diatoms; macroalgae such as "brown algae"), but also economically important pathogens. It is currently divided into two clades, the Alveolata and the Stramenopila.

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

    This group includes the

  • and the hugely diverse ciliates (among the most complex of all protists--and possibly all cell types, in general)

    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 Sometimes subsumed within the Chromalveolata, these are specialized amoebas that 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.



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

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

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


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

    We will talk about the Green Plants as a monophyletic group separately.

    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. And, specifically, to Entamoebas.

    Slime Molds
    But we also share a common ancestor with slime molds, whose systematics are still being worked out.

    Our closest relations, though, are unikont protists known as choanoflagellates. Among all the protists, it is with these small, colonial creatures that we are believed to share a most recent common ancestor.

    And we shall return to them again when we meet the Animalia.