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):
- Multicellularity and true tissues
- Secreted hard body parts
- Organelles that function in defense, prey capture or parasitic invasion via extrusion of internal components.
- Plastids, including chloroplasts and their homologs.
Two processes can be combined to explain eukaryotic origin:
Extensive inpocketing of the external plasma membrane formed a complex internal network of membranes.
First proposed by Lynn Margulis (University of Massachusetts), this model proposes that small,
energy-transducing prokaryotes were either
inside larger prokaryotes, where they survived and thrived.
Eventually, host and symbiont became inextricably linked
in a symbiotic relationship.
- ingested as prey
- internal symbionts
Primary endosymbiosis is believed to have given rise to heterotrophs, and secondary endosymbiosis is believed to have given rise to the photoautotrophic eukaryotes.
- such symbioses exist today (e.g. Giardia, a basal (i.e., very primitive)
flagellated protist has symbiotic, energy-transducing bacteria instead of
mitochondria and two haploid nuclei.)
- there are extant species of cyanobacteria and heterotrophic bacteria
that strongly resemble chloroplasts and mitochondria, respectively.
- the enzymes embedded in mitochondrial and chloroplast internal
membranes 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.
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.
- detritivorous (feeding on dead, organic matter and turning it into
smaller organic molecules, but NOT decomposing it)
- mixotrophs (a little of both)
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"
arch = "beginning"
A Tour of the Protists
See evolving phylogenies at the Tree of Life Project.
The main protist clades (at the moment) are...
- Unikonta (monophyletic with Animals and Fungi)
- Archaeplastida (monophyletic with Plants)
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.
- it has typical eukaryotic flagella
- mitochondria are reduced, and lack DNA
- mitochondria are reduced, and lack DNA
- Giardia lacks any type of plastids
- lacks electron transport chain proteins or any enzymes associated with aerobic respiration.
- usually lives in anaerobic conditions
- has a very simple cytoskeleton
- contains two haploid nuclei
- pathogenic; causes
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
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
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
- economically important Apicomplexans, parasites formerly known as
These all possess a modified Golgi apparatus, the apicoplast, at the apex of the cell, which facilitates invasion of a host cell. All apicomplexans are intracellular parasites of animals (coccidia, gregarines, plasmodia, etc.)
- 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
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.
These manufacture a test of silica (glass!). Found more commonly in cold water and fresh water.
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
sushi wrap (Porphyra sp.)
- lack flagella
- lack centrioles
We will talk about the Green Plants as a monophyletic group separately.
This clade includes several groups of protists as well as the Fungi and Animalia.
Yes, it's true. We are closely related to amoebas. And, specifically, to Entamoebas.
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.