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 endoplasmic reticulum, Golgi bodies, vacuoles, lysosomes, peroxisomes, and the nuclear envelope
a primary genome contained on multiple, linear chromosomes within a membrane-bounded nucleus
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:
Extensive inpocketing of the external plasma membrane formed a complex internal network of membranes.
First proposed by Lynn Margulis (University of Massachusetts), this model describes how 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
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
- 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.
(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.
- Red algae (products of primary endosymbiosis) took up residence/were ingested by heterotrophic eukaryotes and became the precursors of modern taxa
- Green algae (products of primary endosymbiosis) took up residence/were ingested by heterotrophic eukaryotes and became the precursors of modern taxa
- Chlorarachniophyta (green algae and plants)
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
- detritivorous (feeding on dead, organic matter and turning it into
smaller organic molecules, but NOT decomposing it)
- mixotrophs -switch between autotrophy and heterotrophy, as environmental conditions dictate
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"
See evolving phylogenies at the Tree of Life Project.
The main protist clades--still possibly polyphyletic--are
- Archaeplastida (also includes Plants)
- Unikonts (also includes Animals and Fungi)
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.
- it has typical eukaryotic flagella
- mitochondria are reduced, and lack DNA
- no plastids are present
- no electron transport chain proteins or enzymes associated with aerobic respiration.
- usually lives in anaerobic conditions
- has a very simple cytoskeleton
- contains two haploid nuclei
- pathogenic; causes
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.
spp. are the
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
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.
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
These are linked by the presence of alveoli under the plasma membrane,
which is highly complex in function and anatomy.
Important Alveolates include
These organisms are responsible for seasonal red tides that can cause massive die-offs of fish and other marine animals.
They are also indirectly responsible for the most common form of non-bacterial seafood poisoning in the U.S., ciguatera.
Not all dinoflagellates are dangerous.
Genus Symbiodinium forms one of the most vital symbiotic relationships on earth: zooxanthellae and coral.
View some nice dinoflagellate images.
These parasites, formerly known as
"sporozoans", have a modified Golgi apparatus, the apicoplast, at the apex of the cell. This structure facilitates invasion of a host cell.
All apicomplexans are intracellular parasites of animals, and are usually quite host specific.
- Plasmodium spp. cause malaria
- Eimeria and Isospora spp. cause coccidiosis
- Toxoplasma spp. cause toxoplasmosis
- Gregarines are parasites of mollusks and annelids.
Commonly known as ciliates, these are among the most complex of all protists--and possibly all cell types, in general.
- covered in shortened flagella called cilia
- one small, diploid micronucleus for reproduction only; its genes are not expressed
- one large, polyploid macronucleus for regulation of cell functions. Produced by the micronucleus, it is not passed on to daughter cells.
- This are the most diverse protist group, with over 8000 species.
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
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.
These manufacture a test of silica (glass!). Found more commonly in cold water and fresh water. (image on the right)
This clade includes the Red Algae (Rhodophyta) and the Green Plants.
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.
- phycoerythrin (red)
- phycocyanin (blue)
- allophycocyanins (blue)
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
- very deep water
- under algal mats of other types of algae
They are a diverse and beautiful group, and are economically important as the source of
sushi wrap (Porphyra sp.)
- lack flagella
- lack centrioles
(We will visit the Green Plants later. For now, know they are sister taxon to the Rhodophytes.)
This clade includes several groups of protists as well as the Fungi and Animalia.
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.
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.
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.