http://phylogeny.arizona.edu/tree/eukaryotes/eukaryotes.html
What makes a eukaryote a eukaryote?
eukaryotes are distinguished from prokaryotes by
the structural complexity of the cells. - characterized by
having many functions segregated into
semi-autonomous regions (organelles),
and by the cytoskeleton.
The most evident organelle in most cells is the nucleus,
and it is from … this organelle that the eukaryotes get their name.
Nuclei contain most of the genetic material of a cell - with other elements of the
genome located in mitchondria and plastids (if those organelles are also present).
The nucleus is bounded by a membranous envelope.
The nuclear envelope is part of the endomembrane system that extends to include
the endoplasmic reticulum, dictyosomes (Golgi apparatus) and
the plasma membrane that encloses the cell.
Within the nucleus, the genes are located on a number of chromosomes.
The total amount of DNA in a nucleus
measuring less than one hundredth of a millimetre across may stretch to over a metre.
http://phylogeny.arizona.edu/tree/eukaryotes/eukaryotes.html
The cytoskeleton is comprised of a rich array of proteins.
The major ones are
tubulin (which forms microtubules) and
actin (forming microfilaments) and
a myriad of interacting proteins
which effect movement or
create the skeletal architecture of cells.
- enzyme dynein
- enzyme myosin
C&R Fig 7.24
Some antimitotic alkaloids are useful as cancer chemotherapy agents.
Taxol {from yew} increases the formation of microtubules and stabilizes them
so that there is no free tubulin for the formation of mitotic spindles.
Vinblastine {from periwinkel} causes the disassembly of formed microtubules
and causes the aggregation of crystaline tubulin.
Because tumor cells multiply faster than normal cells
they are more susceptible to antimitotic drugs.
The Endosymbiotic Theory of Eukaryote Evolution
This idea was promulgated at the turn of the 20th century (Mereschkowsky, 1910)
and promoted later by Lynn Margulis (Margulis, 1970).
Lynn Margulis
Right: Spirochetes are masters of movement,
corkscrewing about with neither head nor tail.
(Photograpgh by Lyn Margulis)
Bottom: This contemporary protist, Trichonympha,
is pushed through its viscous termite hindgut habitat
by thousands of symbiotic spirochete bacteria
attaching at the rear. (Photography by David Chase)
http://www.earthwise.org/walk/walk_photos/2700.htm
{this is basically the junk drawer}
not animals, true fungi or green plants.
those eukaryotes that are
The protists are a paraphyletic group;
Diplomonadida and Parabasala: lack mitochondria
According to what is called the "archaezoa hypothesis,"
these protists are the modern forms of ancient eukaryotic lineages
that evolved before the acquisition of the endosymbiotic bacteria
that gave rise to mitochondria.
Most protistan systematists have recently rejected that hypothesis
based on the discovery of mitochondrial genes in the nuclear genomes …
the evidence now supports the hypothesis that
these protists lost their mitochondria during their evolution
from ancestors in which mitochondria were present.
the Diplomonads (two nuclei):
http://www.nih.gov/news/pr/aug2001/nichd-15.htm
the Trichomonads (3 flagella)
Trichomoniasis is caused by infection with … Trichomonas vaginalis.
Pregnant women infected with T. vaginalis are more likely to …
give birth prematurely to children of low birth weight, …
children … are more than twice as likely to be stillborn
or to die as newborns…
The euglenoids (such as Euglena) are characterized by an anterior pocket
from which one or two flagella emerge.
Paramylon, a glucose polymer
that functions as a storage molecule,
is also characteristic of euglenoids.
The kinetoplastids have a single large mitochondrion associated with
a unique organelle, the kinetoplast, that houses extranuclear DNA.
Trypanosoma cause African sleeping sickness,
a human disease that is spread by the bite of the tsetse fly
http://usa.biologists.com/JCS/gallery.html
Trypanosoma gambiense
in a blood smear.
Immunofluorescence of the trypanosome mutant snl-1
staining the nucleus and the kinetoplast
(a large mitochondrion),
Protist: Euglenozoa: Kinetoplasts: Trypanosoma
http://www.biosci.ohio-state.edu/~parasite/chagas.html
A few species of Trypanosoma are found in the New World.
From the standpoint of human health,
the most important is Trypanosoma cruzi,
causing American trypanosomiasis or Chagas' disease.
The vector for Chagas' disease
is a "true bug" (Hemiptera)
{‘kissing bug’}
In the human host,
Chagas' disease affects primarily the nervous system and heart.
Chronic infections result in various neurological disorders,
including dementia, megacolon, and megaesophagus, and
damage to the heart muscle.
Left untreated, Chagas' disease is often fatal.
Small animals such as dogs, cats, guinea pigs, rats and opossums can serve as reservoirs
for the parasite. T. cruzi generally does not cause the significant disease in animals as it does
in people. …There is no known cure for T. cruzi infection in dogs and cats. Because of this
and the extreme danger to people, it is generally recommended that infected animals be
euthanized. The most effective means of control is the elimination of the kissing bugs.
Alveolata draws together
a group of flagellates (the dinoflagellates),
a group of parasites (apicomplexans),
and a distinctive group of eukaryotes that move by means of cilia (the ciliates).
Alveolates have small membrane-bounded cavities (alveoli)
under their cell surfaces. The function of the alveoli is unknown;
the Cilliates - Paramecium - Fig 28.14
macronucleus (approx 50n) controls ‘everyday functions’
& micronuclei - swapped in sexual conjugation - Fig 28.15
the Apicomplexans (sporozoan parasites)
Plasmodium - malaria- Fig 28.7
At present, at least
300,000,000 people are affected by malaria globally,
and there are between
1,000,000 and 1,500,000 malaria deaths per year.
the Dinoflagellates: phytoplankton - Red Tides
Pfiesteria piscicida (‘the Cell from Hell!’)
Haplotype diversity and linkage disequilibrium at human G6PD:
Recent origin of alleles that confer malarial resistance.
Tishkoff et al SCIENCE 293 (5529): 455-462 JUL 20 2001
Abstract:
The frequencies of Low-activity alleles of glucose-6-phosphate dehydrogenase
in humans are highly correlated with the prevalence of malaria.
These "deficiency" alleles are thought to provide reduced risk from infection
by the Plasmodium parasite and are maintained at high frequency
despite the hemopathologies that they cause.
Haplotype analysis of "A-" and "Med" mutations at this Locus indicates that
they have evolved independently and have increased in frequency
at a rate that is too rapid to be explained by random genetic drift.
Statistical modeling indicates that
the A- allele arose within the past 3840 to 11,760 years and
the Med allele arose within the past 1600 to 6640 years.
These results support the hypothesis that
malaria has had a major impact on humans
only since the introduction of agriculture within the past 10,000 years
and provide a striking example
of the signature of selection
on the human genome.
Alveolata: Dinoflagellates:
PhD - MSU 1986
What is Pfiesteria ?
http://www.epa.gov/owow/estuaries/pfiesteria/fact.html
Pfiesteria piscicida (fee-STEER-ee-uh pis-kuh-SEED-uh)
is a toxic dinoflagellate that has been associated with fish lesions
and fish kills in coastal waters from Delaware to North Carolina.
… dinoflagellates are microscopic, free-swimming,
single-celled organisms, usually classified as a type of alga.
Although many dinoflagellates are plant-like
and obtain energy by photosynthesis, others, including
Pfiesteria, are more animal-like and acquire some or all of their energy
by eating other organisms.
Discovered in 1988 by researchers at North Carolina State University,
Pfiesteria piscicida is now known to have a highly complex life-cycle
with 24 reported forms, a few of which can produce toxins.
… Pfiesteria only becomes toxic in the presence of fish,
triggered by their secretions or excrement in the water.
At that point, Pfiesteria cells shift forms and
begin emitting a powerful toxin that stuns the fish …
Other toxins are believed to break down fish skin tissue, …
As fish are incapacitated, the Pfiesteria cells feed on their tissues …
HEADLINE: The 'Cell From Hell'
Newsweek August 25, 1997
… what triggers the proliferation of Pfiesteria.
In Maryland, environmentalists are blaming the
PhD - MSU 1986
manure runoff from the region's 600 million chickens.
In North Carolina, environmentalists say the culprit
is mostly runoff from hog farms.
… Burkholder's struggles to get North Carolina to recognize Pfiesteria are legion.
A new book, "And the Waters Turned to Blood,"
recounts how corporations and state officials attacked Burkholder's credibility
HEADLINE: Scary monsters super creeps
New Scientist June 3, 2000
She calls it the career from hell - and she's only half joking.
She's been hounded by powerful industries …and watched her funding disappear.
North Carolina had tried for years to deny there was a problem,
in Maryland action was swift. The governor … asked for medical tests on about 15
people who had been exposed to the toxins. Some of them were diagnosed in the
bottom 2 per cent of the US population in their ability to learn and remember.
North Carolina was, I think, embarrassed and the state legislature provided generous
funding for a new biohazard level 3 lab. With that we are now making good progress
…
A ‘target article’
Pfiesteria piscicida and P. shumwayae reportedly secrete potent exotoxins
thought to cause fish lesion events, acute fish kills and human disease…
Pfiesteria toxins have never been isolated or characterized8.
We investigated mechanisms by which P. shumwayae kills fish …
Here we show that larval fish bioassays conducted in tissue culture plates
fitted with polycarbonate membrane inserts exhibited mortality (100%)
only in treatments where fish and dinospores were in physical contact.
No mortalities occurred in treatments where the membrane
prevented contact between dinospores and fish. …
Videomicrography and electron microscopy show
dinospores swarming toward and attaching to skin,
actively feeding, and rapidly denuding fish of epidermis.
We show here that our cultures of actively fish-killing P. shumwayae
do not secrete potent exotoxins; rather,
fish mortality results from micropredatory feeding. …
A digression on your News & Views assignment
A cute, catchy title
Certain episodes of mass fish mortality in coastal waters off the
eastern United States have been ascribed to
a planktonic organism called Pfiesteria.
There are now fresh clues to how these fish are killed.
In James Powlik's novel Sea Change1, a colony of mutant cells called
Pfiesteria grows into a carpet-like monster stretching over several
square miles of the Strait of Juan de Fuca, a body of water separating
the west coast of the United States and Canada. Its slow movement
south threatens the residents of Seattle with a nightmarish fate — death
following inhalation or ingestion of a toxin produced by the microbes.
The good news is that the Sea Change story is fiction; the bad news is
that Pfiesteria is fact. But what is the truth about this 'phantom of
the ocean'? It is thought that Pfiesteria, which belongs to a
group of planktonic organisms called dinoflagellates, releases
a toxin that ultimately kills fish2. But using simple methods,
Vogelbein and his colleagues (page 967 of this issue3)
conclude that the toxin is not released into the environment
and that the organism kills by direct methods.
continued …
Establishing a problem
Luring in the reader
with a question (below)
& a teaser (above)
The ‘established wisdom’’
The ‘bottom line’’ on how
the target article is going
to change the
‘established wisdom’’
In reality, the presence of Pfiesteria has not been documented along the
US west coast but rather along the eastern seaboard, with a range from
Delaware to Alabama4. It has been associated with massive fish kills,
especially off North Carolina2. Because of the harm that results when
Pfiesteria multiplies to high numbers, it is categorized with other
plankton known to cause harmful 'blooms'. Many of these bloom-forming
organisms produce potent toxins, so the assumption that Pfiesteria
releases a toxin is not unfounded. Moreover, the population explosions
of various single-celled plankton, including diatoms and dinoflagellates,
are frequently associated with the production of nerve toxins —
saxitoxin, brevetoxin, maitotoxin and the like5 — that are damaging
when ingested by higher organisms.
These toxins are the key for researchers wishing to unlock the secrets of
harmful algal blooms. By knowing the chemical identity of the toxins,
highly specific detection methods can be developed to track the location
of blooms and to discover where they originate, how they spread and
when they are most toxic. The impact of harmful algae on living
creatures, such as seabirds, marine mammals and even humans, can
be assessed by determining the levels of toxin in their tissues and body
fluids. But first the toxins must be isolated from the clusters of cells that
release them into the environment.
This is typically done by picking a single suspect cell from the mixture of
microbes in a water sample from an estuary or ocean during a harmful
event, and allowing it to multiply in the laboratory. Chemical constituents
are then extracted from several litres of the cells, and toxic components
are identified using a bioassay, a test that assesses whether a particular
fraction is poisonous, usually to a fish or mouse. For almost a decade,
The basic story
before the
target article
Vogelbein et al.3 and — in a related study, with some authors in
common — Berry et al.7 have taken a different approach to the quest for
a toxin released by Pfiesteria. First, they grew Pfiesteria in the lab and
determined that the cultures could kill fish through direct contact. Then
they separated the cells from the rest of the material using several
methods, including filtration, centrifugation and dialysis (Fig. 1). They
theorized that, if Pfiesteria cells release a toxin into sea water, this
should be present in the cell-free fractions. But the experiments showed
that the cell-free fractions do not cause fish death, and that Pfiesteria is
lethal only when cells are in direct contact with the fish. The researchers
therefore concluded that a toxin is not released into the surrounding sea
water.
Figure 1 The dialysis
technique
used by Vogelbein
et
al.3
If they are not killed by a toxin, how do fish exposed to Pfiesteria die? It
has been known for many years that Pfiesteria cells extend a suctioncup-like appendage called a peduncle to digest fish tissue2. Vogelbein et
al. go a step further and propose that fish die because Pfiesteria literally
sucks the life out of them. It attaches to fish skin using the peduncle,
extending finger-like protrusions called filopodia, then ingests cell matter
from the fish. This parasitic feeding behaviour by Pfiesteria is detailed in
high-magnification microscope images in Vogelbein et al.3, and in a
video clip available in their Supplementary Information.
A nice, simple
summary of
what the authors
did & found
The new work promises to bring us closer to unveiling the true
nature of this phantom of the ocean. Many loose ends remain,
however, and it is possible nonetheless that a toxin is involved.
Humans have suffered from memory impairment thought to
stem from exposure to Pfiesteria8. Are these memory problems
caused by a toxin, in aerosol form, produced by another
organism that often coexists with Pfiesteria, as Berry et al.7
suggest? Do the rod-shaped granules that Vogelbein and
colleagues identified in Pfiesteria (see Fig. 4 on page 969)
contain toxins that are released only after the peduncle
becomes attached? Do only certain Pfiesteria isolates produce
toxins, as Burkholder et al.9 have proposed, indicating that the
Pfiesteria studied by Vogelbein, Berry and their colleagues
were simply non-toxic strains? Can contamination by a fungus
or other microbe explain why Pfiesteria cultures routinely
grown in the presence of fish do indeed appear to produce a
toxin10?
Assembling these pieces of the complex puzzle posed by
Pfiesteria will require exceptional cooperation among
researchers of differing expertise. The opportunity to discuss
the outstanding questions and to establish new collaborations
will engage scientists at the Tenth International Conference on
Harmful Algal Blooms, to be held in St Petersburg, Florida, this
October.
What questions
are left
unanswered
& what needs to
be done next?
You only need to cite
the target article
Stramenopila includes several heterotrophic groups
as well as a variety of photosynthetic protists (algae).
The term stramenopila refers to the numerous fine, hairlike projections
on the flagella that are characteristic of these organisms
the Water Molds
(not fungi because cell walls cellulose, not chitin)
‘ick’ on fish - Fig 28.16
downy mildews on plants:
potato blight, vine root rot
the Diatoms: - Fig 28.17
have unique glasslike walls
consisting of hydrated silica
embedded in an organic matrix.
Each wall is in two parts that overlap like a shoe box and lid –
most of the year, diatoms reproduce asexually by mitotic cell divisions,
with each daughter cell receiving half of the cell wall of its parent
and regenerating a new second half.
Most golden algae are unicellular, their cells biflagellated.
Some species are mixotrophic, absorbing dissolved organic compounds
or ingesting food particles and bacteria by phagocytosis,
Many of the ‘seaweeds’ are brown algae (ex: kelp).
Unlike other eukaryotic algae, red algae
have no flagellated stages in their life cycle.
They are reddish because of an accessory pigment
called phycoerythrin also found in cyanobacteria.
Molecular systematics and cellular morphology leave little doubt
that green algae and land plants are closely related.
Some systematists advocate
an expanded kingdom, the Viridiplantae.
Fig 28.24
Amoeboid things
‘A diversity of protists that use pseudopodia for movement and feeding
Little is known about their phylogeny,
although it is clear that they represent several distinct eukaryotic lineages.
Because of taxonomic uncertainties,
we have not included these protists on the phylogenetic tree
the Rhizopods (amoebas) ‘a polyphyletic group’
The cytoskeleton of microtubules and microfilaments
functions in amoeboid movement.
Entamoeba histolytica causes amoebic dysentery.
Actinopod means "ray foot," a reference to
the slender pseudopodia that radiate from these protists. Each axopodium is
reinforced by a bundle of microtubules covered by a thin layer of cytoplasm.
Most actinopods are planktonic, and their projections
place an extensive area of cellular surface in contact with the surrounding water,
Foraminiferans, or forams, are named for their porous shells.
Almost all are marine, most species live in the sand or on rocks
and algae, but some are also abundant in plankton, deriving
nourishment from the photosynthesis of symbiotic algae
that live within the shells.
?
Mycetozoa translates as "fungus animals,"
though these protists are neither fungi nor animals.
Acellular Plasmodial slime molds - diploid - Fig 28.29
http://www.ucmp.berkeley.edu/protista/slimemolds.html
Plasmodial slime molds, like Physarum shown here,
are formed when individual flagellated cells
swarm together and fuse into
one large bag of cytoplasm with many diploid nuclei.
Dictyostelida (Cellular Slime Molds) – haploid Fig 28.30
Although the feeding stage of the life cycle
consists of solitary cells that function individually,
when food is depleted the cells form an aggregate
that functions as a unit.
Poses questions about what it means to be an individual organism.
Serves as a model for evolution of multicellularity & cooperation
http://www.ucmp.berkeley.edu/protista/slimemolds.html
the cellular slime molds,
spend most of their lives as separate single-celled amoeboid protists,
but upon the release of a chemical signal {cAMP}, the individual cells aggregate …
they provide a comparatively simple and easily manipulated system
for understanding how cells interact to generate a multicellular organism.
C&R Fig 28.30
Ennis et al 2000. PNAS 97:3292-3297.
Dictyostelium amoebae feed on bacteria during growth,
but when the food supply is exhausted, they aggregate to form a … cylindrical slug …
about 80% of the amoebae form spore cells, whereas 20% form stalk cells.
Spore cells germinate to give rise to the next generation, but stalk cells die.
The stalk cells have been described as altruistic,
sacrificing themselves for the improved dispersal of spores.
One of the problems … is that in the wild,
parasitic forms of Dictyostelium should arise that form only spore cells.
Such parasites, not having to tithe 20% to the stalk cells,
would increase in the population and
eventually impede the cooperativeness that leads to fruiting body formation.
Altruism and social cheating in the social amoeba Dictyostelium discoideum
Strassmann JE, Zhu Y, Queller DC
NATURE 408 (6815): 965-967 DEC 21 2000
Abstract:
The social amoeba, Dictyostelium discoideum, is widely used
as a simple model organism for multicellular development(1,2),
but its multicellular fruiting stage is really a society.
Most of the time, D. discoideum lives as haploid, free-living, amoeboid cells
that divide asexually. When starved, 10(4)-10(5) of these cells aggregate into a slug.
The anterior 20% of the slug altruistically differentiates into a non-viable stalk,
supporting the remaining cells, most of which become viable spores(3-5).
If aggregating cells come from multiple clones,
there should be selection for clones to exploit other clones by
contributing less than their proportional share to the sterile stalk.
Here we use microsatellite markers to show that different clones
collected from a field population readily mix to form chimaeras.
Half of the chimaeric mixtures show a clear cheater and victim.
Thus, unlike the clonal and highly cooperative development of most multicellular
organisms, the development of D. discoideum is partly competitive, with conflicts of
interests among cells. These conflicts complicate the use of D. discoideum
as a model for some aspects of development, but they make it highly attractive
as a model system for social evolution.
Continued …
In social situations, opportunities arise
for some individuals to take advantage of others.
This happens in wild populations of the social amoeba
Dictyostelium discoideum.
Most of the organisms studied by developmental biologists
— the fruitfly Drosophila, for instance — arise from a fertilized egg.
So, apart from the mature reproductive cells, the cells from which these organisms
are made are genetically identical. No cell has a genetic advantage
over the others, and any variants that do arise are excluded from the germ line.
Because of the lack of genetic diversity among their cells and the presence of a
germ line, cheating by individual cells is restricted in most multicellular organisms.
But there is another route to multicellularity, taken by organisms such as
Dictyostelium and Myxococcus, a group of social bacteria.
As they describe on page 965 of this issue1, Strassmann and colleagues
have studied the curious cooperative behaviour of Dictyostelium,
and find that some genetic variants cheat on their partners
during the process of spore formation (Fig. 1).
continued …
Vol. 96, Issue 16, 8801-8803, August 3, 1999
Commentary {on Ennis et al 2000. PNAS 97:3292-3297.}
Slime molds, ascidians,
and the utility of evolutionary theory
Leo W. Buss
replicating
DNA molecule
Consider a social insect colony.
The colony as a unit reproduces.
The ants within a colony reproduce.
The cells within each ant reproduce.
The mitochondria with each cell reproduce, as does
the chromosome of the mitochondria and those of the nucleus.
And within those chromosomes are likely transposable elements …
Any modern organism is a Russian doll
of actually or potentially reproducing units.
Selection can, in principle, act on each such unit.
We will, of course, not see its operation in most modern contexts, because
conflict between units of selection are evident only when a chimera is formed.
Chimeras that pit one unit of selection against another
arise in specific ecological contexts, as exemplified by … slime molds …
They also can arise by mutation during the life of an individual,
as any oncologist knows well.