Jim's Discussion Notes -
THE ORIGINS OF
EUKARYOTIC
DIVERSITY
(Campbell, Ch.28 and Johnson/Raven, Ch.22)
·
Protists
are eukaryotes and thus are much more complex than the prokaryotes.
·
The
first eukaryotes were unicellular.
·
Not
only were they the predecessor to the great variety of modern protists, but
also to all other eukaryotes - plants, fungi, and animals.
· The origin of the eukaryotic cell and the emergence of multicellularity unfolded during the evolution of protists.
·
Eukaryotic
fossils date back 2.1 billion years and “chemical signatures” of eukaryotes
date back 2.7 billion years.
· For about 2 billion years, eukaryotes consisted of mostly microscopic organisms known by the informal name “protists.”
1. Systematists have split protists into many kingdoms
·
In
the five-kingdom system of classification, the eukaryotes were distributed
among four kingdoms: Protista, Plantae, Fungi, and Animalia.
·
So
far, the plant, fungus, and animal kingdoms are surviving the taxonomic
remodeling though their boundaries have been expanded to include certain groups
formerly classified as protists.
·
However,
systematists have split protists into many kingdoms.
·
Modern
systematists have crumbled the former kingdom of protists beyond repair.
·
Protista
was defined partly by structural level (mostly unicellular eukaryotes) and
partly by exclusion from the definitions of plants, fungi, or animals.
·
However,
this created a group including single-celled microscopic members, simple
multicellular forms, and even complex giants like seaweeds.
·
The
kingdom Protista formed a paraphyletic group, with some members more closely
related to animals, plants, or fungi than to other protists.
·
Systematists
have split the former kingdom Protista into as many as 20 separate kingdoms.
·
Still,“protist”
is used as an informal term for this great diversity of eukaryotic kingdoms.
2. Protists
are the most diverse of all eukaryotes
·
Protists
are so diverse that few general characteristics can be cited without
exceptions.
·
Most
of the 60,000 known protists are unicellular, but some are colonial and others
multicellular.
·
While
unicellular protists would seem to be the simplest eukaryotic organisms, at the
cellular level they are the most elaborate of all cells.
·
A
single cell must perform all the basic functions performed by the collective of
specialized cells in plants and animals.
·
Protists
are the most nutritionally diverse of all eukaryotes,
·
Most
protists are aerobic, with mitochondria for cellular respiration.
·
Some
protists are photoautotrophs with chloroplasts.
·
Still
others are heterotrophs that absorb organic molecules or ingest larger food
particles.
·
A
few are mixotrophs, combining
photosynthesis and heterotrophic nutrition.
·
Euglena, a single celled
mixotrophic protist, can use chloroplasts to undergo photosynthesis if light is
available or live as a heterotroph by absorbing organic nutrients from the
environment.
·
These
various modes of nutrition are scattered throughout the protists.
·
The
same group may include photosynthetic species, heterotrophic species, and
mixotrophs.
·
While
nutrition is not a reliable taxonomic characteristic, it is useful in understanding the adaptations of protists and the
roles that they play in biological communities.
·
Protists
can be divided into three ecological categories:
·
Protozoa—ingestive, animal-like
protists
·
Absorptive,
fungus-like protists
·
Algae—photosynthetic, plant-like
protists.
·
Most
protists move with flagella or cilia during some time in their life cycles.
·
The
eukaryotic flagella are not homologous to those of prokaryotes.
·
The
eukaryotic flagella are extensions of the cytoplasm with a support of the 9 + 2
microtubule system.
·
Cilia
are shorter and more numerous than flagella.
·
Cilia
and flagella move the cell with rhythmic power strokes, analogous to the oars
of a boat.
·
Reproduction
and life cycles are highly varied among protists.
·
Mitosis
occurs in almost all protists, but there are many variations in the process.
·
Some
protists are exclusively asexual or at least employ meiosis and syngamy (the union of two gametes),
thereby shuffling genes between two individuals.
·
Others
are primarily asexual but can also reproduce sexually at least occasionally.
·
Protists
show the three basic types of sexual life cycles, with some other variants,
too.
·
The
haploid stage is the vegetative stage of most protists, with the zygote as the
only diploid cell.
·
Many
protists form resistant cells (cysts)
that can survive harsh conditions.
·
Protists
are found almost anywhere there is water.
·
This
includes oceans, ponds, and lakes, but also damp soil, leaf litter, and other
moist terrestrial habitats.
·
In
aquatic habitats, protists may be bottom-dwellers attached to rocks and other
anchorages or may creep through sand and silt.
·
Protists
are also important parts of plankton,
communities of organisms that drift passively or swim weakly in the water.
·
Phytoplankton (including planktonic
eukaryotic algae and prokaryotic cyanobacteria) are the bases of most marine
and freshwater food chains.
·
Many
protists are symbionts that inhabit the body fluids, tissues, or cells of
hosts.
·
These
symbiotic relationships span the continuum from mutualism to parasitism.
·
Some
parasitic protists are important pathogens of animals, including those that
cause potentially fatal diseases in humans.
B. The Origin and Early Diversification of Eukaryotes
·
The
evolution of the eukaryotic cell led to the development of several unique
cellular structures and processes.
·
These
include the membrane-enclosed nucleus, the endomembrane system, mitochondria,
chloroplasts, the cytoskeleton, 9 + 2 flagella, multiple chromosomes of linear
DNA with organizing proteins, and life cycles with mitosis, meiosis, and sex.
1. Endomembranes contributed to larger, more complex cells
·
The
small size and simple construction of a prokaryote imposes limits on the number
of different metabolic activities that can be accomplished at one time.
·
The
relatively small size of the prokaryote genome limits the number of genes
coding for enzymes that control these activities.
·
In
spite of this, prokaryotes have been evolving and adapting since the dawn of
life, and they are the most widespread organisms even today.
·
One
trend was the evolution of multicellular prokaryotes, where cells specialized
for different functions.
·
A
second trend was the evolution of complex communities of prokaryotes, with
species benefiting from the metabolic specialties of others.
·
A
third trend was the compartmentalization of different functions within single
cells, an evolutionary solution that contributed to the origins of eukaryotes.
·
Under
one evolutionary scenario, the endomembrane system of eukaryotes (nuclear
envelope, endoplasmic reticulum, Golgi apparatus, and related structures) may
have evolved from infoldings of plasma membrane.
·
Another
process, called endosymbiosis, probably led to mitochondria, plastids, and
perhaps other eukaryotic features.
2. Mitochondria and plastids evolved from endosymbiotic bacteria
·
The
evidence is now overwhelming that the eukaryotic cell originated from a
symbiotic coalition of multiple prokaryotic ancestors.
·
A
mechanism for this was originated by a Russian biologist C. Mereschkovsky and
developed extensively by Lynn Margulis of the University of Massachusetts.
·
The
theory of serial endosymbiosis
proposes that mitochondria and chloroplasts were formerly small prokaryotes
living within larger cells.
·
Cells
that live within other cells are called endosymbionts.
·
The
proposed ancestors of mitochondria were aerobic heterotrophic prokaryotes.
·
The
proposed ancestors of chloroplasts were photosynthetic prokaryotes.
·
These
ancestors probably entered the host cells as undigested prey or internal
parasites.
·
This
process would be facilitated by the presence of an endomembrane system and
cytoskeleton, allowing the larger host cell to engulf the smaller prokaryote
and to package them within vesicles.
·
This
evolved into a mutually beneficial symbiosis.
·
A
heterotrophic host could derive nourishment from photosynthetic endosymbionts.
·
In
an increasingly aerobic world, an anaerobic host cell would benefit from
aerobic endosymbionts that could exploit oxygen.
·
As
host and endosymbiont evolved, both would become more interdependent, evolving
into a single organism, its parts inseparable.
·
All
eukaryotes have mitochondria or genetic remnants of mitochondria.
·
However,
not all eukaryotes have chloroplasts.
·
The
serial endosymbiosis theory supposes that mitochondria evolved before
chloroplasts.
·
Many
examples of symbiotic relationships among modern organisms are analogous to
proposed early stages of the serial endosymbiotic theory.
·
Several
lines of evidence support a close similarity between bacteria and the
chloroplasts and mitochondria of eukaryotes.
·
These
organelles and bacteria are similar in size.
·
Enzymes
and transport systems in the inner membranes of chloroplasts and mitochondria
resemble those in the plasma membrane of modern prokaryotes.
·
Replication
by mitochondria and chloroplasts resembles binary fission in bacteria.
·
The
single circular DNA in chloroplasts and mitochondria lacks histones and other
proteins, as in most prokaryotes.
·
Both
organelles have transfer RNAs, ribosomes, and other molecules for transcription
of their DNA and translation of mRNA into proteins.
·
The
ribosomes of both chloroplasts and mitochondria are more similar to those of
prokaryotes than to those in the eukaryotic cytoplasm that translate nuclear
genes.
·
A
comprehensive theory for the origin of the eukaryotic cell must also account
for the evolution of the cytoskeleton and the 9 + 2 microtubule apparatus of
the eukaryotic cilia and flagella.
·
Some
researchers have proposed that cilia and flagella evolved from symbiotic
bacteria (especially spirochetes).
·
However,
the evidence for this proposal is weak.
·
Related
to the evolution of the eukaryotic flagellum is the origin of mitosis and
meiosis, processes unique to eukaryotes that also employ microtublules.
·
Mitosis
made it possible to reproduce the large genomes in the eukaryotic nucleus.
·
Meiosis
became an essential process in eukaryotic sex.
3. The eukaryotic cell is a chimera of prokaryotic ancestors
·
The
chimera of Greek mythology was part goat, part lion, and part serpent.
·
Similarly,
the eukaryotic cell is a chimera of prokaryotic parts:
·
Mitochondria
from one bacteria
·
Plastids
from another
·
Nuclear
genome from the host cell
·
The
search for the closest living prokaryotic relatives to the eukaryotic cell has
been based on molecular comparisons because no morphological homologies connect
species so diverse.
·
Sequence
comparisons of the small ribosomal subunit RNA (SSU-rRNA) among prokaryotes and
mitochondria have identified the closest relatives of the mitochondria as the
alpha proteobacteria group.
·
Sequence
comparisons of SSU-rRNA from plastids of eukaryotes and prokaryotes have
indicated a close relationship with cyanobacteria.
·
While
mitochondria and plastids contain DNA and can build proteins, they are not
genetically self-sufficient.
·
Some
of their proteins are encoded by the organelles’ DNA.
·
The
genes for other proteins are located in the cell’s nucleus.
·
Other
proteins in the organelles are molecular chimeras of polypeptides synthesized
in the organelles and polypeptides imported from the cytoplasm (and ultimately
from nuclear genes).
·
A
reasonable hypothesis for the collaboration between the genomes of the
organelles and the nucleus is that the endosymbionts transferred some of their
DNA to the host genome during the evolutionary transition from symbiosis to
integrated eukaryotic organism.
·
Transfer
of DNA between modern prokaryotic species is common (for example, by
transformation).
4. Secondary endosymbiosis increased the diversity of algae
·
Taxonomic
groups with plastids are scattered throughout the phylogenetic tree of
eukaryotes.
·
These
plastids vary in ultrastructure.
·
The
chloroplasts of plants and green algae have two membranes.
·
The
plastids of others have three or four membranes.
·
These
include the plastids of Euglena (with
three membranes) that are most closely related to heterotrophic species.
·
The
best current explanation for this diversity of plastids is that plastids were
acquired independently several times during the early evolution of eukaryotes.
·
Those
algal groups with more than two membranes were acquired by secondary endosymbiosis.
·
It
was by primary endosymbiosis that
certain eukaryotes first acquired the ancestors of plastids by engulfing
cyanobacteria.
·
Secondary
endosymbiosis occurred when a heterotrophic protist engulfed an algae
containing plastids.
·
Each
endosymbiotic event adds a membrane derived from the vacuole membrane of the
host cell that engulfed the endosymbiont.
·
In
most cases of secondary endosymbiosis, the endosymbiont lost most of its parts,
except its plastid.
·
In
some algae, there are remnants of the secondary endosymbionts.
·
For
example, the plastids of cryptomonad algae contain vestiges of the
endosymbiotic nucleus, cytoplasm, and even ribosomes.
·
Thus,
a cryptomonad is a complex chimera, like a box containing a box containing a
box.
5. Research on the relationships between the three domains is changing ideas about the deepest branching in the tree of life
·
The
chimeric origin of the eukaryotic cell contrasts with the classic Darwinian view
of lineal descent through a “vertical” series of ancestors.
·
The
eukaryotic cell evolved by “horizontal” fusions of species from different
phylogenetic lineages.
·
The
metaphor of an evolutionary tree starts to break down at the origin of
eukaryotes and other early evolutionary episodes.
·
The
conventional model of relationships among the three domains places the archaea
as more closely related to eukaryotes than they are to prokaryotes.
·
Similarities
include proteins involved in transcription and translation.
·
This
model places the host cell in the endosymbiotic origin of eukaryotes as
resembling an early archaean.
·
The
conventional cladogram predicts that the only DNA of bacterial origin in the
nucleus of eukaryotes are genes that were transferred from the endosymbionts
that evolved into mitochondria and plastids.
·
Surprisingly,
systematists have found many DNA sequences in the nuclear genome of eukaryotes
that have no role in mitochondria or chloroplasts.
·
Also,
modern archaea have many genes of bacterial origin.
·
All
three domains seem to have genomes that are chimeric mixes of DNA that was
transferred across the boundaries of the domains.
·
This
has led some researchers to suggest replacing the classical tree with a
web-like phylogeny.
·
In
this new model, the three domains arose from an ancestral community of primitive cells that swapped DNA promiscuously.
·
This
explains the chimeric genomes of the three domains.
·
Gene
transfer across species lines is still common among prokaryotes.
·
However,
this does not appear to occur in modern eukaryotes.
6. The origin of eukaryotes catalyzed a second great wave of diversification
·
The
first great adaptive radiation, the metabolic diversification of the
prokaryotes, set the stage for the second.
·
The
second wave of diversification was catalyzed by the greater structural
diversity of the eukaryotic cell.
·
The
third wave of diversification followed the origin of multicellular bodies in
several eukaryotic lineages.
·
The
diversity of eukaryotes ranges from a great variety of unicellular forms to
such macroscopic, multicellular groups as brown algae, plants, fungi, and
animals.
·
The
development of clades among the diverse groups of eukaryotes is based on
comparisons of cell structure, life cycles, and molecules.
·
This
includes both SSU-rRNA sequences and amino acid sequences for some cytoskeletal
proteins.
·
If
plants, animals, and fungi are designated as kingdoms, then each of the other
major clades of eukaryotes probably deserves kingdom status as well.
·
However,
protistan systematics is still so unsettled that any kingdom names assigned to
these other clades would be rapidly obsolete.
·
In
fact, some of the best-known protists, such as the single-celled amoebas, are
not even included in this tentative phylogeny because it is so uncertain where
they fit into the overall eukaryotic tree.
·
As
tentative as our eukaryotic tree is, the current tree is an effective tool to
organize a survey of the diversity found among protists.
1. Diplomonadida and Parabasala: Diplomonads and parabasalids lack mitochondria
·
A
few protists, including the diplomonds and the parabasalids, lack mitochondria.
·
According
to the “archaezoa hypothesis,” these protists are derived from ancient
eukaryotic lineages before the acquisition of endosymbiotic bacteria that
evolved into mitochondria.
·
This
hypothesis has largely been discarded because of the presence of mitochondrial
genes in the nuclear genomes of both groups.
·
This
evidence suggests a new hypothesis, that these protists lost their mitochondria during their evolution.
·
Other
details of cell structure and data from molecular systematics still place the
diplomonads and parablastids on the phylogenetic branch that diverged earliest
in eukaryotic history.
·
The
diplomonads have multiple flagella,
two separate nuclei, a simple cytoskeleton, and no mitochondria or plastids.
·
One
example is Giardia lamblia, a
parasite that infects the human intestine.
·
The
most common method of acquiring Giardia
is by drinking water contaminated with feces containing the parasite in a
dormant cyst stage.
·
The
parabasalids include trichomonads.
·
The
best known species, Trichomonas vaginalis,
inhabits the vagina of human females.
·
It
can infect the vaginal lining if the normal acidity of the vagina is disturbed.
·
The
male urethra may also be infected, but without symptoms.
·
Sexual
transmission can spread the infection.
2. Euglenozoa: The euglenozoa includes both photosynthetic and heterotrophic flagellates
·
Several
protistan groups, including the euglenoids and kinetoplastids, use flagella for
locomotion.
·
The
euglenoids (Euglenophyta) are
characterized by an anterior pocket from which one or two flagella emerge.
·
They
also have a unique glucose polymer, paramylon, as a storage molecule.
·
While
Euglena is chiefly autotrophic, other
euglenoids are mixotrophic or heterotrophic.
·
The
kinetoplastids (Kinetoplastida) have
a single large mitochondrion associated with a unique organelle, the
kinetoplast.
·
The
kinetoplast houses extranuclear DNA.
·
Kinetoplastids
are symbiotic and include pathogenic parasites.
·
For
example, Trypanosoma causes African
sleeping sickness.
3. Alveolata: The alveolates are unicellular protists with subsurface cavities (alveoli)
·
The
Alveolata combines flagellated
protists (dinoflagellates), parasites (apicomplexans), and ciliated protists
(the ciliates).
·
This
clade has been supported by molecular systematics.
·
Members
of this clade have alveoli, small membrane-bound cavities, under the cell
surface.
·
Their
function is not known, but they may help stabilize the cell surface and
regulate water and ion content.
·
The
dinoflagellates are abundant
components of the phytoplankton that are suspended near the water surface.
·
Dinoflagellates
and other phytoplankton form the foundation of most marine and many freshwater
food chains.
·
Other
species of dinoflagellates are heterotrophic.
·
Most
dinoflagellates are unicellular, but some are colonial.
·
Each
dinoflagellate species has a characteristic shape, often reinforced by internal
plates of cellulose.
·
Two
flagella sit in perpendicular grooves in the “armor” and produce a spinning
movement.
·
Dinoflagellate
blooms, characterized by explosive population growth, cause red tides in
coastal waters.
·
The
blooms are brownish-red or pinkish-orange because of the predominant pigments
in the plastids.
·
Toxins
produced by some red-tide organisms have produced massive invertebrate and fish
kills.
·
These
toxins can be deadly to humans as well.
·
One
dangerous dinoflagellate, Pfiesteria
piscicida, is actually carnivorous.
·
This
organism produces a toxin that stuns fish.
·
The
dinoflagellate can then feed on the body fluids of its prey.
·
In
the past decade, the frequency of Pfiesteria
blooms and fish kills have increased in the Mid-Atlantic states of the U.S.
·
One
hypothesis for this change is an increase in the pollution of coastal waters
with fertilizers, especially nitrates and phosphates.
·
Some
dinoflagellates form mutualistic symbioses with cnidarians, animals that build
coral reefs.
·
Photosynthetic
products from the dinoflagellates provide the main food resource for reef
communities.
·
Some
dinoflagellates are bioluminescent.
·
An
ATP-driven chemical reaction gives off light when dinoflagellates are disturbed
by water movements.
·
The
function of bioluminescence may be to attract predators that may eat the
smaller predators that feed on phytoplankton.
·
All
apicomplexans are parasites of
animals and some cause serious human diseases.
·
The
parasites disseminate as tiny infectious cells (sporozoites) with a complex
of organelles specialized for penetrating host cells and tissues at the apex of the sporozoite cell.
·
Most
apicomplexans have intricate life cycles with both sexual and asexual stages
and often require two or more different host species for completion.
·
Plasmodium, the parasite that causes
malaria, spends part of its life in mosquitoes and part in humans.
·
The
incidence of malaria was greatly diminished in the 1960s by the use of
insecticides against the Anopheles
mosquitoes, which spread the disease, and by drugs that killed the parasites in
humans.
·
However,
resistant varieties of the mosquitoes and the Plasmodium species have caused a malarial resurgence.
·
About
300 million people are infected with malaria in the tropics, and up to 2
million die each year.
·
Research
has had little success in producing a malarial vaccine because Plasmodium is evasive.
·
It
spends most of its time inside human liver and blood cells, and continually
changes its surface proteins, thereby changing its “face” to the human immune
system.
·
Identification
of a gene that may confer resistance to chloroquine, an antimalarial drug, may
lead to ways to block drug resistance in Plasmodium.
·
A
second promising approach may attack a nonphotosynthetic plastid in Plasmodium.
·
The
Ciliophora (ciliates), a diverse
protist group, are named for their use of cilia to move and feed.
·
Most
ciliates live as solitary cells in freshwater.
·
Their
cilia are associated with a submembrane system of microtubules that may
coordinate movement.
·
Some
ciliates are completely covered by rows of cilia, whereas others have cilia
clustered into fewer rows or tufts.
·
The
specific arrangement of cilia adapts the ciliates for their diverse lifestyles.
·
Some
species have leg-like structures constructed from many cilia bonded together,
while others have tightly packed cilia that function as a locomotor membranelle.
·
In
a Paramecium, cilia along the oral
groove draw in food that is engulfed by phagocytosis.
·
Like
other freshwater protists, the hyperosmotic Paramecium
expels accumulated water from the contractile vacuole.
·
Ciliates
have two types of nuclei, a large macronucleus and usually several tiny
micronuclei.
·
The
macronucleus has 50 or more copies of the genome.
·
The
macronucleus controls the everyday functions of the cell by synthesizing RNA
and is also necessary for asexual reproduction.
·
Ciliates
generally reproduce asexually by binary fission of the macronucleus, rather
than mitotic division.
·
The
micronuclei (with between 1 and 80 copies) are required for sexual processes
that generate genetic variation.
·
The
sexual shuffling of genes occurs during conjugation,
during which micronuclei that have undergone meiosis are exchanged.
·
In
ciliates, sexual mechanisms of meiosis and syngamy are separate from
reproduction.
4. Stramenopila: The stramenopila clade includes the water molds and heterokont algae
·
The
Stramenopila includes both
heterotrophic and photosynthetic protists.
·
The
name of this group is derived from the presence of numerous fine, hairlike
projections on the flagella.
·
In
most cases a “hairy” flagellum is paired with a smooth flagellum.
·
In
most stramenopile groups, the only flagellated stages are motile reproductive
cells.
·
The
heterotrophic stramenopiles, the oomycotes,
include water molds, white rusts, and downy mildews.
·
Some
are unicellular, others have a fine network of coenocytic hyphae (fine, branching
filaments).
·
These
hyphae have cellulose cell walls and are analogous with the hyphae of true
fungi (with chitin cell walls).
·
Unlike
fungi, the diploid stage dominates in oomycotes and they have biflagellated
cells.
·
These
filamentous bodies have extensive surface area, enhancing absorption of
nutrients.
·
In
the Oomycota, the “egg fungi,” a relatively large egg cell, is fertilized by a
smaller “sperm nucleus,” forming a resistant zygote.
·
Water
molds are important decomposers, mainly in fresh water.
·
They
form cottony masses on dead algae and animals.
·
Some
water molds are parasitic, growing on the skin and gills of injured fish.
·
White
rusts and downy mildews are parasites of terrestrial plants.
·
They
are dispersed by windblown spores.
·
One
species of downy mildew threatened French vineyards in the 1870s and another
species causes late potato blight, which contributed to the Irish famine in the
19th century.
·
The
photosynthetic stramenopile taxa are known collectively as the heterokont
algae.
·
“Hetero”
refers to the two different types of flagella.
·
The
plastids of these algae evolved by secondary endosymbiosis.
·
They
have a three-membrane envelope and a small amount of eukaryotic cytoplasm
within the plastid.
·
The
probable ancestor was a red alga.
·
The
heterokont algae include diatoms, golden algae, and brown algae.
·
Diatoms (Bacillariophyta) have
unique glasslike walls composed of hydrated silica embedded in an organic
matrix.
·
The
wall is divided into two parts that overlap like a shoe box and lid.
·
Most
of the year, diatoms reproduce asexually by mitosis with each daughter cell
receiving half of the cell wall and regenerating a new second half.
·
Some
species form cysts as resistant stages.
·
Sexual
stages are not common, but sperm may be amoeboid or flagellated, depending on
species.
·
Diatoms
are abundant members of both freshwater and marine plankton.
·
Diatoms
store food reserves in a glucose polymer, laminarin, and a few store food as
oils.
·
Massive
accumulations of fossilized diatoms are major constituents of diatomaceous
earth.
·
Golden algae (Chrysophyta), named for
the yellow and brown carotene and xanthophyll pigments, are typically
biflagellated.
·
Some
species are mixotrophic and many live among freshwater and marine plankton.
·
While
most are unicellular, some are colonial.
·
At
high densities, they can form resistant cysts that remain viable for decades.
·
Brown
algae (Phaeophyta) are the largest and most complex algae.
·
Most
brown algae are multicellular.
·
Most
species are marine.
·
Brown
algae are especially common along temperate coasts in areas of cool water and
adequate nutrients.
· They owe their characteristic brown or olive color to accessory pigments in the plastids.
5. Structural and biochemical adaptations help seaweeds survive and reproduce at the ocean’s margins
·
The
largest marine algae, including brown, red, and green algae, are known
collectively as seaweeds.
·
Seaweeds
inhabit the intertidal and subtidal zones of coastal waters.
·
This
environment is characterized by extreme physical conditions, including wave
forces and exposure to sun and drying conditions at low tide.
·
Seaweeds
have a complex multicellular anatomy, with some differentiated tissues and
organs that resemble those in plants.
·
These
analogous features include the thallus
or body of the seaweed.
·
The
thallus typically consists of a rootlike holdfast
and a stemlike stipe, which supports
leaf-like photosynthetic blades.
·
Some
brown algae have floats to raise the blades toward the surface.
·
Giant
brown algae, known as kelps, form forests in deeper water.
·
The
stipes of these plants may be 60 m long.
·
Many
seaweeds have biochemical adaptations for intertidal and subtidal conditions.
·
The
cell walls, composed of cellulose and gel-forming polysaccharides, help cushion
the thalli against agitation by waves.
·
Many
seaweeds are eaten by coastal people, including Laminaria (“kombu” in Japan) in soup and Porphyra (Japanese “nori”) for sushi wraps.
·
A
variety of gel-forming substances are extracted in commercial operations.
·
Algin
from brown algae and agar and carageenan from red algae are used as thickeners
in food, lubricants in oil drilling, or culture media in microbiology.
6. Some algae have life cycles with alternating multicellular haploid and diploid generations
·
The
multicellular brown, red, and green algae show complex life cycles with
alternation of multicellular haploid and multicellular diploid forms.
·
A
similar alternation of generations
evolved convergently in the life cycle of plants.
·
The
life cycle of the brown alga Laminaria
is an example of alternation of generations.
·
The
diploid individual, the sporophyte,
produces haploid spores (zoospores) by meiosis.
·
The
haploid individual, the gametophyte,
produces gametes by mitosis that fuse to form a diploid zygote.
·
In
Laminaria, the sporophyte and
gametophyte are structurally different, or heteromorphic.
· In other algae, the alternating generations look alike (isomorphic), but they differ in the number of chromosomes.
7.
Rhodophyta: Red algae lack flagella
·
Unlike
other eukaryotic algae, red algae have
no flagellated stages in their life cycle.
·
The
red coloration visible in many members is due to the accessory pigment
phycoerythrin.
·
Coloration
varies among species and depends on the depth which they inhabit.
·
The
plastids of red algae evolved from primary endosymbiosis of cyanobacteria.
·
Some
species lack pigmentation and are parasites on other red algae.
·
Red
algae (Rhodophyta) are the most common seaweeds in the warm coastal waters of
tropical oceans.
·
Others
live in freshwater, still others in soils.
·
Some
red algae inhabit deeper waters than other photosynthetic eukaryotes.
·
Their
photosynthetic pigments, especially phycobilins, allow some species to absorb
those wavelengths (blues and greens) that penetrate down to deep water.
·
One
red algal species has been discovered off the Bahamas at a depth of over 260m.
·
Most
red algae are multicellular, with some reaching a size large enough to be
called “seaweeds.”
·
The
thalli of many species are filamentous.
·
The
base of the thallus is usually differentiated into a simple holdfast.
·
The
life cycles of red algae are especially diverse.
·
In
the absence of flagella, fertilization depends entirely on water currents to
bring gametes together.
8. Chlorophyta:
Green algae and plants evolved from a common photoautotrophic ancestor
·
Green algae (chlorophytes and
charophyceans) are named for their grass-green chloroplasts.
·
These
are similar in ultrastructure and pigment composition to those of plants.
·
The
common ancestor of green algae and plants probably had chloroplasts derived
from cyanobacteria by primary endosymbiosis.
·
The
charophyceans are especially closely related to land plants.
·
Most
of the 7,000 species of chlorophytes live in freshwater.
·
Other
species are marine, inhabit damp soil or snow, or live symbiotically within
other eukaryotes.
·
Some
chlorophytes live symbiotically with fungi to form lichens, a mutualistic collective.
·
Chlorophytes
range in complexity, including:
·
Biflagellated
unicells that resemble gametes and zoospores.
·
Colonial
species and filamentous forms.
·
Multicellular
forms large enough to qualify as seaweeds.
·
Large
size and complexity in chlorophytes has evolved by three different mechanisms:
·
1)
Formation of colonies of individual cells (Volvox).
·
2)
The repeated division of nuclei without cytoplasmic division to form
multinucleate filaments (Caulerpa).
·
3)
The formation of true multicellular forms by cell division and cell
differentiation (Ulva).
·
Most
green algae have both sexual and asexual reproductive stages.
·
Most
sexual species have biflagellated gametes with cup-shaped chloroplasts.
·
Photosynthetic
protists have evolved in several clades that also have heterotrophic members.
· Different episodes of secondary endosymbiosis account for the diversity of protists with plastids.
9. A diversity of protists use pseudopodia for movement and feeding
·
Three
groups of protists use pseudopodia,
cellular extensions, to move and often to feed.
·
Most
species are heterotrophs that actively hunt bacteria, other protists, and
detritus.
·
Other
species are symbiotic, including some human parasites.
·
Little
is known of their phylogenetic relationships to other protists and they
themselves are distinct eukaryotic lineages.
·
Rhizopods
(amoebas) are all unicellular and
use pseudopodia to move and to feed.
·
Pseudopodium
emerge from anywhere in the cell surface.
·
To
move, an amoeba extends a pseudopod, anchors its tip, and then streams more
cytoplasm into the pseudopodium.
·
Amoeboid
movement is driven by changes in microtubules and microfilaments in the
cytoskeleton.
·
Pseudopodia
activity is not random but is in fact directed toward food.
·
In
some species pseudopodia extend out through openings in a protein shell around
the organism.
·
Amoebas
inhabit freshwater and marine environments
·
They
may also be abundant in soils.
·
Most
species are free-living heterotrophs.
·
Some
are important parasites.
·
These
include Entamoeba histolytica which
causes amoeboid dysentery in humans.
·
These
organisms spread via contaminated drinking water, food, and eating utensils.
·
Actinopod
(heliozoans and radiolarians), “ray foot,” refers to slender pseudopodia
(axopodia) that radiate from the body.
·
Each
axopodium is reinforced by a bundle of microtubules covered by a thin layer of
cytoplasm.
·
Most
actinopods are planktonic.
·
The
large surface area created by axopodia help them to float and feed.
·
Smaller
protists and other microorganisms stick to the axopodia and are phagocytized by
the thin layer of cytoplasm.
·
Cytoplasmic
streaming carries the engulfed prey into the main part of the cell.
·
Most
heliozoans (“sun animals”) live in
fresh water.
·
Their
skeletons consist of unfused siliceous (glassy) or chitinous plates.
·
The
term radiolarian refers to several
groups of mostly marine actinopods.
·
In
this group, the siliceous skeleton is fused into one delicate piece.
·
After
death, these skeletons accumulate as an ooze that may be hundreds of meters
thick in some seafloor locations.
·
Foraminiferans,
or forams, are almost all marine.
·
Most
live in sand or attach to rocks or algae.
·
Some
are abundant in the plankton.
·
Forams
have multichambered, porous shells, consisting of organic materials hardened
with calcium carbonate.
·
Pseudopodia
extend through the pores for swimming, shell formation, and feeding.
·
Many
forams form symbioses with algae.
·
Over
ninety percent of the described forams are fossils.
·
The
calcareous skeletons of forams are important components of marine sediments.
10. Mycetozoa: Slime molds have structural adaptations and life cycles that enhance their ecological roles as decomposers
·
Mycetozoa (slime molds or “fungus
animals”) are neither fungi nor animals, but protists.
·
Any
resemblance to fungi is analogous, not homologous, for their convergent role in
the decomposition of leaf litter and organic debris.
·
Slime
molds feed and move via pseudopodia, like amoeba, but comparisons of protein
sequences place slime molds relatively close to the fungi and animals.
·
The
plasmodial slime molds
(Myxogastrida) are brightly pigmented, heterotrophic organisms.
·
The
feeding stage is an amoeboid mass, the plasmodium,
that may be several centimeters in diameter.
·
The
plasmodium is not multicellular, but a single mass of cytoplasm with multiple
nuclei.
·
The
diploid nuclei undergo synchronous mitotic divisions, perhaps thousands at a
time.
·
Within
the cytoplasm, cytoplasmic streaming distributes nutrients and oxygen
throughout the plasmodium.
·
The
plasmodium phagocytises food particles from moist soil, leaf mulch, or rotting
logs.
·
If
the habitat begins to dry or if food levels drop, the plasmodium differentiates
into stages that lead to sexual reproduction.
·
The
cellular slime molds (Dictyostelida)
straddle the line between individuality and multicellularity.
·
The
feeding stage consists of solitary cells.
·
When
food is scarce, the cells form an aggregate (“slug”) that functions as a unit.
·
Each
cell retains its identity in the aggregate.
·
The
dominant stage in a cellular slime mold is the haploid stage.
·
Aggregates
of amoebas form fruiting bodies that produce spores in asexual reproduction.
·
Most
cellular slime molds lack flagellated stages.
11. Multicellularity originated independently many times
·
The
origin of unicellular eukaryotes permitted more structural diversity than was
possible for prokaryotes.
·
This
ignited an explosion of biological diversification.
· The evolution of multicellular bodies and the possibility of even greater structural diversity triggered another wave of diversification.