chapter 28 the origins of eukayotic diversity
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CHAPTER 28 THE ORIGINS OF EUKAYOTIC DIVERSITY Introduction
Constituents of plankton (passive drifters inwater); foundation for
aquatic food chains. Free-living and parasitic forms. Eukaryotic
(9+2 flagella, membranousorganelles, nuclei) Primarily unicellular
(eg. Paramecium, Euglena);some colonial (Volvox) some
multicellular(Seaweed) Aerobic Cilia or flagella at some time in
life cycle. Sexual (syngamy- meiosis and union of gametes)and
asexually reproducing. Historically Grouped into Three Categories
(representing different phylogeny and nutritional modes,
locomotion) Animal-like (protozoa): ingestive; chemoheterotrophs
Fungus-like, absorptive; chemoheterotrophs Plant-like (algae):
photosynthetic autotrophs; Chlorophyll a and accessory pigments
(carotene, xanthophylls, phycobilins) Eukaryotic fossils date back
2.1 billionyears and chemical signatures ofeukaryotes date back 2.7
billion years. Copyright 2002 Pearson Education, Inc., publishing
as Benjamin Cummings Evolutionary Steps Required to be a Eukaryotic
Cell
Loss of Cell Wall.Boundary must be flexible. Origin of Cytoskeleton
and MTOC.Must be able tomove and divide.No good plausible theory on
development. Origin of Nuclear Membrane. Appearance of digestive
vesicles and membranousorganelles. Linear DNA wound around
Histones. Whats So Cool About Being A Member of the Eukarya?
Eukaryotes Have MembranousOrganelles! Allows the cell
tosequester/isolate materials. Separate molecules fromcytosol. Keep
important processesisolated and concentrated. Specialization
ofprocesses. How did these characteristics evolve?
Autogenous model of membrane development.Membraneinfolding.
Endosymbiotic hypothesis (serial endosymbiosis) for originof
mitochondria and chloroplasts.Cells that live within other cellsare
called endosymbionts. Proposed by Lynn Margulis Aerobic heterotroph
and photosynthetic prokaryotesbecome endosymbionts. Similar size
Inner membrane transport proteins and enzymes similar Divide by
binary fission Circular DNA without histones
Transcription/translation machinery Evolution of Endomembrane
System and Membrane bound organelles. Each endosymbiotic event adds
a membranederived from the vacuole membrane of the hostcell that
engulfed the endosymbiont. Fig. 28.5 Copyright 2002 Pearson
Education, Inc., publishing as Benjamin Cummings Phylogeny of
Protista. Everything including the Kitchen Sink!
In the five-kingdom system of classification, theeukaryotes were
distributed among four kingdoms:Protista, Plantae, Fungi, and
Animalia. The plant, fungus, and animal kingdoms are survivingthe
taxonomic remodeling so far, though theirboundaries have been
expanded to include certaingroups formerly classified as protists.
However, systematists have split protists into manykingdoms. Modern
systematists has crumbled the former kingdomof protists beyond
repair. Copyright 2002 Pearson Education, Inc., publishing as
Benjamin Cummings Protista was defined partly by structural
level(mostly unicellular eukaryotes) and partly byexclusion from
the definitions of plants, fungi, oranimals. However, this created
a group ranging from single-celled microscopic members, simple
multicellular forms, and complex giants like seaweeds. Copyright
2002 Pearson Education, Inc., publishing as Benjamin Cummings The
kingdom Protista formed a paraphyleticgroup, with some members more
closely relatedto 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. Fig. 28.2 Fig. 28.8 1.
Diplomonadida and Parabasala (the Archaezoa)
lack mitochondria. Ribosomes similar toprokaryotes. Trichomonas
vaginalis Giardia lamblia The diplomonads have multiple flagella,
twoseparate nuclei, a simply cytoskeleton, and nomitochondria or
plastids. One example is Giardia lamblia, a parasite thatinfects
the human intestine causing giardiasis, or"hiker's diarrhea" 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.
Parabasalians are found only in association with animals. Some
pathogenic, others commensal The best known species, Trichomonas
vaginalis, inhabitsthe vagina of human females.In men this is
oftensymptomless, but in women it causes painfulinflammation of the
vagina (vaginitis). It can infect the vaginal lining if the normal
acidity of the vaginais disturbed. Sexual transmission can spread
the infection. Also, found in gut of termites and roaches digest
cellulose 2. Euglenozoa: aka: Flagellates.
Several protistan groups,including the euglenoidsand
kinetoplastids, useflagella for locomotion. The
euglenoids(Euglenozoa) arecharacterized by ananterior pocket
fromwhich one or two flagellaemerge. While Euglena is
chieflyautotrophic, othereuglenoids are mixotrophicor
heterotrophic. Euglena spp. Many photosynthetic with Chlorophyll a
& b.
They also have a unique glucosepolymer, paramylon, as a
storagemolecule. Kinetoplastids are symbiotic and include
pathogenic parasites.
The kinetoplastids (Kinoplastida) have a singlelarge mitochondrion
associated with a uniqueorganelle, the kinetoplast. The kinetoplast
houses extranuclear DNA. Kinetoplastids are symbiotic and
includepathogenic parasites. For example, Trypanosoma causes
African sleeping sickness.Another human form oftrypanosomiasis
occurs in the Americas and isknown as Chagas disease. African
trypanosomiasis or sleeping sickness
The early phase entails bouts of fever,headaches, pains in the
joints and itching. infests the central nervous system. This iswhen
the characteristic signs and symptomsof the disease appear:
confusion, sensorydisturbances and poor coordination,disturbance of
the sleep cycle. Without treatment, the disease is fatal. If
thepatient does not receive treatment before theonset of the second
phase, neurologicaldamage is irreversible even after treatment.
Sleeping sickness has become the first orsecond greatest cause of
mortality, ahead ofHIV/AIDS, in Angola, the DemocraticRepublic of
Congo and southern Sudan . 3. Alveolata: The Alveolata combines
flagellatedprotists (dinoflagellates), parasites(apicomplexans),
and ciliated protists (theciliates). Members of this clade have
alveoli, smallmembrane-bound cavities, under the cellsurface. Their
function is not known, but they may helpstabilize the cell surface
and regulate water andion content. The dinoflagellates are abundant
components ofthe phytoplankton that are suspended near thewater
surface. Dinoflagellates and other phytoplankton form thefoundation
of most marine and many freshwater foodchains. Triple membrane
around the plastids.Someheterotrophic. Most dinoflagellates are
unicellular, but some arecolonial. Some parasites on fish or on
other protists. Each dinoflagellate specieshas a characteristic
shape,often reinforced by internalplates of cellulose. Two flagella
sit inperpendicular grooves in thearmor and produce aspinning
movement. Blooms of dinoflagellatesproduce "red tides" whichinjure
marine life. Dinoflagellate blooms, characterized by
explosivepopulation (20 million per liter), cause red tides in
coastalwaters. The blooms are brownish-red or pinkish-orange
because of thepredominant pigments in the plastids. Toxins produced
by some red-tide organisms have producedmassive invertebrate and
fish kills. These toxins can be deadly to humans as well.
Saxitoxin, aneurotoxin 100,000 times more potent than
cocaine.Prevents Na+movement at neuromuscular junction leading to
respiratory failureand cardiac arrest. Some dinoflagellates are
bioluminescent.
Some dinoflagellates form mutualistic symbioseswith cnidarians,
animals that build coral reefs. Photosynthetic products from the
dinoflagellatesprovide the main food resource for reef communities.
Some dinoflagellates are bioluminescent. An ATP-driven chemical
reaction gives off light whendinoflagellates are disturbed by water
movements. The function of bioluminescence may be to
attractpredators that may eat the smaller predators that feedon
phytoplankton. All apicomplexans are parasites of animals andsome
cause serious human diseases.
The parasites disseminate as tiny infectious cells(sporozoites)
with a complex of organelles specializedfor penetrating host cells
and tissues at the apex of thesporozoite cell. Most apicomplexans
have intricate life cycles withboth sexual and asexual stages and
often require two ormore different host species for completion.
Plasmodium, the parasite that causes malaria, spends partof its
life in mosquitoes and part in humans. Fig The incidence of malaria
was greatly diminishedin the 1960s by the use of insecticides
against theAnopheles mosquitoes, which spread the disease,and by
drugs that killed the parasites in humans. However, resistant
varieties of the mosquitoes and thePlasmodium species have caused a
malarialresurgence. About 300 million people are infected with
malariain the tropics, and up to 2 million die each year. Malaria
Facts Today approximately 40% of the world's populationmostly those
living in the world's poorest countries is atrisk of malaria. 90%
of deaths due to malaria occur in Africa south of theSahara mostly
among young children. Malaria kills anAfrican child every 30
seconds. Malaria can kill by infecting and destroying red
bloodcells (anaemia) and by clogging the capillaries that
carryblood to the brain (cerebral malaria) or other vital organs.
Malaria, together with HIV/AIDS and TB, is one of themajor public
health challenges undermining developmentin the poorest countries
in the world. The Ciliophora (ciliates), a diverse protist group,
isnamed for their use of cilia to move and feed. Free-living,
primarily solitary. Their cilia are associated with a
submembranesystem of microtubules that may
coordinatemovement.
Some ciliates are completely covered by rows of cilia,whereas
others have cilia clustered into fewer rows ortufts. The specific
arrangement of cilia adapts the ciliates fortheir diverse
lifestyles. Some species have leglike structures constructedfrom
many cilia bonded together, while others havetightly packed cilia
that function as a locomotormembranelle. In a Paramecium, cilia
along the oral groove draw in foodthat are engulfed by
phagocytosis.
Like otherfreshwater protists,the hyperosmoticParameciumexpels
accumu-lated water fromthe contractilevacuole. Fig c Ciliates have
two types of nuclei, a largemacronucleus and usually several tiny
micronuclei.
The macronucleus has 50 or more copies of the genome. The
macronucleus controls the everyday functions of hecell by
synthesizing RNA and is also necessary forasexual reproduction.
Ciliated generally reproduce asexually by binary fissionof the
macronucleus, rather than mitotic division. The micronuclei (with
between 1 and 80 copies) arerequired for sexual processes that
generate geneticvariation. The sexual shuffling of genes occurs
duringconjugation, during which micronuclei that haveundergone
meiosis are exchanged. In ciliates, sexual mechanisms of meiosis
and syngamyare separate from reproduction. Fig 4. Stramenopila:
aka. Chromista
The Stramenopila includes both heterotrophicand photosynthetic
protists. The name of this group is derived from the presence
ofnumerous fine, hairlike projections on the flagella. In most
cases a hairy flagellum is paired with asmooth flagellum. almost
all aquatic organisms, planktonic and sessileforms. Photosynthetic
forms have chlorophyll c (unique tothis clade), brown pigment
fucoxanthin. The heterotrophic stramenopiles, the oomycotes,include
water molds, white rusts, and downymildews.
Some are unicellular, others have a fine network ofcoenocytic
hyphae (fine, branching filaments). These hyphae have cellulose
cells walls and areanalogous with the hyphae of true fungi (with
chitincell walls). Unlike fungi, the diploid stage dominates
inoomycotes 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
eggcell is fertilized by a smaller sperm nucleus, forming
aresistant zygote. Fig Water molds are important decomposers,
mainly in fresh water.
They form cottony masses on dead algae and animals.Someparasites of
fish (skin and gills) White rusts and downy mildews are parasites
of terrestrialplants. They are dispersed by windblown spores.
Spreads rapidly undercool and damp conditions One species of downy
mildew threatened French vineyards inthe 1870s (brought from
American vines) and another speciescauses late potato blight, which
contributed to the Irish famine(nearly 1 mill. deaths) in the 19th
century. Downy mildew on Squash Stripe Rust Include diatoms, golden
algae, and brown algae.
The photosynthetic stramenopile taxa are knowncollectively as the
heterokont algae. Hetero refers to the two different types of
flagella. The plastids of these algae evolved by
secondaryendosymbiosis. They have a three-membraneenvelope and a
small amount of eukaryoticcytoplasm within the plastid. The
probable ancestor was a red alga. Include diatoms, golden algae,
and brown algae. The wall is divided into two parts.
Diatoms (Bacillariophyta) have unique glasslike wallscomposed of
hydrated silicaembedded in an organic matrix. The wall is divided
into twoparts. Diatom are abundant members of both freshwater and
marine plankton.
Sexual stages are not common, but sperm may beamoeboid or
flagellated, depending on species. Diatom are abundant members of
both freshwaterand marine plankton. estimated that 20% -- 25% of
all carbon fixation on theplanet is carried out by diatoms. Diatoms
store food reserves in a glucose polymer,laminarin, and a few store
food as oils. Massive accumulations of fossilized diatoms are
majorconstituents of diatomaceous earth. Golden algae
(Chrysophyta), named for theyellow and brown carotene and
xanthophyllpigments, are typically biflagellated. Some species are
mixotrophic (can becomeheterotrophic in absence of light, feeding
onbacteria) and many live among freshwater andmarine plankton.
While most are unicellular, some are colonial. At high densities,
they can form resistant cysts thatremain viable for decades. Brown
algae (Phaeophyta) are the largest and most complex algae.
are multicellular. Most species are marine. Traditionally
classified as plants. Brown algae are especially common
alongtemperate coasts in areas of cool water andadequate
nutrients.Largest forms calledseaweeds. They owe their
characteristic brown or olive colorto accessory pigments
(carotenoids) in theplastids. Seaweeds inhabit the intertidal and
subtidal zones of coastal waters.
Macrocystis integrifolia Bladder Kelp Egregia menziesii.Feather Boa
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
multicellularanatomy, with some differentiated tissues andorgans
that resemble those in plants. These analogous features include the
thallus or bodyof the seaweed. The thallus typically consists of a
rootlike holdfastand a stemlike stipe, which supports
leaflikephotosynthetic blades. Blade Holdfast Some brown algae have
floats to raise the blades toward the surface.
Giant brown algae, known as kelps, form forests indeeper water. The
stipes of these plants may be 60 m long. Many seaweeds have
biochemical adaptations forintertidal and subtidal
conditions.
The cells walls, composed of cellulose and
gel-formingpolysaccharides (Alginic Acid, help cushion the
thalliagainst agitation by waves; also help to retain water.) Many
seaweeds are eaten by coastal people,including Laminaria (kombu in
Japan) andPorphyra (Japanese nori) for sushi wraps. A variety of
gelforming substances are extracted incommercial operations. Algin
from brown algae and agar and carageenan fromred algae are used as
thickeners in food, lubricants in oildrilling, 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 showcomplex life
cycles with alternation of multicellularhaploid and multicellular
diploid forms. A similar alternation of generations evolved
convergently in thelife cycle of plants. The life cycle of the
brown alga Laminaria is an exampleof 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 gametophyteare structurally
different, called heteromorphic.
In other algae, the alternating generations lookalike (isomorphic),
but they differ in the numberof chromosomes. 7. Rhodophyta: Red
algae lack flagella
Unlike other eukaryotic algae,red algae have no flagellatedstages
in their life cycle. The red coloration visible inmany members is
due to theaccessory pigmentphycoerythrin. Coloration varies
amongspecies and depends on thedepth which they inhabit. Unique red
pigment used aschemical tags of molecules,used in research. Agars
and carageenan foundin cell walls. Red algae (Rhodophyta) are the
most commonseaweeds in the warm coastal waters of
tropicaloceans.
Others live in freshwater, still others in soils. Some red algae
inhabit deeper waters than otherphotosynthetic eukaryotes. Their
photosynthetic pigments, especially phycobilins,allow some species
to absorb those wavelengths (bluesand greens) that penetrate down
to deep water. Reflectsred light and absorbs blue light One red
algal species has been discovered offBahamas at a depth of over
260m. Most red algae aremulticellular, with somereaching a size to
becalled seaweeds.
The thalli of many species are filamentous. The base of the thallus
is usually differentiated into a simple holdfast. Red coralline
algae depositcalcium carbonate outsideof cell wall Fig 8.
Chlorophyta: the proposed ancestor of terrestrial plants.
Green algae (chlorophytes andcharophyceans) are named for their
grass- green chloroplasts. These are similar in ultrastructure
andpigment composition to those of plants. The common ancestor of
green algae andplants probably had chloroplasts derivedfrom
cyanobacteria by primaryendosymbiosis. Caulerpa Most of the 7,000
species of chlorophytes live in freshwater.
Other species are marine, inhabit damp soil or snow, or
livesymbiotically within other eukaryotes. Some chlorophytes live
symbiotically with fungi to form lichens, amutualistic collective.
Chlorophytes range in complexity, including: biflagellated unicells
that resemble gametes and zoospores colonial species and
filamentous forms multicellular forms large enough to qualify
asseaweeds. Spirogyra Large size and complexity in chlorophytes has
evolved bythree different mechanisms:
(1) formation of colonies of individual cells (Volvox) (2) the
repeated division of nuclei without cytoplasmic division to
formmultinucleate filaments (Caulerpa) (3) formation of true
multicellular forms by cell division and celldifferentiation
(Ulva). Art imitates Life! Most green algae have both sexual and
asexual reproductive stages.
Most sexual species have biflagellated gametes withcup-shaped
chloroplasts. Fig 9. A diversity of protists use pseudopodia for
movement and feeding
Three groups of protists use pseudopodia, cellularextensions, to
move and often to feed. Most species are heterotrophs that actively
hunt bacteria, otherprotists, and detritus (detritivore-feed on
dead organic matter). Other species are symbiotic, including some
human parasites. pinocytosis Pseudopodium emerge from anywhere in
the cell surface.
Rhizopods (amoebas) are all unicellular and usepseudopodia to move
and to feed. Pseudopodium emerge from anywhere in the cellsurface.
To move, an amoeba extends a pseudopod, anchors itstip, and then
streams more cytoplasm into thepseudopodium. Amoeboid movement
isdriven by changes inmicrotubules andmicrofilaments in
thecytoskeleton.
In some speciespseudopodia extend outthrough openings in aprotein
shell around theorganism. 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 causesamoeboid dysentery in humans.Symptoms
includediarrhea, abdominal pains, and potentially liver disease
These organisms spread via contaminated drinkingwater, food, and
eating utensils. Actinopod (heliozoans and radiolarians), rayfoot,
refers to slender pseudopodia (axopodia)that radiate from the body.
Each axopodium is reinforced by a bundle ofmicrotubules covered by
a thin layer of cytoplasm. Most actinopods are planktonic.
The large surface area created by axopodia help them tofloat and
feed. Smaller protists and other microorganisms stick to
theaxopodia and are phagocytized by the thin layer ofcytoplasm.
Cytoplasmic streaming carries the engulfed prey intothe main part
of the cell. Most heliozoans (sun animals) live in fresh
water.
Their skeletons consist of unfused siliceous (glassy) orchitinous
plates. The term radiolarian refers to several groups ofmostly
marine actinopods. In this group, the siliceous (SiO2)skeleton is
fused intoone delicate piece. After death, these skeleton
accumulate as an ooze thatmay be hundreds of meters thick in some
seafloorlocations. 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
oforganic materials hardened with calcium carbonate. Over ninety
percent of the described forams are fossils.
Pseudopodia extend through the pores forswimming, shell formation,
and feeding. Many forams form symbioses with algae. Over ninety
percent of the described forams arefossils. The calcareous
skeletons of forams are importantcomponents of marine sediments.
Fossil forams are often used as chronological markersto correlate
the ages of sedimentary rocks from differentparts of the world. 10.
Mycetozoa: The Fungus-Like Protists
Mycetozoa (slime molds or fungusanimals) are neither fungi nor
animals,but protists. Any resemblance to fungi is analogous,
nothomologous, for their convergent role in thedecomposition of
leaf litter and organic debris. Slime molds feed and move
viapseudopodia, like amoeba, butcomparisons of protein sequences
placeslime molds relatively close to the fungiand animals. The
plasmodial slime molds (Myxogastrida) arebrightly pigmented,
heterotrophic organisms.
The feeding stage is an amoeboid mass, theplasmodium, that may be
several centimeters indiameter. The plasmodium is not
multicellular, but a single mass of cytoplasm with multiple nuclei.
The diploid nuclei undergo synchronous mitoticdivisions, perhaps
thousands at a time.
Within the cytoplasm, cytoplasmic streamingdistributes nutrients
and oxygen throughout theplasmodium. The plasmodium phagocytises
food particles frommoist soil, leaf mulch, or rotting logs. If the
habitat begins to dry or if food levels drop,the plasmodium
differentiates into stages that leadto sexual reproduction. The
cellular slime molds (Dictyostelida) straddlethe 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. Fig Copyright 2002 Pearson Education,
Inc., publishing as Benjamin Cummings The dominant stage in a
cellular slime mold is the haploid stage.
Aggregates of amoebas form fruiting bodies thatproduce spores in
asexual reproduction. Most cellular slime molds lack flagellated
stages.
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