Eukaryotic Diversity

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I.                     Eukaryotes originated by symbiosis among prokaryotes

A.     There is a greater difference between prokaryotic and eukaryotic cells than between the cells of plants and animals.

B.     The cellular structures and process unique arose during the genesis of the protists.  These cellular structures are:

1.  A membrane bound nucleus

2. Mitochondria, chloroplasts, and the endomembrane system.

3.  A cytoskeleton

4. 9+2 flagella

5. Multiple chromosomes with linear DNA compactly arranged around proteins.

6.     Diploid life cycle stages.

7.     Mitosis.

8.     Meiosis.

9.     Sexual reproduction

C.    The small size and simpler construction of the prokaryotic cell has many advantages but also imposes a number of limitations.

1.     The number of metabolic activities that can occur at one time is smaller.

2.     The small size of the genome limits the number of genes that control activities.

D.    While prokaryotes are extremely successful, natural selection resulted in increasing complexity in some groups.  Three trends were:

1.     Multicellular forms with different cells with specialized functions.

2.     Complex bacteria communities where each species benefits.

3.     The compartmentalization of different functions within single cells.

E.     The evolution of the compartmentalization nature of eukaryotic cells may have resulted from two processes.

1.     Specialization of plasma membrane invaginations.

a.      These invaginations gave rise to the nuclear membrane, ER, Golgi, and etc.

2.     Endosymbiotic associations of prokaryotes may have resulted in the appearance of some organelles.

a.      Mitochondria and chloroplasts evolved from prokaryotes living within other prokaryotes.

3.     The endosymbiotic theory proposes that certain prokaryotic species called endosymbionts lived within larger prokaryotes. 

a.      Developed by Lynn Margulis

b.      Focuses on mitochondria and chloroplasts.

c.      Chloroplasts may have descended from cyanobacteria living in larger cells.

d.      Mitochondria are prokaryotic aerobic heterotrophs.

e.      May have been parasites or undigested prey of larger prokaryotes.

f.        The association progressed to mutualism.

F.     Evidence for the endosymbiotic origin of mitochondria and chloroplasts includes the similarities between these organelles and eubacteria.

1.     Are of appropriate size to be descendants of eubacteria.

2.     Have inner membranes containing several enzymes and transport systems.

3.     Replicate by splitting processes similar to binary fission.

4.     Have DNA which is circular and not associated with histones.

5.     Contain their own tRNA, ribosomes, and other components.

6.     Chloroplasts have ribosomes more similar to prokaryotic ribosomes.

7.     Mitochondrial ribosomes are more like prokaryotic ribosomes.

G.    Molecular systematics lends even more evidence to support the endosymbiotic theory.

1.     The rRNA of chloroplasts is more similar in base sequence to RNA from certain photosynthetic eubacteria.

2.     Chrloroplast rRNA is transcribed from genes in the chloroplasts.

3.     Mitochondrial rRNA also had a base sequence the supports an eubacterial origin.

H.     A comprehensive theory for the origin of eukaryotic cells must also include the evolution of:

1.     9+2 flagella and chili which are like prokaryotic flagella

2.     The origins of mitosis and meiosis which also utilize microtubules.

a.      Mitosis made it possible for large eukaryotic genomes to be reproduced.

b.      Meiosis is essential to sexual reproduction.

c.      Protists have the most varied sexual life histories.

II.                   Archezoans provide clues to the early evolution of eukaryotes.

A.     An ancient lineage of eukaryotes branched away from the eukaryotic tree very early, perhaps as long as two billion years ago.  This group is referred to as the archezoa and contains only a few phyla.

1.     These organisms lack mitochondria and plastids.

2.     Have simple cytoskeletons

3.     Ribosomes are more like prokaryotes

4.     Modern representative is Giardia intestinalis

a.      Flagellated unicellular parasite of the human intestine.

b.      Transmitted in a cyst through feces contaminated water.

III.                  The diversity of protists represents different experiments in the evolution of eukaryotic organization.

A.     Precambrian rock dated to about 2.1 billion years of age contain acritarchs, the oldest commonly accepted fossils of protists.

1.     Remnants of the proper size and structure to be ruptured coats of cysts.

2.     Adaptive radiation produced a large diversity.

3.     The variations are representative of the structure and function in eukaryotic cells.

B.     Protists are found in almost all moist environments, the seas, freshwater systems, and moist terrestrial habitats.

1.     They are important components of marine and freshwater plankton.

2.     Many are bottom dwellers

3.     Photosynthetic species from mats at the still water edges of lakes and ponds where they provide a food source for other protists.

4.     Damp soil, leaf litter, and other moist habitats contain large numbers.

5.     Symbiotic species found in the body fluids, tissues, and cells of the host.

C.    Almost all protists are aerobic, using mitochondria for cellular respiration.

1.     Anaerobic forms lack mitochondria and live in anaerobic environments.

2.     May be photoautotrophic, heterotrophic or mixotrophic.

D.    Most protists have flagella or cilia at some time in the life cycle.

1.     Eukaryotic cilia and flagella are extensions of the cytoplasm.

2.     These cilia and flagella have the same basic 9+2 ultrastructure.

E.     Cell division and nuclear structure are very variable in the protists.

1.     Unique mitotic divisions occur in many groups.

2.     All can reproduce asexually

3.     Some can reproduce sexually or use syngamy to trade genes.

4.     Some for resistant cysts in harsh environments.

F.     Protists are considered the simplest eukaryotic organisms because most are unicellular.

IV.               Diverse modes of locomotion and feeding evolved among protozoa

A.     The term protozoa is an informal reference to a diverse groups of heterotrophic protists.

1.     These organisms seek and consume bacteria, other protists, and detritus.

a.      Detritus is the dead organic matter.

2.     Some are symbiotic and many are pathogens.

3.     The subdivision of this group into different phyla is based in part on how they feed and move.

B.     Rhizopoda (Amoebas)

1.     Includes amoebas and their relatives.

2.     Simplest of protists

3.     Unicellular

4.     No flagellated stages in life cycle.

5.     Pseudopodia are cellular extinctions that function in feeding and movement.

a.      They cytoskeleton of microtubules and microfilaments functions here.

6.     All reproduction is by asexual mechanisms.

7.     During mitosis, spindle fibers form but stages are not apparent.

a.      Nuclear membrane persists during cell division.

8.     Inhabit freshwater, marine and soil habitats

9.     Most are free living

10. Some are parasitic.

C.    Actinopoda (Heliozoans and Radiozoans)

1.     Actinopoda = ray feet.

2.     Possess axopodia

a.      Projections reinforced by bundles of microtubules covered by cytoplasm.

3.     Axopodia increase the surface area.

4.     Help organisms float and feed.

5.     Microorganisms stick to the axopodia and are phagocytized.

6.     Most are planktonic.

7.     Heliozoans live in fresh water.

8.     Radiozoans are primarily marine and have delicate shells of silica.

D.    Foraminifera (Forams)

1.     Have porous, multi-chambered shells of organic material hardened by calcium carbonate.

2.     Exclusively marine living in sand or attached to algae.

3.     Cytoplasmic strands extend through the shell’s pores and function in swimming, feeding, and shell formation.

4.     Many have a symbiotic algae living beneath the shell.

5.     90% of described species are fossils.

6.     Shells are important parts of sedimentary rocks.

E.     Apicomplexa (Sporozoans)

1.     All species are parasites.

2.     The infections cells produced during the life cycle are sporozoites.

3.     The apex of sporozoites ahs organelles for penetrating host cells and tissues.

4.     Life cycles are intricate

a.      Have both sexual and asexual reproduction.

5.     Example is Plasmodium.

a.      Cause malaria

b.      Anopheles mosquitoes serves as the intermediate host and humans the final host.

c.      The human immune system has little effect on the parasite.

d.      Spends most of its life cycle in blood cells or liver cells.

e.      Has the ability to alter surface proteins.

f.        Vaccination available.

F.     Zoomastigophora (Zooflagellates)

1.     All species are heterotrophs and absorb organic molecules or phagocytize prey.

2.     Use whiplike flagella to move

3.     Solitary cells

4.     Free living or symbiotic

a.      Symbiotic live in the gut of termites to digest cellulose.

5.     Example is Trypanosoma

a.      Causes African sleeping sickness.

b.      Spreads by the tsetse fly.

c.      Evade the hosts immune system and rearranges the genes coding for coat proteins.

G.     Cilophora (Ciliates)

1.     Use cilia to move and feed.

2.     Most are solitary cells in fresh water.

3.     Cilia are relatively short and beat in synchrony.

4.     Cilia are a part of a submembranous system that coordinates movement.

5.     Are dispersed over the surface or clustered.

6.     Some species move on leg-like cirri which are cilia bonded together.

7.     Have two nuclei

a.      Large macronucleus

1.      Has over 50 copies of the genome

2.      Genes are packed in a large number of small units.

3.      Controls everyday functions

4.      Necessary for asexual reproduction during binary fission.

b.      Small micronucleus

1.      Small and may number from 1 to 80 micronuclei dependent on species.

2.      Does not function in growth, maintenance or asexual reproduction.

3.      Functions in conjugations

V.                 Funguslike protists have morphological adaptations and life cycles that enhance their ecological role as decomposers.

A.     The resemblance of slime molds and water molds to true fungi is a result of convergent evolution of filamentous body structure.

1.     a filamentous body increases exposure to the environment and enhances their roles as decomposers.

2.     Slime molds differ from true fungi in their cellular organization, reproduction, and life cycles.

3.     Lack chloroplasts.

B.     Myxomycota (Plasmodial Slime Molds)

1.     Heterotrophic

2.     Brightly pigmented

3.     Plasmodium is the feeding stage of life cycle

4.     Nuclei are diploid and exhibit synchronous mitotic divisions

5.     Cytoplasmic streaming within distributes nutrients and oxygen.

6.     Engulfs food by phagocytosis

7.     Grows by extending pseudopodia.

8.     Lives in moist soil, leaf mulch and rotting longs.

9.     When environment changes, forms a fruiting body or sporangia.

C.    Acrasiomycota (Cellular Slime Molds)

1.     Feeding stage consists of individual, solitary haploid cells.

2.     When food supply is depleted, cells aggregate but remain separate and move to new supplies.

3.     Fruiting bodies function in asexual reproduction.

4.     Only have a few flagellated stages.

D.    Oomycota

1.     Includes water molds, white rusts, and downy mildews

2.     Have coenocytic hyphae that are fine branching filaments.

3.     Cell walls are made of cellulose

4.     Diploid condition prevails in most species.

5.     Biflagellated cells are present in life cycles.

6.     In water molds

a.      A large egg is fertilized by a smaller sperm to form a resistant zygote.

b.      Usually decomposers which grow on dead algae and animals in fresh water.

c.      Some are parasitic.

7.     White rusts

a.      Usually parasitic on terrestrial plants.

b.      Disperse by windblown spores

c.      Also have flagellated zoospores at some point.

d.      Some of the most important plant pathogens are in this group.

VI.               Eukaryotic algae are key producers in most aquatic ecosystems.

A.     A majority of the eukaryotic algae are photosynthetic with only a few of the phyla having heterotrophic or mixotrophic members.

B.     Algae are relatively simple photoautotrophic aquatic organisms.

1.     Account for 50% of the global photosynthetic production

2.     Forms include fresh water plankton, marine plankton, and seaweeds.

3.     All have chlorophyll a

4.     Phylogenic relationships are determined by

a.      Accessory pigments.

b.      Chloroplast structure

c.      Cell wall chemistry

d.      Number, type and position of flagella.

e.      Food storage product

C.    Dinoflagellata (Dinoflagellates)

1.     Foundation of most marine food chains.

2.     Make up phytoplankton

3.     May cause red tides by explosive growth

a.      produce a toxin that is concentrated by invertebrates including shellfish.

b.      Toxin is dangerous to human and causes paralytic shellfish poisoning.

4.     Most are unicellular, some are colonial

5.     Cell surface is reinforce by cellulose plates

6.     Flagella found in perpendicular grooves creating a whirling movement.

7.     Some are photosynthetic symbionts

8.     Some lack chloroplasts and live as parasites.

9.     Have brownish plastids containing chlorophyll a, c, and a mix of carotenoids including peridinin

10. Food is stored as starch.

11. Chromosomes lack histones and are condensed.

12. Have no mitotic stages.

13. Kinetochores are attached to the nuclear envelope

14. Chromosomes are distributed by splitting the nucleus.

D.    Bacillariophyta (Diatoms)

1.     Yellow or brown in color due to the presence of brown plastids.

2.     Move by a gliding motion.

3.     Usually reproduce asexually

4.     Sexual stages are rare.

5.     Produce resistant cysts.

6.     Mostly unicellular

7.     Overlapping glasslike walls of hydrated silica

8.     Have chlorophyll a, c, yellow and brown carotenoids, and xanthophyll

9.     Found in fresh water and marine habitats.

10. Store food in form of oil

a.      Also makes cells buoyant.

E.     Chrysophyta (Golden Algae)

1.     Plastids have chlorophyll a, c, yellow and brown carotenoids, and xanthophyll.

2.     Live in freshwater.

3.     Mostly colonial

4.     Flagellated cells with both flagella attached near one end of the cell.

5.     Store carbohydrates as laminarin, a polysaccharide.

6.     Form cysts in harsh environments.

a.      cysts found in Precambrian rocks.

F.     Phaeophyta (Brown Algae)

1.     Largest and most complex of the algae.

2.     Multicellular

3.     Most are marine inhabitants.

4.     Have chlorophyll a, c and fucoxanthin.

a.      Fucoxanthin is a carotenoid

5.     Store carbohydrate food reserves as laminarin.

6.     Cell walls made of cellulose and algin.

G.    Evolutionary Adaptations of Seaweed

1.     Seaweeds are large, multicellular marine algae which are found in the intertidal and subtidal zones of coastal waters.

a.      Diverse group including Phaeophyta, Rhodophyta, and Chlorophyta

b.      Emphasizes adaptations in the red algae.

2.     The habitat poses challenges for survival.

a.      Movement of water due to waves and wind are physically active.

b.      Tidal rhythms result in the seaweeds being alternately covered by water and exposed to sunlight and drying conditions.

3.     Have evolved several structures to survive.

4.     Structural adaptations result from the multicellular anatomy.

5.     Some have differentiated tissues and organs analogous to a plant.

a.      The thallus is the body of the seaweed. 

1.      Has no roots, stems, or leaves.

b.      The thallus consists of a rootlike holdfast to maintain position.

c.      A stemlike stipe to support the blades

d.      A leaflike blade that gives a large surface for photosynthesis.

e.      Floats help suspend the blades near the water surface.

f.        Brown algae are known as giant kelp.

1.      Live in less harsh conditions.

6.     Biochemical adaptations enhance survival.

a.      Cellulose cell walls contain gel-forming polysaccharides that cushion the blow from the waves.

b.      Some red algae incorporate calcium carbonate into their cells.

7.     Seaweeds are used by human in a variety of ways.

a.      Brown and red alga are used as food.

b.      Marine algae are nutrient supplements due to the iodine content.

c.      Algin, agar and carageenan are extracted and used as thickeners.

d.      Agar is used as a microbiological culture media.

H.     Alternation of Generations in the Life Cycles of some algae

1.     Alternation of generations is a variety of life cycles.

2.     Alternate between multicellular haploid forms and multicellular diploid forms.

3.     The diploid individual is a sporophyte

a.     Produces reproductive cells called spores.

4.     The haploid individual is a gametophyte.

a.     Produces gametes.

5.     The sporophyte and gametophyte take turns.

a.      Spores release from sporophyte develop into gamtophytes.

b.      Gametophytes produce gametes which fuse to form a diploid zygote that develops into a sporophyte.

c.      Heteromorphic means the generations look different.

d.      Isomorphic means the generations look alike.

I.         Rhodophyta (Red Algae)

1.     Primarily warm tropical marine inhabitants

2.     Some found in freshwater and soil.

3.     Contain chlorophyll a, carotenoids, phycobilins, and chlorophyll d

4.     Red color of plastids due to accessory pigment called phycoerythrin.

a.      Phycoerythrin is a phycobilin found only in red algae and cyanobacteria

5.     Color of thallus may vary

a.      Deep water is black

b.      Moderate is red

c.      Shallow is green

6.     Store carbohydrates as floridean starch similar to glycogen.

7.     Cell walls are cellulose with agar and carageenan.

8.     Multicellular

9.     Reproduce sexually

10. Have no flagellated stages.

11. Alternation of generations is common.

J.      Chlorophyta (Green Algae)

1.     Contain plant-like chloroplasts

2.     7000 freshwater species

3.     Unicellular live as plankton.

4.     Live mutualistically with fungi to form lichens.

5.     Colonial forms are filamentous (pond scum)

6.     Have both sexual and asexual reproductive stages.

a.      Some conjugate with amoeboid gametes

b.      Majority produce biflagellated gametes.

VII.              Systematists continue to refine their hypotheses of eukaryotic phylogeny

A.     New debate about the protist kingdoms

1.     Obsolete

2.     Does not reflect the current understanding of phylogeny

B.     Proposed eight-kingdom system

1.     Prokaryotes split into two kingdoms.

2.     Archeazonas have their own kingdom.

3.     Kingdom protista is maintained but a new kingdom Chromista is added for algae.

4.     Organisms in the kingdom Chromisa are distinguished by

a.      Presence of unusual chloroplasts with two additional membranes

b.      Small amount of cytoplasm.

c.      Presence of a vestigial nucleus.

5.     Green and red algaes are moved to the plant kingdom.

VIII.            Multicellularity originated independently many times

A.     Early eukaryotes were more complex than prokaryotes and this increase allowed for variations in morphology.

B.     Multcellularity evolved several times to give rise to multicellular algae, plants, fungi, and animals.

C.    Most researchers believe the earliest forms were colonies or loose aggregates of interconnected cells.

1.     Evolution of multicellularity involved cellular specialization and division of labor in aggregates.

2.     Earliest specialization may have been locomotor abilities.

3.     Separate sex cells from somatic cells.

D.    Multicellular forms appeared approximately 700 million years ago.

E.     Seaweeds and other complex algae were abundant during the Cambrian period.

F.     Primitive plants are believed to evolved from  green algae living in shallow waters about 400 million years ago.