Biology 1215 Lecture Notes
Chapter 28: The origins of eukaryotic
- Eukaryotes originated by symbiosis among
- Evidence for the endosymbiotic theory of
mitochondria and chloroplasts
- Origin of eukaryotic cell concomitant with
- Archezoans provide clues to the early evolution
- The diversity of protists represents different
"experiments" in the evolution of eukaryotic organization
- Almost all protists are aerobic, using mitochondria
for cellular respiration.
- Most protists have flagella or cilia at some
time in their life cycle.
- Cell division and nuclear structure are very
variable in the protists.
- Protists are considered the simplest eukaryotic
organisms because most are unicellular.
- Diverse modes of locomotion and feeding evolved
- Phylum Rhizopoda (Amoebas)
- Phylum Actinopoda
- Phylum Foraminifera
- Phylum Apicomplexa (Sporozoans)
- Phylum Zoomastigophora (Zooflagellates)
- Phylum Ciliophora (Ciliates)
- Fungus like protists have morphological adaptations
and life cycles that enhance their ecological role as decomposers
- Myxomycota (Plasmodial slime molds)
- Acrasiomycota (Cellular slime molds)
- Eukaryotic algae are key producers in most
- Phylum Dinoflagellata (Dinoflagellates)
- Phylum Bacillariophyta (Diatoms)
- Phylum Chrysophyta (Golden Algae)
- Phylum Paeophyta (Brown Algae)
- Evolutionary adaptations of seaweed
- Alternation of generations in the life
cycles of some algae.
- Phylum Rhodophyta (Red algae)
- Phylum Chlorophyta (Green algae)
- Multicellularity originated independently
- Eukaryotes are cells containing a nucleus. Include animals, plants,
fungi, and protists.
- Protists are the most diverse and ancient of the eukaryotes. Most
of the 60,000 species of extant protists are unicellular, although some
(algae) can be quite complex multicellular organisms.
- Some of the most important episodes in the evolution of the eukaryotic
cell unfolded during the evolution of protists.
Eukaryotes originated by symbiosis
- During the genesis of protists, a number of cellular structures and
processes unique to eukaryotes arose:
- A membrane-enclosed nucleus
- The endomembrane system
- The cytoskeleton
- 9+2 flagella
- Multiple chromosomes consisting of linear DNA compactly arranged
- Diploid stages in life cycles
- Mitosis, Meiosis, and Sex
- The evolution of the compartmentalized nature of the eukaryotic cells
may have resulted from two processes (Fig 28.2):
- Specialization of plasma membrane invaginations
- Invaginations and subsequent specializations may have given
rise to the nuclear envelope, endoplasmic reticulum, Golgi apparattus,
and other components of the endomembrane system.
- Endosymbiotic associations of prokaryotes resulted in appearance
of some organelles:
- Mitochondria and chroloplasts evolved from prokaryotes living
within other cells.
- The endosymbiotic theory proposes
that certain prokaryotes, called endosymbionts, lived within larger
- Chloroplasts are descended from endosymbiotic photosynthesizing
prokaryotes, such as cyanobacteria, living within larger cells.
- Mitochondria are postulated to be descended from prokaryotic
- May have been parasites or undigested prey of larger prokaryotes.
- The association progressed from parasitism or predation to
- As the host and endosynbiont became more interdependent, they
integrated into a single organism.
- Many extant organisms are involved in endosymbiotic relationships.
Evidence for the endosymbiotic theory
of mitochondria and chloroplasts
- Appropriate size.
- Inner membranes containing many enzymes and transport systems similar
to those on prokaryotic plasma membranes.
- Replicate by binary fission.
- Have their own circular genomes (not associated histones )
- Have their own ribosomes, tRNAs and other components of transcription
- Have ribosomes which are more similar to those of prokaryotes (antibiotic
- Phylogenetic evidence (rRNA).
Origin of eukaryotic cell concomitant
with origins of:
- 9+2 flagella and cilia (analogous to prokaryotic flagella)
- Mitosis and meiosis, which also utilize microtubules
- Mitosis made it possible for large eukaryotic genomes to be reproduced.
- Meiosis is essential to sexual reproduction (genetic variation).
- Protist have the most varied sexual life histories of the eukaryotes.
Archezoans provide clues to the early
evolution of eukaryotes
- The earliest divergence (2 billion yrs?) within the eukaryotic lineage
resulted in the archezoa.
- Lack mitochondria, plastids, endoplasmic reticulum and Golgi apparatus
and have relatively simple cytoskeletons.
- Ribosomes have some characteristics more closely aligned with prokaryotes
Giardia intestinalis is a modern
representative of the archezoa.
- Flagelated unicellular anaerobic obligate parasite of human intestine.
- Most commonly transmitted in cyst form through water contaminated
with human feces.
- Giardia is important to evolutionary biology.
- Have two separate haploid nuclei, which may be a vestige of early
- Prokaryotes are haploid. Some researchers believe that early
eukaryotes had a simple haploid nucleus bounded by a nuclear membrane.
- Diplomonads diverged from eukaryotic lineage before the process
of nuclear fusion and meiosis evolved. Thus, their dual nuclei may
be a clue to the past.
The diversity of protists represents
different "experiments" in the evolution of eukaryotic organization
- Oldest protist fossils (acritarchs) date to about 2.1 billion yrs.
- Acritarchs are of proper size and structure to be ruptured coats
of cysts similar to those of extant protists.
- Adaptive radiation produced a diversity of protists over the
next billion yrs.
- The variations present in these organisms were representative
of the structure and function possible in eukaryotic cells. Today
we recognize over 60 different protist lineages.
- Protists are found in almost all moist environments, the seas, freshwater
systems, and moist terrestrial habitats.
- Important components of marine and freshwater plankton.
Plankton = communities of organisms,
mostly microscopic, that drift passively or swim weakly near the surface
of oceans, ponds, and lakes.
- Many are bottom dwellers in freshwater and marine habitats where they
attach to rocks or live in the sand and silt.
- Photosynthetic species form mats at the still-water edges of lakes,
ponds where they provide a major food source for other protists.
- Large numbers of protists live in moist terrestrial habitats: damp
soil, leaf litter
- Symbiotic species are found in the body fluids, tissues and cells
of many types of hosts.
Almost all protists are aerobic, using
mitochondria for cellular respiration.
- Anaerobic forms lack mitochondria and live in anaerobic environments
or have mutualistic respiring bacteria.
- Protists may be photoautotrophic, heterotrophic, or mixotrophic (combining
photosysnthesis with heterotrophy).
- The different modes of nutrition are used to separate protists
into 3 convenient (but not phylogenetic) categories:
- 1. Algae (photosynthetic) (Plant-like)
- 2. Protozoa (ingestive)(Animal-like)
- 3. Absorptive protists. (Fungal-like)
Most protists have flagella or cilia
at some time in their life cycle.
- Not homologous to prokaryotic flagella.
- Eukaryotic flagella and cilia are extensions of the cytoplasm (surrounded
by plasma membrane)
- These cilia and flagella have 9+2 microtubular microstructure.
- Cilia and flagella differ in that cilia are shorter and more numerous.
Cell division and nuclear structure
are very variable in the protists.
- Unique mitotic divisions occur in many groups.
- All can reproduce asexually; some can also reproduce sexually or
at least use syngamy (cell fusion)
to trade genes between asexual reproductive episodes.
- Some form resistant cysts when
stressed by harsh environments.
Protists are considered the simplest
eukaryotic organisms because most are unicellular
- At the cellular level, protists are extremely complex.
- The unicellular protist is not analogous to a single plant or animal
cell, but is a complete organism.
- The single cell of a protist must perform all the basic functions
carried out by the specialized cells of plants and animals.
- Protistan taxonomy is in a state of flux
- Classification schemes and phylogeny they represent are only tentative
and change with new information.
- Whittakers popular 5-kingdom taxonomic system placed only unicellular
eukaryotes in kingdom protista.
- Sea-weeds have since been included as protists. More closely
related to algae than plants.
- Slime molds and water molds (once thought to be fungi) also placed
in protista kingdom.
- Molecular systematics, especially those based on rRNA, is revamping
taxonomy of protists into a more "natural" classification.
There are currently 60 lineages protists. Many of these represent diffferent
kingdoms in the domain eukaryoa.
Diverse modes of locomotion and feeding
evolved among protozoa
- Protozoa is an informal reference to diverse group of heterotrophic
- Seek and consume bacteria and other protists and detritus (dead
- Some are symbiotic and many are pathogens.
- Classification of protozoa into different phyla based in part
on how they feed and move (classification ongoing process).
Phylum Rhizopoda (Amoebas)
- Simplest of protists; all are unicellular
- No flagellated stages in life cycle
- Pseudopodia form as cellular extensions and function in feeding and
movement (cytoskeleton involved in this).
- All reproduction is asexual; no meiosis
- Nuclear envelope persists during cell division in many genera
- Inhabitants of fresh water, marine and soil habitats.
- Most are free-living, although some are parasitic.
Read textbook, but not on exam.
Read textbook, but not on exam.
Phylum Apicomplexa (Sporozoans)
- All species of the phylum Apicomplexa are parasites of animals
- The infectious cells produced in the life cycles are called sporozoites
- The apex of sporozoites has organelles for penetrating host cell
and tissues (Phylum named after this trait).
- Life cycles are intricate having both sexual and asexual reproduction;
often requiring two or more host species.
- Several species of Plasmodium cause malaria
- Anopheles mosquitoes serve as intermediate host and human as
the final host.
- Incidence of malaria greatly reduced by insecticide against mosquito
(1960s). Resistant strains have appeared (both mosquito and
- Malaria still a major killer.
- Little success in developing vaccine against Plasmodium.
- The human immune system has little effect on the parasite.
- Plasmodium spends most of its life cycle in blood cells and
- Plasmodium alternates surface proteins rapidly.
- Vaccine strategy: mix of synthetic proteins which mimic parasite
Phylum Zoomastigophora (Zooflagellates)
- All are heterotrophs that absorb organic molecules or phagocytise
- Use whip-like flagella to move.
- Most are solitary, but some colonial.
- Many are freeliving and some are synbiotic.
- Zooflagellate synbionts in gut of termites digest cellulose
in wood eaten by host.
- Some Trypanosomes cause African sleeping
sickness, and are spread by tsetse
- Large reservoir for the disease since many tropical African mammals
harbor the parasite.
- Trypanosomes can evade host immune response by rapidly changing
genes encoding surface coat proteins.
- Zooflagellates are closely related to flagellated protists which include
Phylum Ciliophora (Ciliates)
- Ciliates use celia to move and feed.
- Most ciliates exist as solitary cells in fresh water.
- Cilia are short and beat synchronously.
- Cilia associated with a submembranous system that coordinates
the movement of thousands of cilia.
- Cilia can be dispersed or clustered in rows or tufts. Some move
on leg-like cirri. (many cilia bonded together).
- Other species have rows of tightly packed cilia that function
together as locomotor membranelles (ie Stentor).
- Ciliates possess two types of nuclei: one large macronucleus and from
one to several small micronuclei. (Fig 28.9)
- Characteristics of macronucleus:
- Large, over 50 per genome.
- Genes are packaged in large number of small units, each with
hundreds of copies of just a few genes.
- Controls everyday functions of cell by synthesizing RNA.
- Macronucleus function in asexual reproduction during binary
fission. The macronucleus elongates and splits instead of undergoing
- Characteristics of micronucleus
- Small and may number between 1 to 80 micronuclei, depending
- Does not function in growth, maintenance or asexual reproduction.
- Functions in conjugation, a sexual process which produces
- Note that meiosis and syngamy are separate from reproduction
Fungus like protists have morphological
adaptations and life cycles that enhance their ecological role as decomposers
- The resemblance of slime molds and water molds to true fungi is a
result of convergent evolution of filamentous body structure.
- Filamentous body structure increases exposure to the environment
and enhance their roles as decomposers.
- Slime molds differ from true fungi in their cellular organization,
reproduction, and life cycles.
- Slime molds more closely related to amoeboid protists than
to true fungi.
- Molecular phylogenetic data indicate that water molds more
closely ralated to certain algae, although they lack chloroplasts.
- Fungal-like protists fall into three phylum:
- Myxomycota (Plasmodial slime molds)
- Acrasiomycota (Cellular slime molds)
Myxomycota (Plasmodial slime
- This phylum consists of the plasmodial slime molds which are all heterotrophs.
Plasmodium: Feeding stage of life cycle consisting of an amoeboid, coenocytic
mass (multinucleated cytoplasm undivided by membrane).
- Life cycle of plasmodium (Fig 28.13)
- In most species, the nuclei of the plasmodia are diploid and
exibit synchronous mitotic divisions.
- Cytoplasmic streaming within the plasmodium helps distribute
nutrients and oxygen.
- Engulfs food by phygocytosis as it grows by extending pseudopodia.
- Live in moist soil, leaf litter and rotting logs.
- When stressed by drying or lack of food, the plasmodium ceases
growth and forms sexually reproductive structures called fruiting
bodies, or sporangia.
Acrasiomycota (Cellular slime molds)
- Life cycle (Fig 28.14)
- Feeding stage of life cycle consists of solitary, individual haploid
- When food supply depleted, cells aggregate to form a mass similar
to those of plasmodial slime molds, but cells remain separate (not coenocytic)
- Fruiting bodies function in asexual reproduction (unlike plasmodial
- Only a few have flagellated stages.
- This phylum includes water molds, white rusts and downy mildews.
- Have coenocytic hyphae (fine, branching filaments) that are analogous
to those found in fungi.
- Cell walls made of cellulose, rather than chitin.
- Diploid condition in the life cycle prevails in most species.
- Biflagellated cells are present in the life cycles, while fungi
lack flagellated cells)
- In Water molds:
- A large egg is fertilized by a smaller sperm cell to form a resistant
zygote (Fig 26.17)
- Usually decomposers living on dead algae and animals in fresh
- Some are parasitic, growing on injured tissue but may also grow
on the skin and gills of fish
- White rusts and downy mildews:
- Usually parasitic on terrestrial plants.
- Disperse by windblown spores but also flagellated zoospores at
some point in their life cycle.
Eukaryotic algae are key producers
in most aquatic ecosystems
- The majority of algae are photosynthetic with only few of the phyla
having heterotrophic or mixotrophic members
Algae = relatively simple photoautotrophic
- Algae are of great ecological significance:
- Account for 50% of global photosynthate production.
- Forms include fresh water plantkton, marine plankton, and intertidal
seaweeds that form the basis of aquatic food webs.
- All algae have chlorophyl a (like plants). The phylogenetic
relationships among algal phyla have been determined by difference in:
- Accessory pigments such as carotenoids, xanthophyls, phycobilins,
and other forms of chlorophyl.
- Chloroplast structure.
- Cell wall chemistry
- Number, type and position of flagella
- Food storage product
Phylum Dinoflagellata (Dinoflagellates)
- Dinoflagellates are components of phytoplankton which provide the
foundation of most marine food chains.
- Some cause red tides by explosive growth (bloom)
- produce toxin which is concentrated by invertebrates, including
- Toxin is dangerous to humans, and causes paralytic shellfish
- Most unicellular, some are colonial.
- Cell surface is reinforced by cellulose plates with flagella in perpendicular
grooves creating its whirling movement and resulting in a characteristic
- Some live as photosynthetic symbionts of the cnidarians that build
- Some lack chloroplasts and live as parasites; a few carniverous species
- Food is stored as starch.
- Have brownish plastids containing chlorophyll a, chlorophyll c, and
a mix of carotenoids, including peridinin (found only in this phylum).
- Chromosomes lack histones and are always condensed.
- Has no mitotic stages.
- Kinetochores are attached to the nuclear envelope and chromosomes
distributed to daugher cells by the splitting of the nucleus.
- Dinoflagellates are probably more closely related to zooflagellates
than to any phylum of algae.
Phylum Bacillariophyta (Diatoms)
- Mostly unicellular organisms with overlapping glasslike walls of
hydrated silica in organic matrix. (glass houses)
- Usually reproduce asexually.
- Components of freshwater and marine plankton.
- Store food in a form of oil, which also makes cells more buoyant.
- Same photosynthetic pigments as chrysophyta.
Phylum Chrysophyta (Golden Algae)
- Plastids have chlorophyl a, chlorophyl c, yellow and
brown carotenoids, and xanthophyl.
- Live among freshwater plankton; most are colonial
- Have flagellated cells with both flagella attached near one end of
- Store carbohydrates in the form of laminarin,
- Survive environmental stress by forming resistant cysts.
- microfossils resembling these cysts have been found in Precambrian
Phylum Paeophyta (Brown Algae)
- Largest and most complex algae (kelps).
- All are multicellular and most are in marine habitats.
- Have chlorophyl a, chlorophyl c, and the carotenoid
- Store carbohydrate food reserves in the form of laminarin.
- Cell walls made of cellulose and algin.
Evolutionary adaptations of seaweed
- Seaweeds are large, multicellular marine algae which are found in
the intertidal and subtidal zones of coastal waters.
- Term "seaweed" include a diverse group of algae including
brown algae, red algae, and green algae.
The habitat of seaweeds, particularly the intertidal zone, poses
several challenges to the survival of these organisms.
- Motion of water creates physically active habitat.
- Tides result in seaweed being alternatively covered by seawater
and exposed to direct sunlight and the drying conditions of the
- Seaweeds have evolved several unique structural and biochemical adaptations
to survive the conditions of their habitats.
- Structural adaptations found in seaweeds are a result of their complex
multicellular anatomy. Some forms have differentiated tissues and organs
analogous to those of plants.
- Thalus = the body of a seaweed.
Its plant like in appearance, but has no true roots, stems, or leaves.
- A thalus consists of a rootlike holdfast (maintains position),
a stem-like stipe (supports
the baldes), and leeflike blades (large
surfaces for photosynthesis).
- Some alage produce floats, which help suspend blades near the
- Brown algae known as kelps, occur beyond intertidal zone where
conditions are less harsh and may have stipes which may reach a
length of 100 meters.
- Biochemical adaptations in some seaweeds reinforce the anatomical
adaptations and enhance survival.
- Cellulose cell walls also contain gel-forming polysaccharides
(algin in brown algae, carageenan in red algae)
- Seaweeds are used by humans:
- Brown and red alga are used as food in many parts of orient.
- Marine algae are used as nutrient supplements due to high content
of iodine and other minerals.
- Algin, agar, and carageenan are extracted and used as thickners
for processed foods and lubricants in oil drilling.
Alternation of generations in the
life cycles of some algae.
- A variety of life cycles in brown algae, the most complex having alternation
- Alternation of generations = alternation between multicellular haploid
forms and multicellular diploid forms in a life history.
- Alternation of generations also involved in certain groups of
red and green algae.
- The life cycle of laminaria is
an example of a complex life cycle with alternation of generations (Fig
- The diploid individual is called a sporophyte because it produces
reproductive cells called spores.
- The haploid individual is called the gametophyte because it produces
- The sporophyte and gametophyte generations of the life cycles
take turn producing one another.
- Spores released from sporophyte develop into gametophytes.
- Gametophytes produce gametes which fuse (fertilization) to
form a diploid zygote that develops into a sporophyte.
- In Laminaria the sporophyte and gametophyte generations
are said to be heteromorphic because they are morphologically different.
- In Ulva, a green algae exhibiting alteration of generations,
the generations are referred to as isomorphic because they look alike.
Phylum Rhodophyta (Red algae)
- Found primarilty in warm, marine habitats, although some also found
in fresh water and soil.
- Contain chlorophyl a, carotenoids, phycobilins and chlorophyl
d in some.
- Red colour of plastids due to ecessory pigment, phycoerythrin
- Carbohydrate food reserves stored as
floridean starch (similar to glycogen).
- Cell walls made of cellulose with agar and carageenan.
- Most red algae are multicellular and are known as seaweeds.
- Most thalli are filamentous and are often branched.
- All red algae reproduce sexually:
- Have no flagellated stages, unlike other algal protists.
- Alternation of generations is common.
Phylum Chlorophyta (Green algae)
- Contain plant-like chloroplasts and are believed to be most recent
common ancestor of plants.
- 7000 species known, most are fresh water; fewer are marine.
- Many live as unicellular plankton. Inhabit damp soil, symbionts
with protozoa or invertebrates.
- Some live mutualistically with fungi and are known as lichens.
- Colonial forms are often filamentous (pond scum).
- Some multicellular forms also known as seaweeds.
- Evolutionary trends that probably produced colonial and multicellular
forms from flagellated unicellular ancestors:
- Formation of colonies of individual cells (as seen in Volvox).
- Repeated division of nuclei with no cytoplasmic division (as
- Formation of true multicellular forms as in ulva.
- Most chlorophytes have complex life histories involving sexual and
asexual reproductive stages.
- The life cycle of Chlamydomonas is a good example of the life history
of a unicellular chlorophyte (Figure 28.24).
- During asexual reproduction, the flagella are reabsorbed and
the cell divides twice by mitosis to form four cells.
- The daughter cells develop and emerge as swimming zoospores.
Zoospore development includes formation of flagella and cell
- Zoospores grow into mature cells, thus completing asexual
- Sexual reproduction is stimulated by environmental stress from
such things as shortage of nutrients, drying of the pond, or others..).
- During sexual reproduction, many gametes are produced by mitotic
division within the wall of the parent. The gametes escape the
parent cell wall.
- Gametes of opposite mating strains (+ and -) pair off and
cling together by tips of flagella.
- The gametes are morphologically indistinguishable and their
fusion is known as isogamy.
- The slow fusion of gametes forms a diploid zygote which secretes
a resistant coat for protection.
- When dormancy of the zygote is broken, four haploid individuals
(two of each mating type) are produced by meiosis.
- These haploid cells emerge from the coat and develop into
mature cells, thus completing life cycle.
- Many features of chlamydomonas sex are believed to have evolved early
in the chlorophyte lineage. Using this basic life cycle, many refinements
that evolved among the chlorophytes have been identified.
- Some green algae produce gametes that differ from vegetative
cells, and in some species, the male gamete differs in size or morphology
from the female gamete (anisogamy)
- Many species exhibit oogamy,
a type of anosogamy in which a flagellated sperm fertilizes a nonmotile
- Some multicellular species also exhibit alternation of generations.
- Ulva produces isomorphic thalli for its diploid sporophyte
and haploid gametophyte.
Multicellularity originated independently
- Early eukaryotes were more complex than prokaryotes. This increase
in complexity allows for greater morphological variations to evolve.
- Extant protists are more complex in structure and show a greater
diversity of morphology than the simpler prokaryotes.
- The ancestral stock which gave rise to the new waves of adaptive
radiations were the protists with multicellular bodies.
- Multicellularity evolved several times among the early eukaryotes
and gave rise to the multicellular algae, plants, fungi, and animals.
- Its believed that earliest multicellular forms arose from unicellular
ancestors as colonies or loose aggregates of interconected cells.
- Multicellular algae, fungi, plants and animals probably evolved from
several lineages of protists that formed by amalgamations of individual
- Evolution of multicellularity from colonial aggregates involved cellular
specialization and division of labor.
- The earliest specializations may have been locomotor capabilities
provided by flagella.
- As cells became more interdependent, some lost their flagella
and performed other functions.
- Further division of labor may have separated sex cells from somatic
cells (eg. Volvox).
- Multicellular forms more complex than filamentous algae appeared
approximately 700 million years ago.
- A variety of animal fossils has been found in late precambrian strata
and many new forms evolved in cambrian period (570 million yr bp).
- Seaweeds and other complex algae were also abundant during cambrian
- Primitive plants are believed to have evolved from certain green
algae living in shallow waters about 400-450 MY bp.
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