Protists and the Dawn of the Eukarya

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Protists and theDawn of the Eukarya
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Protists and the Dawn of the Eukarya

• Protists Defined
• The Origin of the Eukaryotic Cell
• General Biology of the Protists
• Protist Diversity
• Diplomonads and Parabasalids
• Euglenozoans
• Alveolates
• Stramenopiles
• Red Algae
• Chlorophytes
• Choanoflagellates
• A History of Endosymbiosis
• Some Recurrent Body Forms

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Protists Defined

• Many members of the Eukarya do not fit into the
  three familiar kingdoms of the Plantae, Animalia,
  and Fungi.
• The eukaryotes that are neither plants, animals,
  nor fungi are called protists.
• The protists are a polyphyletic group some are
  more closely related to the animals than they are
  to other protists.

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Figure 28.1 Three Protists
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The Origin of the Eukaryotic Cell

• The eukaryotic cell differs in many ways from the
  prokaryotic cell.
• The nature of the evolutionary process dictates
  that these differences could not have arisen
  simultaneously.
• The global environment underwent an enormous
  changefrom anaerobic to aerobicduring the
  course of the evolution of the eukaryotes.
• We can make only reasonable guesses as to what
  the steps in this evolutionary process were.

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The Origin of the Eukaryotic Cell

• The evolution of eukaryotic cells included the
  following components
• The origin of a flexible cell surface
• The origin of a cytoskeleton
• The origin of a nuclear envelope
• The appearance of digestive vesicles
• The endosymbiotic acquisition of certain
  organelles

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The Origin of the Eukaryotic Cell

• The first step toward the eukaryotic condition
  may have been the loss of the cell wall by an
  ancestral prokaryotic cell.
• A surface that is flexible enough to allow for
  infolding lets the cell exchange materials with
  its environment rapidly enough to sustain a
  larger volume and more rapid metabolism.
• A flexible surface also allows endocytosis.
• An infolded plasma membrane attached to a
  chromosome within an ancestral prokaryote may
  have led to the formation of the nuclear envelope.

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Figure 28.2 Membrane Infolding
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The Origin of the Eukaryotic Cell

• The early steps in the evolution of the
  eukaryotic cell likely included three advances
• The formation of ribosome-studded internal
  membranes, some of which surrounded the DNA
• The appearance of a cytoskeleton
• The evolution of digestive vesicles

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Figure 28.3 From Prokaryotic Cell to Eukaryotic
Cell (Part 1)
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The Origin of the Eukaryotic Cell

• A cytoskeleton allowed the now much larger cell
  to manage changes in its shape, distribute
  daughter chromosomes, and move materials from one
  part of the cell to another.
• The origin of the cytoskeleton is a mystery the
  genes that encode it are found in neither
  bacteria nor archaea.
• A controversial hypothesis suggests that these
  genes may have originated in a long-extinct
  fourth domain of life that transferred them
  laterally to an ancestor of the early eukaryotes.

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The Origin of the Eukaryotic Cell

• From an intermediate kind of cell, the next
  advance was likely to have been a motile
  phagocyte.
• The first true eukaryotic cell possessed a
  cytoskeleton and a nuclear envelope it also may
  have had an associated endoplasmic reticulum and
  Golgi apparatus and perhaps one or more flagella.

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The Origin of the Eukaryotic Cell

• During the early stages of eukaryotic evolution,
  the O2 levels in the atmosphere were increasing
  as a result of the photosynthetic activities of
  the cyanobacteria.
• Most living things were unable to tolerate this
  new aerobic, oxidizing environment, but some
  prokaryotes and ancient phagocytes were able to
  survive.
• One hypothesis suggests that the key to the
  survival of the early phagocytes was the
  ingestion of a prokaryote that became symbiotic
  and evolved into the peroxisomes of today.

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The Origin of the Eukaryotic Cell

• Peroxisomes are organelles that are able to
  disarm the toxic products of oxygen, such as
  hydrogen peroxide.
• The crucial endosymbiotic event that marked the
  completion of the modern eukaryotic cell was the
  incorporation of a proteobacterium that evolved
  into the mitochondrion.

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The Origin of the Eukaryotic Cell

• There are still several uncertainties surrounding
  the origins of eukaryotic cells.
• Lateral gene transfer may not have been extensive
  enough to account for the increasing number of
  genes of bacterial origin that are found in
  eukaryotes.
• The endosymbiotic origin of the mitochondria and
  chloroplasts accounts for the presence of
  bacterial genes that encode enzymes for
  respiration and photosynthesis, but it does not
  explain the presence of many other bacterial
  genes.

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The Origin of the Eukaryotic Cell

• It is clear that the eukaryotic genome is a
  mixture of genes with two distinct origins.
• Recently, it has been suggested that the Eukarya
  may have arisen from the mutualistic fusion of a
  Gram-negative bacterium and an archaean.

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General Biology of the Protists

• Most protists are aquatic, occupying a variety of
  environments including marine and fresh waters,
  the body fluids of other organisms, and soil
  water.
• Most are unicellular, but some are multicellular,
  and a few are very large.
• Some protists are heterotrophs, some are
  autotrophs, and some switch between these two
  modes of nutrition.
• The terms protozoan and algae actually lump
  together many phylogenetically distant protist
  groups.

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General Biology of the Protists

• Most protist groups include motile cells.
• Amoeboid motion involves the formation of
  pseudopods, extensions of the cells constantly
  changing body mass.
• The coordinated beating of tiny, hairlike
  organelles called cilia can move cells forward or
  backward.
• The eukaryotic flagella move like a whip some
  flagella push the cell, while others pull the
  cell.

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Figure 28.4 An Amoeba
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General Biology of the Protists

• One reason cells are small is that they need a
  high surface area-to-volume ratio to support the
  exchange of materials required for their
  existence.
• The presence of membrane-enclosed vesicles of
  various types increases the effective surface
  area in large, unicellular protists.
• Several protists that are hypertonic to their
  environments have contractile vacuoles that
  excrete excess water.
• Food vacuoles are vesicles in which ingested food
  is digested.

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Figure 28.5 Contractile Vacuoles Bail Out Excess
Water
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Figure 28.6 Food Vacuoles Handle Digestion and
Excretion
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General Biology of the Protists

• The cell surfaces of protists are diverse.
• Some protists are surrounded only by a plasma
  membrane, such as an amoeba.
• Most have stiffer surfaces to maintain the
  structural integrity of the cell.
• Some protists have complex cell walls.
• Some protists have internal shells.

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Figure 28.7 Diversity among Protist Cell
Surfaces (Part 1)
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Figure 28.7 Diversity among Protist Cell
Surfaces (Part 2)
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General Biology of the Protists

• Many protists contain endosymbionts.
• Endosymbiosis is very common in the protists, and
  in some cases both the host and the endosymbiont
  are protists.

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Figure 28.8 Protists within Protists
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General Biology of the Protists

• Most protists practice both asexual and sexual
  reproduction some groups practice only asexual.
• Asexual reproductive processes in the protists
  include binary fission, multiple fission,
  budding, and the formation of spores.
• Sexual reproduction in the protists also takes
  various forms.

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Protist Diversity

• The diversity found among the protists reflects
  the diversity of avenues pursued during the early
  evolution of the eukaryotes.
• Molecular biology techniques, such as rRNA
  sequencing, are making it possible to explore the
  evolutionary relationships among the protists in
  greater detail.

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Figure 28.9 Major Protist Groups in an
Evolutionary Context
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Diplomonads and Parabasalids

• The diplomonads and the parabasalids appear to
  represent the earliest surviving branches in
  todays tree of eukaryotic life.
• Both clades are unicellular organisms that lack
  mitochondria. Their ancestors possessed
  mitochondria, but they were lost in the course of
  evolution.
• Giardia lamblia is a parasitic diplomonad that
  contaminates water supplies and causes
  giardiasis.
• Trichomonas vaginalis is a parabasilid
  responsible for a sexually transmitted disease in
  humans.

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Figure 28.10 Two Protist Groups Lack Mitochondria
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Euglenozoans

• The euglenozoans are a clade of unicellular
  protists with flagella.
• The euglenoids and the kinetoplastids are the two
  subgroups of the euglenozoans.

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Euglenozoans

• The euglenoids possess flagella arising from a
  pocket at the anterior end of the cell.
• The Euglena propels itself through the water with
  one of its two flagella.
• Many species of Euglena are heterotrophic,
  whereas others are photoautotrophs.
• These autotrophic Euglena can become
  heterotrophic when kept in the dark, and they
  resume their autotrophic behavior when returned
  to light.

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Figure 28.11 A Photosynthetic Euglenoid
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Euglenozoans

• The kinetoplastids are unicellular, parasitic
  flagellates with a single, large mitochondrion.
• The mitochondrion contains a kinetoplast, a
  unique structure that houses multiple, circular
  DNA molecules and associated proteins.
• The trypanosomes are human pathogens that cause
  sleeping sickness, leishmaniasis, Chagas
  disease, and East Coast fever.

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Figure 28.12 A Parasitic Kinetoplastid
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Alveolates

• The alveolates are a clade of unicellular
  organisms characterized by the possession of
  cavities called alveoli just below their plasma
  membranes.
• Alveolates include the dinoflagellates,
  apicomplexans, and the ciliates.

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Alveolates

• The dinoflagellates are unicellular, aquatic
  organisms they are among the most important
  primary producers in the oceans.
• Many dinoflagellates are endosymbionts, while
  some live as parasites within other marine
  organisms.
• The dinoflagellates have a distinctive appearance
  with two flagella.
• They are responsible for toxic red tides.
• Many are bioluminescent.

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Figure 28.13 A Red Tide of Dinoflagellates
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Alveolates

• The apicomplexans are exclusively parasitic.
• The apical complex is a mass of organelles
  contained within the apical end of their spores.
  These organelles help the apicomplexan spore
  invade its host tissue.
• Apicomplexans of the genus Plasmodium are the
  cause of malaria.
• This parasite enters the human circulatory system
  by way of the Anopheles mosquito.
• It is an extracellular parasite in the insect
  vector and an intracellular parasite in the human
  host.

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Figure 28.14 The Life Cycle of an Apicomplexan
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Alveolates

• The ciliates are named for their characteristic
  hairlike cilia.
• Almost all are heterotrophic, and they have a
  complex body form.
• The ciliates possess two types of nuclei a
  single macronucleus and one or more micronuclei.
• The micronuclei are typical eukaryotic nuclei.
• The macronuclei are derived from the micronuclei
  and contain DNA that is transcribed and
  translated to regulate the life of the cell.

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Figure 28.15 Diversity among the Ciliates (Part
1)
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Figure 28.15 Diversity among the Ciliates (Part
2)
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Alveolates

• Paramecium is a frequently studied ciliate.
• The cell is covered by an elaborate pellicle
  composed of an outer membrane and an inner layer
  of membrane-enclosed alveoli that surround the
  bases of the cilia.
• Defensive trichocysts in the pellicle are
  expelled in response to threat.
• The cilia on a Paramecium provide a form of
  locomotion that is more precise than locomotion
  by flagella or pseudopods.

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Figure 28.16 Anatomy of Paramecium
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Alveolates

• Paramecia reproduce asexually by binary fission,
  in which the micronuclei divide mitotically and
  the macronuclei divide by an unknown mechanism.
• Paramecia also exhibit a form of genetic
  recombination called conjugation. It is not a
  reproductive process no new cells are created.
• Each member of a pair of cells gets two haploid
  micronuclei, which fuse to form a new diploid
  micronucleus.
• Experiments have shown that in species not
  permitted to conjugate, the clones can survive
  only a limited number of divisions.

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Figure 28.17 Paramecia Achieve Genetic
Recombination by Conjugating
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Stramenopiles

• The stramenopiles typically have two flagella of
  unequal length at some point in their life cycle.
• The longer of the two flagella bears rows of
  tubular hairs.
• There are photosynthetic and nonphotosynthetic
  stramenopile groups.
• Some stramenopiles lack flagella, but are
  presumed to be descended from ancestors that had
  flagella.
• The stramenopiles include the diatoms, the brown
  algae, and the oomycetes.

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Stramenopiles

• Diatoms are single-celled organisms, but some
  species form filaments. Diatoms have carotenoids
  in their chloroplasts to give them a yellow or
  brownish color.
• Diatoms deposit silicon in their cells walls,
  which gives them their characteristically
  intricate appearance.
• Certain sedimentary rocks are almost entirely
  composed of diatom skeletons, called diatomaceous
  earth.
• Diatoms reproduce both sexually and asexually.
• Diatoms are major photosynthetic producers in
  coastal waters and in fresh waters.

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Figure 28.18 Diatom Diversity
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Stramenopiles

• The brown algae are multicellular organisms
  composed of either branched filaments or leaflike
  growths called thalli.
• The carotenoid fucoxanthin in the chloroplasts
  gives brown algae their color.
• The brown algae are exclusively marine, and most
  are attached to rocks near the shore.
• The holdfast is a specialized structure that
  glues the attached forms to rocks.
• Some brown algae have stalks and blades, and some
  develop gas-filled cavities or bladders.

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Figure 28.20 Brown Algae
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Figure 28.21 Brown Algae in a Turbulent
Environment
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Stramenopiles

• The oomycetes are a nonphotosynthetic group that
  consists largely of the water molds and their
  terrestrial relatives, such as the downy mildews.
  
• The oomycetes are coenocytes (many nuclei
  enclosed in a single plasma membrane).
• The oomycetes are diploid for most of their life
  cycle and have flagellated reproductive cells.
• The water molds are aquatic and saprobic.
• Most terrestrial oomycetes are decomposers,
  although some are serious plant parasites.
• Phytophthora infestans water mold was the cause
  of the Irish potato famine.

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Figure 28.23 An Oomycete
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Red Algae

• Almost all red algae are multicellular.
• Their characteristic red color results from the
  photosynthetic pigment phycoerythrin.
• Most species of red algae are marine-dwelling,
  from shallow tide pools to deep in the ocean.
• The red algae have the ability to change the
  relative amounts of their various photosynthetic
  pigments depending on the light conditions.

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Figure 28.24 Red Algae
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Red Algae

• The red algae have characteristics that make them
  unique among protists.
• They contain the pigments phycoerythrin and
  phycocyanin and store the products of
  photosynthesis as floridean starch.
• They produce no motile, flagellated cells at any
  stage in their life cycle.
• Some produce a mucilaginous polysaccharide
  substance which is the source of agar.
• Certain red algae became endosymbionts long ago
  within the cells of other, nonphotosynthetic
  protists, eventually giving rise to chloroplasts.

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Chlorophytes

• The chlorophytes are a monophyletic group with a
  sister lineage consisting of other green algal
  lineages and the plant kingdom.
• Like plants, the chlorophytes contain
  chlorophylls a and b, and store photosynthetic
  products as starch in plastids.
• There are terrestrial, marine, and freshwater
  chlorophyte species.
• There is an incredible variety in shape and
  construction of the algal body within the
  chlorophytes.

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Figure 28.25 Chlorophytes (Part 1)
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Figure 28.25 Chlorophytes (Part 2)
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Chlorophytes

• There is great diversity within the life cycles
  of the chlorophytes.
• The sea lettuce Ulva lactuca exhibits an
  isomorphic life cycle.
• Most species of Ulva have structurally
  indistinguishable male and female gametes, and
  are categorized as isogamous.

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Chlorophytes

• Other chlorophytes are anisogamous, having female
  gametes that are distinctly larger than male
  gametes.
• Many other chlorophytes have a heteromorphic life
  cycle, with some exhibiting a variation of the
  heteromorphic life cycle called the haplontic
  life cycle.
• Other chlorophytes have a diplontic life cycle
  like that of many animals, where every cell
  except the gametes is diploid.

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Chlorophytes

• There are green algae other than chlorophytes.
• The chlorophytes are the largest lineage of green
  algae, but there are other lineages as well.
• These lineages are branches of a lineage that
  also includes the charophytes and the plant
  kingdom.

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Choanoflagellates

• The choanoflagellates are a group of colonial,
  flagellated protists that are thought to comprise
  the closest relatives of the animals.
• Choanoflagellates bear a striking resemblance to
  the most characteristic type of cell found in the
  sponges.

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Figure 28.28 A Link to the Animal Kingdom
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A History of Endosymbiosis

• Chloroplasts are found in many distantly related
  protist lineages.
• Some of these groups differ from others in terms
  of the photosynthetic pigments in their
  chloroplasts and the number of membranes
  surrounding their chloroplasts.
• These differences can be traced back to whether
  the group acquired its chloroplast through
  primary, secondary, or tertiary endosymbiosis.

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Some Recurrent Body Forms

• The amoeboid body plan includes pseudopods for
  locomotion.
• Amoebas appear in many protist groups.
• Amoebas are specialized protists many are
  adapted for life on the bottoms of lakes, ponds,
  and other bodies of water.
• Most are predators, parasites, or scavengers. A
  few are photosynthetic.
• Some have two-stage life cycles.
• Some amoebas have shells.

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Some Recurrent Body Forms

• The actinopods are have thin, stiff pseudopods,
  reinforced by microtubules.
• The pseudopods increase the surface area of the
  cell, help the cell float, provide locomotion in
  some species, and are the cells feeding organs.
• Radiolarians are exclusively marine and secrete a
  glassy endoskeleton.
• Heliozoans are primarily freshwater actinopods
  that lack an endoskeleton.

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Figure 28.30 Two Actinopods
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Some Recurrent Body Forms

• Foraminiferans are marine protists that secrete
  shells of calcium carbonate.
• The parent shell is abandoned after foraminiferan
  reproduction. The discarded skeletons of ancient
  foraminiferans make up extensive limestone
  deposits.
• The shells of individual foraminiferan species
  have been preserved as fossils in marine
  sediments and are valuable as indicators in the
  classification and dating of sedimentary rocks.

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Figure 28.7 (a) Diversity among Protist Cell
Surfaces
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Some Recurrent Body Forms

• Initially, the three groups of slime molds were
  seen as so similar they were placed in a single
  phylum.
• In actuality, they are so different that some
  biologists now classify them in separate
  kingdoms.
• Slime molds share only general characteristics
• All are motile.
• All ingest particulate food by endocytosis.
• All form spores on erect fruiting bodies.

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Some Recurrent Body Forms

• Acellular slime molds form a multinucleate mass
  with diploid nuclei (a coeonocyte) during the
  vegetative phase.
• This mass moves over its substrate in a network
  of strands called a plasmodium.
• Changes in the fluidity of the outer cytoplasmic
  regions within acellular slime molds allow them
  to move by cytoplasmic streaming.

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Figure 28.3 (a) Acellular Slime Molds