Origin of Eukaryotes

Prokaryotes

• No true nucleus• No plastids

• Internal membrane systems are folds of plasma membrane

• True nucleus• Specialized plastids

• Internal membrane systems independent of plasma membrane

Eukaryotes

Trends in Increased Prokaryote Complexity

• Multicellular prokaryotes with specialized cells

• Complex bacterial communities

• Compartmentalization of different functions within single cells

Heterocyst of Anabaena

Trends in Increased Prokaryote Complexity

• Multicellular prokaryotes with specialized cells

• Complex bacterial communities

• Compartmentalization of different functions within single cells

• These trends are important because eukaryotes had to evolve from prokaryotes

Origins of Eukaryotes

• Earliest evidence - 1.5 billion years

• Acritarchs resemble cysts produced by living autotrophic protists

• Development of oxygen atmosphere

Electron micrograph of an acritarch.

Models Proposed for the Evolution of Eukaryotes

Autogenous Model - eukaryotic cells evolved from specialization of internal membranes

derived from plasma membrane of prokaryotes

Autogenous Model

• Single-membranes organelles formed by folding of inner membrane only

• Double-walled organelles by complete invagination

Autogenous Model

Endosymbiotic Model

Predecessors of eukaryotes where symbionts, with small specialized species (endosymbionts)

living within larger prokaryotes

Fig. 22.12b

Endosymbiotic Model

Model uses chloroplasts and mitochondria as examples

• Chloroplasts were photosynthesizing prokaryotes

• Mitochondria evolved from aerobic heterotrophs (emphasis on role of Krebs cycle)

Chloroplast

Mitochondrion

The Model was Controversial

The endosymbiotic model differs from evolution as we discussed earlier

It is a merger of evolutionary lineages giving rise to a new form of life

But,

Supporting evidence has strengthened validity

e.g., mitochondrial and chloroplast DNA

Also, symbiosis is a common phenomenon in nature

Kingdom Protista

• First eukaryotic organisms

• Typically thought of as the unicellular eukaryotes

• Some colonial and multicellular species

• Addition of multicellular forms justified by similarities in cell structure and life cycles

Unicellular, but Complex

• Genesis of protists reveals rise of – true nucleus– specialized organelles– 9 + 2 flagella and cilia– mitosis– meiosis

• Share common ancestry with multicellular eukaryotes

Protistan Systematics

• As would be expected, it is difficult to develop phylogenetic relationships among the protistans– Poor fossils (except those with external covering– Some features (e.g., flagella and autotrophy)

arose and have been lost more than once over their evolution

• Phyla are placed into supergroups (Table 28.1)

Protistan Systematics

• Evolutionary Relationships– Cellular structure– Gene sequences

• Evolutionary relationships constantly changing

• Systematics

• Relationships into supergroups

Informal Classification – Ecological Roles

Protozoa – animal-like heterotrophic Protista

Ciliate consuming diatoms

Algae – autotrophic protists

Informal Classification – Ecological Roles

Fungus-like protists

Informal Classification – Motility

Ciliates

Flagellates

Informal Classification – Motility

Amoeboid

Life Processes??

Although unicellular, protistans can carry out all life processes

Osmoregulation – Water Balance

Vacuoles increase effective surface area in large cells.

Contractile vacuoles in freshwater microbial eukaryotes such as Paramecium are used to excrete excess water.

Figure 27.10 Contractile Vacuoles Bail Out Excess Water

Nutrition

• Phagotrophy

• Osmotrophy

• Autotrophy

• Mixotrophy

Defense

• Mucilage

• Trichocysts

• Bioluminescence

• Toxins

27.3 How Did the Microbial Eukaryotes Diversify?

Food vacuoles are formed by protists when solid food particles are ingested by endocytosis.

The food is digested in the vacuole. Smaller vesicles pinch off—increasing surface area for products of digestion to be absorbed by the rest of the cell.

Cell surfaces

Many microbial eukaryotes have diverse means of strengthening their surfaces.

Cytoskeleton

Internal structures that provide support and rigidity

27.3 How Did the Microbial Eukaryotes Diversify?

Some amoebas make a “shell” or test from bits of sand beneath the plasma membrane.

Diatoms form glassy cell walls of silica. These walls are exceptionally strong, and perhaps enhanced defense against predators. Frustule

Figure 27.12 Cell Surfaces in the Microbial Eukaryotes

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Asexual Reproduction

• All protists can reproduce asexually

• Many produce cysts with thick, protective walls that remain dormant in bad conditions

• Many protozoan pathogens spread from one host to another via cysts

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Sexual Reproduction

• Eukaryotic sexual reproduction with gametes and zygotes arose among the protists

• Generally adaptive because it produces diverse genotypes

• Zygotic and sporic life cycles

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• Zygotic life cyclesMost unicellular sexually

reproducing protistsHaploid cells transform into

gametes+ and – mating strainsThick-walled diploid zygotes

Survive like cysts

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• Sporic life cycle Many multicellular green and brown

seaweeds Also known as alternation of generations 2 types of multicellular organisms

Haploid gametophyte produces gametes

Diploid sporophyte produces spores by meiosis

Red seaweed variation involves 3 distinct multicellular generations

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• Gametic life cycleAll cells except the gametes are

diploidGametes produced by meiosisDiatoms

Asexual reproduction reduces the size of the daughter cells

Sexual reproduction restores maximal size

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• Ciliate sexual reproduction – Conjugation Most complex sexual process in protists Have 2 types of nuclei (single

macronucleus and one or more micronuclei)

Macronuclei are the source of the information for cell function

2 cells pair and fuse – conjugation Micronuclei undergo meiosis,

exchange, fusion and mitosis

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Phylum Chlorophyta

• Green algae, diverse group

• unicellular, aggregates, colonial, multicellular

• Believed to be the group that gave rise to plants– multicellular and colonial forms– alternation of generation

Origins of Multicellularity

• Probably arose from colonial protistan– something resembling Volvox– (Volvox is only an example!!!!!!)

• Coordination and cooperation between cells

• Specialized reproductive cells– Volvox - locomotion and reproduction

• Ancestor probably flagellated

Alternation of Generation

• Life cycles that show an alternation between a multicellular haploid form and a multicellular diploid form

• Sporophyte - Diploid; produces reproductive cells (haploid) called spores

• Gametophyte - Haploid; produces haploid gametes. Fusion of gametes produces diploid form

Fig. 28.20

Thus, the presence of alternation of generation and other similarities suggests a linkage between the Chlorophyta and the

Plant Kingdom