Origin of Eukaryotes
Jan 12, 2016
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