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 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 b 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 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 Cell Surfaces in the Microbial Eukaryotes

34 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

35 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

36 Zygotic life cycles  Most unicellular sexually reproducing protists  Haploid cells transform into gametes  + and – mating strains  Thick-walled diploid zygotes  Survive like cysts

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38 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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41 Gametic life cycle  All cells except the gametes are diploid  Gametes produced by meiosis  Diatoms  Asexual reproduction reduces the size of the daughter cells  Sexual reproduction restores maximal size

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43 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

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