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Explore the fascinating world of protists, diverse unicellular eukaryotes that predate plants, animals, and fungi by over a billion years. Discover the diverse structure, life cycles, and nutritional capabilities of protists, as well as their involvement in symbiosis and their role in the evolution of eukaryotic organization. Delve into the systematics and phylogeny of protists, and learn about the emerging monophyletic taxa and alternative proposals for classifying these organisms. Dive into the candidate kingdoms, including Euglenozoa, Alveolata, Stramenopila, and Rhodophyta, and uncover the unique characteristics and adaptations of these diverse protist groups. This chapter also explores the structural and biochemical adaptations of seaweeds and the life cycles of algae with alternating multicellular haploid and diploid generations.
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CHAPTER 28 THE ORIGINS OF EUKARYOTIC DIVERSITY
I. INTRODUCTION TO THE PROTISTS • DIVERSE UNICELLULAR EUKARYOTES (PROTISTS) PREDATE PLANTS, ANIMALS, AND FUNGI BY OVER A BILLION YEARS.
A. PROTISTS ARE THE MOST DIVERSE OF ALL EUKARYOTES • ALL PROTISTS ARE EUKARYOTES, AND THE DIVERSITY OF PROTISTS REPRESENTS DIFFERENT "EXPERIMENTS" IN THE EVOLUTION OF EUKARYOTIC ORGANIZATION. • MOST PROTISTS ARE UNICELLULAR, BUT COLONIAL AND SIMPLE MULTICELLULAR FORMS ALSO EXIST. • PROTISTS ARE FOUND WHEREVER THERE IS WATER, LIVING AS PLANKTON, SUBMERGED BOTTOM-DWELLERS, OR INHABITANTS OF MOIST SOIL OR THE BODY FLUIDS OF OTHER ORGANISMS. • OF ALL EUKARYOTES, PROTISTS ARE THE MOST NUTRITIONALLY DIVERSE; PHOTOAUTOTROPHS, HETEROTROPHS, AND MIXOTROPHS ARE ALL REPRESENTED. • PROTISTS EXHIBIT THE MOST DIVERSE SPECTRUM OF STRUCTURE AND LIFE CYCLES OF ALL KNOWN ORGANISMS.
B. SYMBIOSIS WAS INVOLVED IN THE GENESIS OF EUKARYOTES FROM PROKARYOTES • THE ENDOMEMBRANE SYSTEM OF EUKARYOTES MAY HAVE EVOLVED FROM SPECIALIZED INFOLDINGS OF THE PLASMA MEMBRANE OF ANCESTRAL PROKARYOTES. • CHLOROPLASTS AND MITOCHONDRIA ARE DESCENDANTS OF CYANOBACTERIA AND AEROBIC, HETEROTROPHIC PROKARYOTES, RESPECTIVELY, THAT TOOK UP RESIDENCE WITHIN EVOLVING EUKARYOTE CELLS.
II. PROTIST SYSTEMATICS AND PHYLOGENY • MONOPHYLETIC TAXA ARE EMERGING FROM MODERN RESEARCH IN PROTIST SYSTEMATICS • GENERAL AGREEMENT AMONG BIOLOGISTS THAT CLASSIFYING ALL EUKARYOTES TRADITIONALLY CALLED PROTISTS IN A SINGLE KINGDOM DOES NOT REFLECT PHYLOGENY HAS PROMPTED ALTERNATIVE PROPOSALS, SUCH AS AN EIGHT-KINGDOM SYSTEM. • MONOPHYLETIC TAXA ARE BEGINNING TO EMERGE FROM CURRENT RESEARCH AND DEBATE. CANDIDATE KINGDOMS EUGLENOZOA, ALVEOLATA, STRAMENOPILA, AND RHODOPHYTA ARE MONOPHYLETIC.
B. MEMBERS OF CANDIDATE KINGDOM ARCHAEZOA LACK MITOCHONDRIA AND MAY REPRESENT EARLY EUKARYOTIC LINEAGES • GIARDIA AND OTHER ARCHAEZOANS, WHICH LACK MITOCHONDRIA, SEEM TO REPRESENT THE MOST ANCIENT EUKARYOTIC LINEAGES. • ORGANISMS IN THESE LINEAGES MAY NEVER HAVE HAD MITOCHONDRIA OR MAY HAVE LOST THESE ORGANELLES IN THE COURSE OF EVOLUTION.
C. CANDIDATE KINGDOM EUGLENOZOA INCLUDES BOTH AUTOTROPHS AND HETEROTROPHIC FLAGELLATES • EUGLENOZOANS INCLUDE THE EUGLENOIDS (E.G., EUGLENA), CHARACTERIZED BY AN ANTERIOR POCKET HOUSING THE BASE OF FLAGELLA, AND THE KINETOPLASTIDS, PARASITES WITH A SINGLE LARGE MITOCHONDRION AND AN ASSOCIATED DNA-CONTAINING ORGANELLE.
D. SURFACE CAVITIES (ALVEOLI) ARE DIAGNOSTIC OF CANDIDATE KINGDOM ALVEOLATA • ALVEOLATES INCLUDE DINOFLAGELLATES THAT MOVE IN A SPINNING MOTION BY THE BEATING OF FLAGELLA; • APICOMPLEXANS, PARASITES WITH COMPLEX LIFE CYCLES CHARACTERIZED BY BOTH SEXUAL AND ASEXUAL STAGES THAT OFTEN REQUIRE TWO OR MORE HOST SPECIES; • AND CILIATES, WHICH USE CILIA TO MOVE AND FEED, AND ARE AMONG THE MOST COMPLEX OF CELLS.
E. A DIVERSE ASSEMBLAGE OF UNICELLULAR EUKARYOTES MOVE BY MEANS OF PSEUDOPODIA • RHIZOPODS (UNICELLULAR AMOEBAS AND THEIR RELATIVES) MOVE BY CELLULAR EXTENSIONS CALLED PSEUDOPODIA. • ACTINOPODS (HELIOZOANS AND RADIOLARIANS) HAVE SLENDER, RAYLIKE PSEUDOPODIA CALLED AXOPODIA THAT HELP THEM FLOAT AND FEED. • THE MOSTLY MARINE FORAMS HAVE POROUS SHELLS THROUGH WHICH STRANDS OF CYTOPLASM EXTEND.
F. SLIME MOLDS HAVE STRUCTURAL ADAPTATIONS AND LIFE CYCLES THAT ENHANCE THEIR ECOLOGICAL ROLE AS DECOMPOSERS • PLASMODIAL SLIME MOLDS FEED BY MEANS OF AN AMOEBOID PLASMODIUM CAPABLE OF DIFFERENTIATING INTO SEXUALLY REPRODUCING SPORANGIA WHEN MOISTURE OR FOOD IS SCARCE. • CELLULAR SLIME MOLDS ARE HAPLOID ORGANISMS WITH LIFE CYCLES THAT INCLUDE A MULTICELLULAR AMOEBOID STAGE THAT ERECTS ASEXUAL FRUITING BODIES.
G. DIATOMS, GOLDEN ALGAE, BROWN ALGAE, AND WATER MOLDS ARE MEMBERS OF THE CANDIDATE KINGDOM STRAMENOPILA • A DIVERSE GROUP OF PHOTOAUTOTROPHS AND HETEROTROPHS, THE STRAMENOPILES ARE UNITED BY MOLECULAR SYSTEMATICS, FLAGELLA BEARING HAIRLIKE PROJECTIONS, AND CHLOROPLASTS THAT PROBABLY DERIVED FROM ENDOSYMBIOTIC EUKARYOTIC CELLS. • DIATOMS (BACILLARIOPHYTES) ARE PRIMARILY UNICELLULAR ORGANISMS WITH UNIQUE GLASSLIKE WALLS OF SILICA. • GOLDEN ALGAE (CHRYSOPHYTES) ARE BIFLAGELLATED FRESHWATER AND MARINE PLANKTON. • OOMYCOTES, (WATER WHITE RUSTS, AND DOWNY MILDEWS) ARE HETEROTROPHS WITH CELL OF CELLULOSE AND BIFLAGELLATED STAGES IN THEIR LIFE CYCLES. • BROWN ALGAE (PHAEOPHYTES), WHICH ARE MULTICELLULAR AND PRIMARILY MARINE INCLUDE KELPS • MOST SPECIES SHOW SOME TYPE OF ALTERNATION OF GENERATIONS.
H. STRUCTURAL AND BIOCHEMICAL ADAPTATIONS HELP SEAWEEDS SURVIVE AND REPRODUCE AT THE OCEAN’S MARGINS • SEAWEEDS INCLUDE THALLUS-FORMING MARINE SPECIES A THE BROWN, RED, AND GREEN ALGAE. • THEY ARE WELL ADAPTED TO LIFE ALONG THE TURBULENT MARGINS OF THE OCEANS.
I. SOME ALGAE HAVE LIFE CYCLES WITH ALTERNATING MULTICELLULAR HAPLOID AND DIPLOID GENERATIONS
J. RED ALGAE (CANDIDATE KINGDOM RHODOPHYTA) LACK FLAGELLA • MOSTLY MULTICELLULAR AND MOSTLY MARINE, RED ALGAE POSSESS THE RED ACCESSORY PIGMENT PHYCOEI
K. GREEN ALGAE AND PLANTS PROBABLY HAD A COMMON PHOTOAUTOTROPHIC ANCESTOR • COMMON ANCESTOR OF GREEN ALGAE PROBABLY AROSE FROM AN ENDOSYMBIOTIC ASSOCIATION BETWEEN A FLAGELLATED, HETEROTROPHIC EUKARYOTE AND A CYANOBACTERIUM.
