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Biology 1215 Outline

Biology 1215 Lecture Notes

Chapter 28: The origins of eukaryotic diversity
Outline

  • Eukaryotes originated by symbiosis among prokaryotes
  • Evidence for the endosymbiotic theory of mitochondria and chloroplasts
  • Origin of eukaryotic cell concomitant with origins of:
  • Archezoans provide clues to the early evolution of eukaryotes
  • The diversity of protists represents different "experiments" in the evolution of eukaryotic organization
  • Almost all protists are aerobic, using mitochondria for cellular respiration.
  • Most protists have flagella or cilia at some time in their life cycle.
  • Cell division and nuclear structure are very variable in the protists.
  • Protists are considered the simplest eukaryotic organisms because most are unicellular.
  • Diverse modes of locomotion and feeding evolved among protozoa
    • Phylum Rhizopoda (Amoebas)
    • Phylum Actinopoda
    • Phylum Foraminifera
    • Phylum Apicomplexa (Sporozoans)
    • Phylum Zoomastigophora (Zooflagellates)
    • Phylum Ciliophora (Ciliates)
  • Fungus like protists have morphological adaptations and life cycles that enhance their ecological role as decomposers
    • Myxomycota (Plasmodial slime molds)
    • Acrasiomycota (Cellular slime molds)
    • Oomycota
  • Eukaryotic algae are key producers in most aquatic ecosystems
    • Phylum Dinoflagellata (Dinoflagellates)
    • Phylum Bacillariophyta (Diatoms)
    • Phylum Chrysophyta (Golden Algae)
    • Phylum Paeophyta (Brown Algae)
    • Evolutionary adaptations of seaweed
    • Alternation of generations in the life cycles of some algae.
    • Phylum Rhodophyta (Red algae)
    • Phylum Chlorophyta (Green algae)
  • Multicellularity originated independently many times

 

  • Eukaryotes are cells containing a nucleus. Include animals, plants, fungi, and protists.
  • Protists are the most diverse and ancient of the eukaryotes. Most of the 60,000 species of extant protists are unicellular, although some (algae) can be quite complex multicellular organisms.
  • Some of the most important episodes in the evolution of the eukaryotic cell unfolded during the evolution of protists.

Eukaryotes originated by symbiosis among prokaryotes

  • During the genesis of protists, a number of cellular structures and processes unique to eukaryotes arose:
    • A membrane-enclosed nucleus
    • Mitochondria
    • Chloroplast
    • The endomembrane system
    • The cytoskeleton
    • 9+2 flagella
    • Multiple chromosomes consisting of linear DNA compactly arranged with proteins
    • Diploid stages in life cycles
    • Mitosis, Meiosis, and Sex
  • The evolution of the compartmentalized nature of the eukaryotic cells may have resulted from two processes (Fig 28.2):
    • Specialization of plasma membrane invaginations
      • Invaginations and subsequent specializations may have given rise to the nuclear envelope, endoplasmic reticulum, Golgi apparattus, and other components of the endomembrane system.
    • Endosymbiotic associations of prokaryotes resulted in appearance of some organelles:
      • Mitochondria and chroloplasts evolved from prokaryotes living within other cells.
  • The endosymbiotic theory proposes that certain prokaryotes, called endosymbionts, lived within larger prokaryotes.
    • Chloroplasts are descended from endosymbiotic photosynthesizing prokaryotes, such as cyanobacteria, living within larger cells.
    • Mitochondria are postulated to be descended from prokaryotic aerobic heterotrophs.
      • May have been parasites or undigested prey of larger prokaryotes.
      • The association progressed from parasitism or predation to mutualism.
      • As the host and endosynbiont became more interdependent, they integrated into a single organism.
  • Many extant organisms are involved in endosymbiotic relationships.

Evidence for the endosymbiotic theory of mitochondria and chloroplasts

  • Appropriate size.
  • Inner membranes containing many enzymes and transport systems similar to those on prokaryotic plasma membranes.
  • Replicate by binary fission.
  • Have their own circular genomes (not associated histones )
  • Have their own ribosomes, tRNAs and other components of transcription and translation.
  • Have ribosomes which are more similar to those of prokaryotes (antibiotic sensitivity).
  • Phylogenetic evidence (rRNA).

Origin of eukaryotic cell concomitant with origins of:

  • 9+2 flagella and cilia (analogous to prokaryotic flagella)
  • Mitosis and meiosis, which also utilize microtubules
    • Mitosis made it possible for large eukaryotic genomes to be reproduced.
    • Meiosis is essential to sexual reproduction (genetic variation).
    • Protist have the most varied sexual life histories of the eukaryotes.

Archezoans provide clues to the early evolution of eukaryotes

  • The earliest divergence (2 billion yrs?) within the eukaryotic lineage resulted in the archezoa.
  • Lack mitochondria, plastids, endoplasmic reticulum and Golgi apparatus and have relatively simple cytoskeletons.
  • Ribosomes have some characteristics more closely aligned with prokaryotes than eukaryotes.
    Giardia intestinalis is a modern representative of the archezoa.
    • Flagelated unicellular anaerobic obligate parasite of human intestine.
    • Most commonly transmitted in cyst form through water contaminated with human feces.
  • Giardia is important to evolutionary biology.
    • Have two separate haploid nuclei, which may be a vestige of early eukaryotic evolution.
    • Prokaryotes are haploid. Some researchers believe that early eukaryotes had a simple haploid nucleus bounded by a nuclear membrane.
    • Diplomonads diverged from eukaryotic lineage before the process of nuclear fusion and meiosis evolved. Thus, their dual nuclei may be a clue to the past.

The diversity of protists represents different "experiments" in the evolution of eukaryotic organization

  • Oldest protist fossils (acritarchs) date to about 2.1 billion yrs.
    • Acritarchs are of proper size and structure to be ruptured coats of cysts similar to those of extant protists.
    • Adaptive radiation produced a diversity of protists over the next billion yrs.
    • The variations present in these organisms were representative of the structure and function possible in eukaryotic cells. Today we recognize over 60 different protist lineages.
  • Protists are found in almost all moist environments, the seas, freshwater systems, and moist terrestrial habitats.
  • Important components of marine and freshwater plankton.
    Plankton = communities of organisms, mostly microscopic, that drift passively or swim weakly near the surface of oceans, ponds, and lakes.
  • Many are bottom dwellers in freshwater and marine habitats where they attach to rocks or live in the sand and silt.
  • Photosynthetic species form mats at the still-water edges of lakes, ponds where they provide a major food source for other protists.
  • Large numbers of protists live in moist terrestrial habitats: damp soil, leaf litter
  • Symbiotic species are found in the body fluids, tissues and cells of many types of hosts.

Almost all protists are aerobic, using mitochondria for cellular respiration.

  • Anaerobic forms lack mitochondria and live in anaerobic environments or have mutualistic respiring bacteria.
  • Protists may be photoautotrophic, heterotrophic, or mixotrophic (combining photosysnthesis with heterotrophy).
  • The different modes of nutrition are used to separate protists into 3 convenient (but not phylogenetic) categories:
    • 1. Algae (photosynthetic) (Plant-like)
    • 2. Protozoa (ingestive)(Animal-like)
    • 3. Absorptive protists. (Fungal-like)

Most protists have flagella or cilia at some time in their life cycle.

  • Not homologous to prokaryotic flagella.
  • Eukaryotic flagella and cilia are extensions of the cytoplasm (surrounded by plasma membrane)
  • These cilia and flagella have 9+2 microtubular microstructure.
    • Cilia and flagella differ in that cilia are shorter and more numerous.

Cell division and nuclear structure are very variable in the protists.

  • Unique mitotic divisions occur in many groups.
  • All can reproduce asexually; some can also reproduce sexually or at least use syngamy (cell fusion) to trade genes between asexual reproductive episodes.
  • Some form resistant cysts when stressed by harsh environments.

Protists are considered the simplest eukaryotic organisms because most are unicellular

  • At the cellular level, protists are extremely complex.
  • The unicellular protist is not analogous to a single plant or animal cell, but is a complete organism.
  • The single cell of a protist must perform all the basic functions carried out by the specialized cells of plants and animals.
  • Protistan taxonomy is in a state of flux
  • Classification schemes and phylogeny they represent are only tentative and change with new information.
  • Whittaker’s popular 5-kingdom taxonomic system placed only unicellular eukaryotes in kingdom protista.
    • Sea-weeds have since been included as protists. More closely related to algae than plants.
    • Slime molds and water molds (once thought to be fungi) also placed in protista kingdom.
  • Molecular systematics, especially those based on rRNA, is revamping taxonomy of protists into a more "natural" classification. There are currently 60 lineages protists. Many of these represent diffferent kingdoms in the domain eukaryoa.

Diverse modes of locomotion and feeding evolved among protozoa

  • Protozoa is an informal reference to diverse group of heterotrophic protists.
    • Seek and consume bacteria and other protists and detritus (dead organic matter).
    • Some are symbiotic and many are pathogens.
    • Classification of protozoa into different phyla based in part on how they feed and move (classification ongoing process).

Phylum Rhizopoda (Amoebas)

  • Simplest of protists; all are unicellular
  • No flagellated stages in life cycle
  • Pseudopodia form as cellular extensions and function in feeding and movement (cytoskeleton involved in this).
  • All reproduction is asexual; no meiosis
    • Nuclear envelope persists during cell division in many genera
  • Inhabitants of fresh water, marine and soil habitats.
  • Most are free-living, although some are parasitic.

Phylum Actinopoda
Read textbook, but not on exam.
Phylum Foraminifera
Read textbook, but not on exam.

Phylum Apicomplexa (Sporozoans)

  • All species of the phylum Apicomplexa are parasites of animals
    • The infectious cells produced in the life cycles are called sporozoites
    • The apex of sporozoites has organelles for penetrating host cell and tissues (Phylum named after this trait).
    • Life cycles are intricate having both sexual and asexual reproduction; often requiring two or more host species.
  • Several species of Plasmodium cause malaria (Fig 28.7)
    • Anopheles mosquitoes serve as intermediate host and human as the final host.
    • Incidence of malaria greatly reduced by insecticide against mosquito (1960’s). Resistant strains have appeared (both mosquito and plasmodium).
    • Malaria still a major killer.
  • Little success in developing vaccine against Plasmodium.
    • The human immune system has little effect on the parasite.
      • Plasmodium spends most of its life cycle in blood cells and liver cells.
      • Plasmodium alternates surface proteins rapidly.
    • Vaccine strategy: mix of synthetic proteins which mimic parasite membrane proteins.

Phylum Zoomastigophora (Zooflagellates)

  • All are heterotrophs that absorb organic molecules or phagocytise prey.
    • Use whip-like flagella to move.
    • Most are solitary, but some colonial.
    • Many are freeliving and some are synbiotic.
      • Zooflagellate synbionts in gut of termites digest cellulose in wood eaten by host.
  • Some Trypanosomes cause African sleeping sickness, and are spread by tsetse fly.
    • Large reservoir for the disease since many tropical African mammals harbor the parasite.
    • Trypanosomes can evade host immune response by rapidly changing genes encoding surface coat proteins.
  • Zooflagellates are closely related to flagellated protists which include photosynthetic Euglenoids.

Phylum Ciliophora (Ciliates)

  • Ciliates use celia to move and feed.
    • Most ciliates exist as solitary cells in fresh water.
    • Cilia are short and beat synchronously.
    • Cilia associated with a submembranous system that coordinates the movement of thousands of cilia.
    • Cilia can be dispersed or clustered in rows or tufts. Some move on leg-like cirri. (many cilia bonded together).
    • Other species have rows of tightly packed cilia that function together as locomotor membranelles (ie Stentor).
  • Ciliates possess two types of nuclei: one large macronucleus and from one to several small micronuclei. (Fig 28.9)
    • Characteristics of macronucleus:
      • Large, over 50 per genome.
      • Genes are packaged in large number of small units, each with hundreds of copies of just a few genes.
      • Controls everyday functions of cell by synthesizing RNA.
      • Macronucleus function in asexual reproduction during binary fission. The macronucleus elongates and splits instead of undergoing mitosis.
    • Characteristics of micronucleus
      • Small and may number between 1 to 80 micronuclei, depending on species.
      • Does not function in growth, maintenance or asexual reproduction.
      • Functions in conjugation, a sexual process which produces genetic variation.
    • Note that meiosis and syngamy are separate from reproduction

     

Fungus like protists have morphological adaptations and life cycles that enhance their ecological role as decomposers

  • The resemblance of slime molds and water molds to true fungi is a result of convergent evolution of filamentous body structure.
    • Filamentous body structure increases exposure to the environment and enhance their roles as decomposers.
    • Slime molds differ from true fungi in their cellular organization, reproduction, and life cycles.
      • Slime molds more closely related to amoeboid protists than to true fungi.
      • Molecular phylogenetic data indicate that water molds more closely ralated to certain algae, although they lack chloroplasts.
  • Fungal-like protists fall into three phylum:
    • Myxomycota (Plasmodial slime molds)
    • Acrasiomycota (Cellular slime molds)
    • Oomyota

Myxomycota (Plasmodial slime molds)

  • This phylum consists of the plasmodial slime molds which are all heterotrophs.
    Plasmodium: Feeding stage of life cycle consisting of an amoeboid, coenocytic mass (multinucleated cytoplasm undivided by membrane).
    • Life cycle of plasmodium (Fig 28.13)
    • In most species, the nuclei of the plasmodia are diploid and exibit synchronous mitotic divisions.
    • Cytoplasmic streaming within the plasmodium helps distribute nutrients and oxygen.
    • Engulfs food by phygocytosis as it grows by extending pseudopodia.
    • Live in moist soil, leaf litter and rotting logs.
    • When stressed by drying or lack of food, the plasmodium ceases growth and forms sexually reproductive structures called fruiting bodies, or sporangia.

     

Acrasiomycota (Cellular slime molds)

  • Life cycle (Fig 28.14)
  • Feeding stage of life cycle consists of solitary, individual haploid cells.
  • When food supply depleted, cells aggregate to form a mass similar to those of plasmodial slime molds, but cells remain separate (not coenocytic)
  • Fruiting bodies function in asexual reproduction (unlike plasmodial slime molds).
  • Only a few have flagellated stages.

Oomycota

  • This phylum includes water molds, white rusts and downy mildews.
    • Have coenocytic hyphae (fine, branching filaments) that are analogous to those found in fungi.
    • Cell walls made of cellulose, rather than chitin.
    • Diploid condition in the life cycle prevails in most species.
    • Biflagellated cells are present in the life cycles, while fungi lack flagellated cells)
  • In Water molds:
    • A large egg is fertilized by a smaller sperm cell to form a resistant zygote (Fig 26.17)
    • Usually decomposers living on dead algae and animals in fresh water.
    • Some are parasitic, growing on injured tissue but may also grow on the skin and gills of fish
  • White rusts and downy mildews:
    • Usually parasitic on terrestrial plants.
    • Disperse by windblown spores but also flagellated zoospores at some point in their life cycle.

Eukaryotic algae are key producers in most aquatic ecosystems

  • The majority of algae are photosynthetic with only few of the phyla having heterotrophic or mixotrophic members
    Algae = relatively simple photoautotrophic aquatic organisms.
  • Algae are of great ecological significance:
    • Account for 50% of global photosynthate production.
    • Forms include fresh water plantkton, marine plankton, and intertidal seaweeds that form the basis of aquatic food webs.
  • All algae have chlorophyl a (like plants). The phylogenetic relationships among algal phyla have been determined by difference in:
    • Accessory pigments such as carotenoids, xanthophyls, phycobilins, and other forms of chlorophyl.
    • Chloroplast structure.
    • Cell wall chemistry
    • Number, type and position of flagella
    • Food storage product

     

Phylum Dinoflagellata (Dinoflagellates)

  • Dinoflagellates are components of phytoplankton which provide the foundation of most marine food chains.
  • Some cause red tides by explosive growth (bloom)
    • produce toxin which is concentrated by invertebrates, including shellfish.
    • Toxin is dangerous to humans, and causes paralytic shellfish poisoning.
  • Most unicellular, some are colonial.
  • Cell surface is reinforced by cellulose plates with flagella in perpendicular grooves creating its whirling movement and resulting in a characteristic shape.
  • Some live as photosynthetic symbionts of the cnidarians that build coral reefs.
  • Some lack chloroplasts and live as parasites; a few carniverous species known.
  • Food is stored as starch.
  • Have brownish plastids containing chlorophyll a, chlorophyll c, and a mix of carotenoids, including peridinin (found only in this phylum).
  • Chromosomes lack histones and are always condensed.
  • Has no mitotic stages.
  • Kinetochores are attached to the nuclear envelope and chromosomes distributed to daugher cells by the splitting of the nucleus.
  • Dinoflagellates are probably more closely related to zooflagellates than to any phylum of algae.

Phylum Bacillariophyta (Diatoms)

  • Mostly unicellular organisms with overlapping glasslike walls of hydrated silica in organic matrix. (glass houses)
  • Usually reproduce asexually.
  • Components of freshwater and marine plankton.
  • Store food in a form of oil, which also makes cells more buoyant.
  • Same photosynthetic pigments as chrysophyta.

Phylum Chrysophyta (Golden Algae)

  • Plastids have chlorophyl a, chlorophyl c, yellow and brown carotenoids, and xanthophyl.
  • Live among freshwater plankton; most are colonial
  • Have flagellated cells with both flagella attached near one end of the cell.
  • Store carbohydrates in the form of laminarin, a polysaccharide.
  • Survive environmental stress by forming resistant cysts.
    • microfossils resembling these cysts have been found in Precambrian rocks.

Phylum Paeophyta (Brown Algae)

  • Largest and most complex algae (kelps).
  • All are multicellular and most are in marine habitats.
  • Have chlorophyl a, chlorophyl c, and the carotenoid pigment fucoxanthin.
  • Store carbohydrate food reserves in the form of laminarin.
  • Cell walls made of cellulose and algin.

Evolutionary adaptations of seaweed

  • Seaweeds are large, multicellular marine algae which are found in the intertidal and subtidal zones of coastal waters.
    • Term "seaweed" include a diverse group of algae including brown algae, red algae, and green algae.
      The habitat of seaweeds, particularly the intertidal zone, poses several challenges to the survival of these organisms.
    • Motion of water creates physically active habitat.
    • Tides result in seaweed being alternatively covered by seawater and exposed to direct sunlight and the drying conditions of the air.
  • Seaweeds have evolved several unique structural and biochemical adaptations to survive the conditions of their habitats.
  • Structural adaptations found in seaweeds are a result of their complex multicellular anatomy. Some forms have differentiated tissues and organs analogous to those of plants.
    • Thalus = the body of a seaweed. Its plant like in appearance, but has no true roots, stems, or leaves.
    • A thalus consists of a rootlike holdfast (maintains position), a stem-like stipe (supports the baldes), and leeflike blades (large surfaces for photosynthesis).
    • Some alage produce floats, which help suspend blades near the water surface.
    • Brown algae known as kelps, occur beyond intertidal zone where conditions are less harsh and may have stipes which may reach a length of 100 meters.
  • Biochemical adaptations in some seaweeds reinforce the anatomical adaptations and enhance survival.
    • Cellulose cell walls also contain gel-forming polysaccharides (algin in brown algae, carageenan in red algae)
  • Seaweeds are used by humans:
    • Brown and red alga are used as food in many parts of orient.
    • Marine algae are used as nutrient supplements due to high content of iodine and other minerals.
    • Algin, agar, and carageenan are extracted and used as thickners for processed foods and lubricants in oil drilling.

Alternation of generations in the life cycles of some algae.

  • A variety of life cycles in brown algae, the most complex having alternation of generations.
  • Alternation of generations = alternation between multicellular haploid forms and multicellular diploid forms in a life history.
    • Alternation of generations also involved in certain groups of red and green algae.
  • The life cycle of laminaria is an example of a complex life cycle with alternation of generations (Fig 28.20)
    • The diploid individual is called a sporophyte because it produces reproductive cells called spores.
    • The haploid individual is called the gametophyte because it produces gametes.
    • The sporophyte and gametophyte generations of the life cycles take turn producing one another.
      • Spores released from sporophyte develop into gametophytes.
      • Gametophytes produce gametes which fuse (fertilization) to form a diploid zygote that develops into a sporophyte.
    • In Laminaria the sporophyte and gametophyte generations are said to be heteromorphic because they are morphologically different.
  • In Ulva, a green algae exhibiting alteration of generations, the generations are referred to as isomorphic because they look alike.

 

Phylum Rhodophyta (Red algae)

  • Found primarilty in warm, marine habitats, although some also found in fresh water and soil.
    • Contain chlorophyl a, carotenoids, phycobilins and chlorophyl d in some.
    • Red colour of plastids due to ecessory pigment, phycoerythrin (a phycobilin).
    • Carbohydrate food reserves stored as floridean starch (similar to glycogen).
    • Cell walls made of cellulose with agar and carageenan.
    • Most red algae are multicellular and are known as seaweeds.
    • Most thalli are filamentous and are often branched.
  • All red algae reproduce sexually:
    • Have no flagellated stages, unlike other algal protists.
    • Alternation of generations is common.

     

Phylum Chlorophyta (Green algae)

  • Contain plant-like chloroplasts and are believed to be most recent common ancestor of plants.
    • 7000 species known, most are fresh water; fewer are marine.
    • Many live as unicellular plankton. Inhabit damp soil, symbionts with protozoa or invertebrates.
    • Some live mutualistically with fungi and are known as lichens.
    • Colonial forms are often filamentous (pond scum).
    • Some multicellular forms also known as seaweeds.
  • Evolutionary trends that probably produced colonial and multicellular forms from flagellated unicellular ancestors:
    • Formation of colonies of individual cells (as seen in Volvox).
    • Repeated division of nuclei with no cytoplasmic division (as in Bryopsis)
    • Formation of true multicellular forms as in ulva.
  • Most chlorophytes have complex life histories involving sexual and asexual reproductive stages.
  • The life cycle of Chlamydomonas is a good example of the life history of a unicellular chlorophyte (Figure 28.24).
    • During asexual reproduction, the flagella are reabsorbed and the cell divides twice by mitosis to form four cells.
      • The daughter cells develop and emerge as swimming zoospores. Zoospore development includes formation of flagella and cell walls.
      • Zoospores grow into mature cells, thus completing asexual reproduction.
    • Sexual reproduction is stimulated by environmental stress from such things as shortage of nutrients, drying of the pond, or others..).
      • During sexual reproduction, many gametes are produced by mitotic division within the wall of the parent. The gametes escape the parent cell wall.
      • Gametes of opposite mating strains (+ and -) pair off and cling together by tips of flagella.
      • The gametes are morphologically indistinguishable and their fusion is known as isogamy.
      • The slow fusion of gametes forms a diploid zygote which secretes a resistant coat for protection.
      • When dormancy of the zygote is broken, four haploid individuals (two of each mating type) are produced by meiosis.
      • These haploid cells emerge from the coat and develop into mature cells, thus completing life cycle.
  • Many features of chlamydomonas sex are believed to have evolved early in the chlorophyte lineage. Using this basic life cycle, many refinements that evolved among the chlorophytes have been identified.
    • Some green algae produce gametes that differ from vegetative cells, and in some species, the male gamete differs in size or morphology from the female gamete (anisogamy)
    • Many species exhibit oogamy, a type of anosogamy in which a flagellated sperm fertilizes a nonmotile egg.
    • Some multicellular species also exhibit alternation of generations.
      • Ulva produces isomorphic thalli for its diploid sporophyte and haploid gametophyte.
       

Multicellularity originated independently many times

  • Early eukaryotes were more complex than prokaryotes. This increase in complexity allows for greater morphological variations to evolve.
    • Extant protists are more complex in structure and show a greater diversity of morphology than the simpler prokaryotes.
    • The ancestral stock which gave rise to the new waves of adaptive radiations were the protists with multicellular bodies.
  • Multicellularity evolved several times among the early eukaryotes and gave rise to the multicellular algae, plants, fungi, and animals.
  • Its believed that earliest multicellular forms arose from unicellular ancestors as colonies or loose aggregates of interconected cells.
  • Multicellular algae, fungi, plants and animals probably evolved from several lineages of protists that formed by amalgamations of individual cells.
  • Evolution of multicellularity from colonial aggregates involved cellular specialization and division of labor.
    • The earliest specializations may have been locomotor capabilities provided by flagella.
    • As cells became more interdependent, some lost their flagella and performed other functions.
  • Further division of labor may have separated sex cells from somatic cells (eg. Volvox).
  • Multicellular forms more complex than filamentous algae appeared approximately 700 million years ago.
  • A variety of animal fossils has been found in late precambrian strata and many new forms evolved in cambrian period (570 million yr bp).
  • Seaweeds and other complex algae were also abundant during cambrian period.
  • Primitive plants are believed to have evolved from certain green algae living in shallow waters about 400-450 MY bp.

 

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