The Eukaryotes I. The Fungi Mycology = the study of fungi: Myco is Greek for fungi, ology = the study of A. What are fungi? Primarily terrestrial organisms 100,000 species have been described (Estimates suggest that there are up to 1.5 x 10 6 species) Fungi can represent up to 50% of the biomass in agricultural soils and 90% of the biomass in forest soils Diverse group of organisms Essential to life on earth Fungi are the Primary Decomposers!! Without fungi there would be an excess of biomass Nearly all plants depend on symbiotic fungi mycorrhizae. These fungi aid the plants in absorbing water and nutrients from soil Major cause of disease plants are particularly vulnerable Important for a number of industrial involving fermentation bread, wine, beer but also cheeses, soy sauce. Also used in organic acid production (citric acid) and antibiotics (penicillin). Saccharomyces cerevisiae is the best understood eukaryote. B. Characteristics of fungi 1. They are eukaryotes and contain: a nucleus containing chromosomes membrane bound organelles (mitochondria, ER, golgi, vacuoles, ) 2. Fungi are nonvascular 3. Fungi reproduce by spores Spores are important for several reasons, including surviving environmental stress (desiccation, nutrient limitation) and dissemination (i.e., mass multiplication or r strategy). Spores are typically not motile Both sexual and asexual spores are produced, depends on the fungus and conditions There are several types of sexual spores that tend to be specific to fungal subdivisions. The sexual spores tend to result from the fusion of gametes that may come from the same thallus (i.e., self fertilizing or homothallic species) or from different but sexually compatible mycelia (i.e., heterothallic species) There are several types of asexual spores o Sporangiospores spores that form within a sac (sporangium) at the hyphal tip o Conidiospores spores are not produced in a sac but rather at the tip or on the sides of the hyphae o Blastospores a vegetative mother cell produces these spores by budding o Arthrospores hypha fragments to form cells that behave like spores o Chlamydospores cells surrounded by a thick wall before separation 1
4. Fungal vegetative cells may be unicellular (yeasts) or comprised of a thread-like structure called a thallus the vegetative structure from which they get nutrients 5. Fungi have well-developed cell walls comprised mainly of chitin (N-acetylglucosamine) 6. All fungi are chemoorganotrophs (heterotophic: must feed on preexisting organic matter) 7. Fungi utilize extracellular digestion ( digest then ingest ) 8. Most fungi store their food as glycogen (similar to animals), plants use starch and oils as compounds 9. Fungal membranes possess a unique sterol called ergosterol (replaces cholesterol found in animal membranes) 10. Fungi have a unique lysine biosynthesis pathway 11. Fungal tubulin proteins are unique 12. Most fungi have relatively small genomes with minimal repetitive DNA Mitosis is generally accomplished without dissolution of the nuclear envelope C. The Fungal Thallus Vegetative structure plural - thalli composed of hypha (pl. hyphae): releases enzymes that breakdown food around it, the fungus then absorbs the resulting products of hydrolysis o Thread-like structure o Evolution of terrestrial existence (allowed fungi to grow on land) Mycelium = multiple hyphae. This is a collective term. The extensive root-like system found underground for many fungi and accounts for the bulk of the fungal biomass. Hypha may have cross walls or septa (sing. = septum). In some species the hyphae are aseptate (ceonocytic) Yeast Cell = single celled thallus (an adaptation to an aquatic environment) o Yeasts reproduce asexually via budding. This is a good characteristic for distinguishing between bacteria and yeasts o Some fungi are dimorphic and can change from a yeast form (Y) to a mycelial form (M) in response to environmental factors such as (nutrients, CO 2 tension, temperature, ). This shift is called the YM shift. 2
D. How do fungi grow? Fungal growth occurs always at the tips of hypha. The spore stays stationary and hyphae grow out from it. E. Classification of fungi: Originally fungi were grouped based on phenetic characteristics that did not represent their natural grouping Molecular genetics have revolutionized how we look at the fungi The Fungi includes eight subdivisions: o Chytridiomycetes o Zygomycota o Ascomycota o Basidiomycota o Urediniomycetes o Ustilaginomycetes o Glomeromycota o Microsporidia The first four subdivisions have distinct sexual spores and associated structures that can be used to identify the organism. They also often produce asexual spores that are responsible for the success of the organism. 1. Chytridiomycota Chytrids The chytrids produce motile spores = zoospores Zoospores are typically uniflagellate Flagella are whiplash type and are attached posteriorly Zoospores are typically formed in specialized structures = zoosporangia (zoospore sacs) Zoosporangium holds thousands of the actual zoospores. Cell walls composed of cellulose rather than chitin Decomposers in aquatic ecosystems Pathogens in aquatic ecosystems (e.g., Frog decline caused by chytridiomycosis disease caused by chytrids ) Mutualistic symbiotic relationships with animals o Rumens (such as cattle) o Intestines of hind-gut fermenting animals such as horses e.g., Allomyces 3
2. Zygomycota Zygosporic fungi Produce sexual spores called zygospores in a zygosporangium zygos = yoke and spora = seed Zygospores are typically thick walled and pigmented = a survival structure resistant to UV light and microbial factors This group of fungi also typically produces non-motile asexual spores called sporangiospores Asexual spores are often formed in specialized structures called a sporangium (spore sac; sporangia = pl.) Thousand of spores are produced contribute to the success of fungi. The sporangia are produced on tall stalks in order to introduce the spores to more turbulent wind patterns above the boundary layer. This ensures better wind dispersion. Coenocytic hyphae Decomposers in the soil - decaying plant and animal matter in the soil A few are parasites of plants, insects, other animals and humans e.g., Rhizopus, Mucor 3. Ascomycota Sac Fungi A very large group of fungi that produce sexual spores = ascospores These are typically formed in sac-like structures termed asci (singular = ascus) Ascospores are typically forcibly discharged (via pressure, such as being stepped on) Asci are borne naked or in specialized fruiting structures called ascomata (or ascocarp) Ascomata are often formed in stroma hyphae produced by a fungus organize into an interwoven tissue Many Ascomycetes fungi are yeast Asexual reproduction Many members of the ascomycota also reproduce asexual spores are termed conidiospores or conidia This is an ecological adaptation that allow fungi to access and exploit new niches Fungi as a rule are good examples of r-selection reproductive strategies: o Small size o Many offspring o Reduced energy o Early maturity o Most individuals die within a short period of time There is tremendous diversity in asexual spores and the way they are formed This is such a successful strategy, that some fungi have even lost the ability to reproduce asexually 4
Important decomposers in freshwater, marine and terrestrial habitats because they degrade many recalcitrant materials, including cellulose, lignin and collagen Many are parasites on higher plants - cause important plant diseases such as powdery mildew and Dutch elm disease Many are edible morels and truffles e.g., Neurospora, Aspergillus, Saccharomyces cerevisiae 4. Basidiomycota Basidiosporic fungi Commonly known as the club fungi, contains the mushrooms, boletes, bracket fungi, rusts and smuts Have gills (underside of cap) that maximize surface area Produce basidiospores on specialized structures called basidium. May be held in fruiting bodies known as basidiocarps Important terrestrial decomposers that decompose plant material (cellulose and lignin) Many mushrooms are cultivated for food Agaricus Some mushrooms produce specific secondary metabolites alkaloids that are either poisonous (Amanita phalloides death angel) or hallucinogenic e.g., Agaricus, Amanita 5. Urediniomycetes and Ustilaginomycetes Often considered as Basidiomycota but they do not produce large basidiocarps but rather produce small basidia that arise at the tip of the hyphae at the surface of the host plant Important plant pathogens causing rust and smut Some Urediniomycetes cause disease in humans e.g., Ustilago 6. Glomeromycota Often considered as zygomycetes by some Only asexual reproduction is known to occur in this group spores are produced and germinate when in contact with roots of a suitable host plant Important plant symbionts most are endomycorrhizal symbionts of vascular plants. The fungus helps protect the host plant from stress and delivers soil nutrients to the plant which in turn provides carbohydrate to the fungus 5
7. Microsporidia Used to considered as protists; however, molecular analysis of rrna and α and β-tubulin show that this group is most closely related to fungi Unlike other fungi, this group lacks mitochondria, peroxisomes and centrioles Small spores are produced and these are viable outside the host Obligate intracellular parasites that infect insects, fish, and humans e.g., Encephalitozoon Fungal Classification The Eukaryote Kingdoms: Plantae Animalia Protista Fungi Domain = Eukarya Kingdom = Fungi Phylum = -mycota Class = -mycetes Order = -ales Family = -aceae Genus = ------- Species = ------- II. The Algae Relatively simple eukaryotic photoautotrophs Mostly found in aquatic environments but some are found in most soils or on trees is sufficient moisture is available Identification of unicellular or filamentous algae requires microscopy It is estimated that 80% of the earth s oxygen is produced by planktonic algae A. Structure Unicellular or multicellular Body of a multicellular alga is called a thallus. The thalli of seaweeds consist of branched holdfasts (anchor the alga to the substrate i.e., a rock), stem-like stipes and leaf-like blades. The cells covering the thallus can carry out photosynthesis. No conductive or vascular tissues are found in algae and they absorb nutrients from the water over their entire surfaces. 6
B. Life cycle All algae can reproduce asexually (i.e., fragments of a seaweed or filamentous algae can form a new thallus or filament, respectively). Unicellular algae divide by mitosis followed by cytokinesis to produce two new daughter cells Sexual reproduction occurs in algae. This process may alternate with asexual reproduction C. Nutrition Most algae are photoautotrophs but one group of fungal like algae (Oomycota) are chemoheterotrophs D. Algal Taxonomy Algae are classified according to their rrna gene sequences, structures, pigments and cellular arrangements (multicellular vs unicellular) Refer to Table 12.4 for some of the characteristics of select groups of algae. Review the description of members of these groups in textbook and prepare your own notes. III. The Protozoa Unicellular, chemoheterotrophic eukaryotes found in kingdom protista Represent a mixed bag of organisms that some claim contains more diversity than all other eukaryote groups combined The feeding and growing stage is known as the trophozoite. This stage tends to feed on bacteria and small particulate nutrients A. Life cycle Protozoa reproduce asexually by fission, budding or schizogony. Schizogony is multiple fission: the nucleus undergoes several rounds of divisions before the cell divides. After many nuclei are formed, a portion of cytoplasm concentrates around each nucleus prior to the cell separating into daughter cells Sexual reproduction has been observed in some protozoa Some protozoa will produce protective cysts during adverse conditions permitting the organisms to survive when i) nutrients, water or oxygen are lacking, ii) temperatures are unsuitable, or iii) toxic chemicals are present B. Nutrition Protozoa are mostly aerobic heterotrophs. Some intestinal protozoa are capable of anaerobic growth. There are two groups of protozoa contain chlorophyll: dinoflagellates and euglenoids. All protozoa live in areas with a large supply of water. Some absorb nutrients across the plasma membrane. Others have a strong protective covering under the plasma membrane known as the pellicle and require specialized structure to take in nutrients. Similarly others engulf food by phagocytosis (e.g., amoeba) or have a mouthlike opening called a cytostome that is used to collect food particles. Digestion occurs in membrane enclosed vacuoles in all protozoa. 7
C. Protozoal Taxonomy This is a large and diverse group of organisms. Classification into a number of phyla is based upon rrna gene sequencing as well as morphological characteristics Microspora are no longer considered protozoans. Molecular methods have determined these organisms to be fungi 1. Archaezoa Lack mitochondria but have a unique organelle (i.e., mitosome) that appears to be a remnant of the mitochondria. Many live a symbionts in the digestive tracts of animals Typically spindle shaped with flagella projecting from one end e.g., Trichomonas vaginalis Giardia lamblia forms cysts in feces that persist in the environment 2. Amoebozoa Move by pseudopodia Lack cell walls e.g., Entamoeba histolytica causes amoebic dysentry 3. Apicomplexa Nonmotile in mature forms Obligate intracellular parasites Characterized by the presence of a complex of special organs at the cell apex Usually have complex life cycles involving transmission between several hosts e.g., Plasmodium causes malaria 4. Ciliophora Cells are covered with cilia that are arranged in precise rows. The cilia are used for locomotion as well as to propel food into the cytostome 5. Euglenozoa Two groups of flagellated cells are included in this group o Euglenoids photoautotrophs with a flagellum at the anterior end. The membrane is a semi-rigid pellicle e.g., Euglena o Hemoflagellates blood parasites transmitted by blood feeding insects e.g., Trypanosoma brucei causes African sleeping sickness 8