CHAPTER 31 FUNGI Most fungi have complex multicellular bodies They are found in every imaginable terrestrial and aquatic habitat They break down organic material and recycle nutrients allowing other organism to assimilate essential chemical elements 31 1 FUNGI ARE HETEROTROPHS THAT FEED BY ABSORPTION The most important trait fungi share is the way they derive nutrition NUTRITION AND ECOLOGY They are heterotrophs like animals but do not ingest food they absorb nutrients from the environment outside of its body Many fungi secrete hydrolytic enzymes to break down complex molecules to smaller organic compounds that fungi can absorb Other fungi use enzymes to secrete walls of cells to absorb nutrients from the cells Food sources also correspond to varied ecological roles parasites decomposers mutualists BODY STRUCTURE Most common body structures are multicellular filaments and single cells yeast Many species can grow as both filaments and yeasts but even more grow as filaments Yeasts usually inhabit moist environments The morphology of multicellular fungi is important to growing and obtaining nutrition HYPHAE tubular cell walls surrounded by plasma membrane and cytoplasm of cells form a tubular network CHITIN surrounds the cell wall and aids in absorption of nutrients preventing fungal cells from bursting SEPTA cross walls that divide cells allow for organelles to flow from cell to cell COENOCYTIC FUNGI lack septa and are a continuous cytoplasmic mass with thousands of nuclei MYCELIUM fungal hyphae that form an interwoven mass increases surface volume ratio and make feeding more efficient HYPHAE length allows for fungi to obtain nutrition without having to move SPECIALIZED HYPHAE IN MYCORRHIZAL FUNGI Fungi have specialized hyphae that allow them to feed on living animals In addition HAUSTORIA allow fungi to extract nutrients from or exchange nutrients with plant hosts This type of symbiotic relationship is called MYCORRHIZAE This relationship is vital because MYCORRHIZAL fungi are more efficient in delivering phosphate ions and other minerals to plants while in return the fungi get organic nutrients The 2 main types of mycorrhizal fungi are ECTOMYCORRHIZAL FUNGI form sheaths of hyphae over surface of root and grow into extracellular spaces of root cortex ARBUSCULAR MYCORRHIZAL FUNGI branching hyphae extend through root cell wall and inwards into root cell plasma membrane IMPORTANT BECAUSE ALL VASCULAR PLANTS HAVE MYCORRHIZAE and rely on fungal partners for nutrients 31 2 FUNGI PRODUCE SPORES THROUGH SEXUAL OR ASEXUAL LIFE CYCLES Mycorrhizial fungi colonize soils by spreading haploid cells called spores that form new mycelia Spore production can be done asexually or sexually Spores can be carried for long distances via wind or water Spores are very successful and can form mycelia almost anywhere mycelia on melons after 1 week SEXUAL REPRODUCTION The nuclei of fungal hyphae and spores of are mostly haploid Sexual reproduction occurs when hyphae from 2 mycelia release PHERMONES sexual signaling molecules If mycelia from different mating types meet pheromones from each partner bind to receptors and hyphae extend towards source of Pheromones Some species have compatibility tests to make sure the hyphae are from different mycelium to increase genetic variation If the mating is successful a PLASMOGAMY union of 2 parent cytoplasms occurs However the fusing may not occur right away and instead parts of the fused mycelium coexist with genetically different nuclei HETEROKARYON In some species this does not occur the mycelium are DIKARYOTIC haploid nuclei pair off 2 to a cell one from each parent As a dikaryotic mycelium grows each cell divides in tandem without fusing which results in 2 separate haploid cells and not a diploid cell with homologus chromosomes within a singular nucleus A long time may pass between plasmogamy and Karyogamy which results in diploid cells Meiosis then restores the spores to haploids KARYGOGAMY AND MEIOSIS IS THE KEY SOURCE OF GENTEIC VARIATION IN FUNGI WHICH DRIVES NATRUAL SELECTION HETEROKARYOTIC ALSO OFFERS SOME ADVANTAGES OF DIPLOID IN ONE HAPLOID GENOME THAT MAY COMPENSATE FOR HARMFUL MUTATIONS ASEXUAL REPRODUCTIONS Many fungi reproduce asexually by growing as filamentous fungi that produce haploid spores moles if visible through mitosis Molds can grow rapidly and colonize food sources Other fungi reproduce asexually as single celled yeasts through mitosis or pinching of small bud cells Many yeasts and filamentous fungi have no known sexual stage All fungi lacking Sexual REPRODCUTION are grouped into DEUTEROMYCETES 31 3 THE ANCESTOR OF FUNGI WAS AN AQUATIC SINGLE CELLED FLAGELLATED PROTIST Fungi and animals are said to be more closely related to each other than Fungi and Plants THE ORIGIN OF FUNGI Phylogenetic analysis suggests that fungi evolved from a flagellated ancestor Most fungi lack flagella but the earliest diverging lineages did DNA evidence shows that all EUKARYOTES form a clade called OPISTHOKONTS and that fungi are more closely related to single celled protists than they are to animals Animals closest relative to protists are the CHOANOFLAGELLATES which means that multi cellularity arose independently in plants and animals THE MOVE TO LAND Fungi may have colonized land before plants Terrestrial environments were made up of green slime that consisted of extraterrestrial organisms and fungi may have been well suited to feed on them Plants and fungi formed symbiotic associations with each other Evidence includes fossils of hyphae penetrating plant cells and forming structures that resemble mycorrhizae Further evidence supports this claim because the earliest land plants may have need fossils to absorb nutrients because they lacked fossils For a mycorrhizial fungi and plant to form a symbiotic relationship certain genes need to be expressed by fungi and plants Plants need sym genes which allows for mycorrhizae to form in flowering plants and evidence shows that sym genes were present in the common ancestor of land plants 31 4 FUNGI HAVE RADIATED INTO A DIVERSE SET OF LINEAGES CHYTRIDS The globular fruiting body forms multicellular branched hyphae LM Ubiquitous in lakes and soil have flagellated spores and are thought to have been one of the earliest fungal groups to diverge from other fungi Unique among fungi in having flagellated spores ZOOSPORES ZYGOMYCETES Include species of fast growing olds parasites and commensal symbionts Life cycle can be seen