THE WORLD OF LIFE with Douglas Drenkow

Lichens: A Fungus (typically a sac fungus but sometimes a club fungus) Living Together in One Body with An Alga (typically a unicellular green alga but sometimes a blue-green alga)

Note that before modern genetic research, oomycetes and slime molds were also classified as fungi; and fungi were classed as plants (which also have cell walls and often reproduce by means of spores).

Biology

ENVIRONMENTS

Various fungi live in salt- and freshwater, in and on the land (typically in moist habitats), and in and on other living organisms.

Growing together as the body of a lichen, the fungus provides a "foothold" (as on a rock), moisture, and shade (from over-intense sunlight); and the alga provides food (via photosynthesis) -- the alga can live alone, and often better alone; but the fungus cannot survive without the alga.

The mutually beneficial living arrangement ("symbiosis") of a fungus growing in close association with the roots of a higher plant is called a "mycorrhiza": The fungus helps control mineral uptake by the roots, which in turn feed the fungus.

OVERALL STRUCTURE

Although some fungi, notably yeasts, are unicellular, most fungi are mutlicellular, forming often branched filaments, called "hyphae".

The cell walls of fungi are reinforced with "chitin" -- a nitrogen-enriched "polysaccharide" (a "polymer", chain-like molecule, composed of sugars) -- as in the "exoskeleton" of various arthropods. Only certain chytrid fungi (presumably like the ancestors of all fungi) have cell walls with "cellulose" (a different polysaccharide), as in various algae and all higher plants. Fungi are truly neither plants nor animals.

The cross-walls within the hyphae of a fungal body are either porous or absent: Each hypha typically has many nuclei (like likewise "coenocytic" oomycetes or golden-green algae, from which fungi probably evolved).

The hyphae of the "vegetative body" of a fungus (as opposed to the "fruiting bodies", as described below) typically grow together as a weblike or leatherlike structure, known collectively as the "mycelium".

ENERGY CAPTURE

Fungi, like animals but unlike plants, are "heterotrophic" (not able to produce their own food molecules). Most fungi are "saprophytic", consuming dead or other nonliving organic matter and, thus, sometimes rotting valuable products but typically contributing immensely to the absolutely vital recycling of waste materials in the environment. Many fungi are parasitic, causing minor to life-threatening diseases of plants and animals.

EXCHANGE OF MATERIALS WITH THE ENVIRONMENT

Typically, the hyphae of a fungus secrete enzymes, which break large food molecules in the environment down into molecules small enough to be absorbed.

Some fungi produce "haustoria", specialized hyphae that grow into and absorb materials from the cells of other organisms.

INTERNAL TRANSPORT

Materials are moved within each cell and throughout all the hyphae of the mycelium (with porous or absent cross-walls) by the active process of cytoplasmic streaming.

DEVELOPMENTAL CONTROL

The growth and development of a fungus is under genetic control, and hormones play a role in at least multicellular forms.

ASEXUAL REPRODUCTION

Fungi reproduce asexually by various means. Unicellular forms, such as yeasts, reproduce by "budding" (simple fission). Some multicellular forms reproduce asexually from pieces of hyphae, such as hardened, unicellular "chlamydospores". Most fungi reproduce asexually by means of spores, which may be mobile or nonmobile, and either borne in "sporangia" or not encased.

Fungi for which no sexual means of reproduction has been observed were traditionally classed as "imperfect fungi", a completely artificial category; with genetic research etc., these species are being reclassified into the other, naturally significant groups.

SEXUAL REPRODUCTION

The chytrids are undoubtedly the most primitive fungi: Their "gametes" (sex cells) have propeller-like "flagella", with which they swim through their aquatic environment. All other fungi must grow together to affect sexual fertilization.

The sexual reproduction of a fungus typically involves three distinct life stages. The "diploid" stage (with both sets of chromosomes) produces the "haploid" stage" (with just one set of chromosomes), via "meiosis" (the form of cell division that cuts the number of chromosomes in half). The haploid stage which produces the "dikaryotic" stage (each cell or hypha with twice as many (haploid) nuclei as normal). The dikaryotic stage finally reproduces the diploid stage, via the fusion of the nuclei -- if and only if the nuclei are compatible, which in some species means from different hyphae (however, because they appear and grow similarly, such hyphae are known as "plus" or "minus" strains, not distinctly "male" or "female" sexes).

Typically in Zygomycota (such as bread molds), if the tips of short hyphae (branching off the main mycelia) of plus and minus strains happen to meet, they will enlarge and be walled off from the rest of the hyphae, forming "gametangia", each with many nuclei (haploid, like the rest of the mycelium). The gametangia then fuse, producing a diploid "zygote". The zygote develops a thick wall, becoming a "zygospore", which eventually germinates into another haploid hypha.

Typically in sac fungi, a haploid mycelium produces female and male gametangia, in close contact with one another. "Gametes" (haploid male and female cells) are produced within each gametangium. The cytoplasm of a male gamete fuses with that of a female gamete, but their nuclei do not fuse: The resulting, dikaryotic hyphae grows, intermingled with the original, haploid mycelium, forming a ball-, bottle-, or dish-like body (the "ascocarp"), which bears tiny dikaryotic sacs ("asci") on its inner surface. Within each ascus, the nuclei finally fuse; immediately, within this diploid cell, haploid "ascospores" form, by meiosis (cell division halving the number of chromosomes) followed by mitosis (the typical form of cell division, not changing the number of chromosomes). After being released, these ascospores germinate into other haploid hyphae.

Typically in yeasts (unicellular, atypical sac fungi), the ascus is in effect the diploid parent cell, in which are formed (via meiosis) the haploid ascospores.

Typically in club fungi, two haploid hyphae (or special cells from them) fuse, producing a dikaryotic cell.

In club fungi like mushrooms, the dikaryotic cell typically produces an inconspicuous mycelium that grows into the visible mushroom, puffball, or bracket-like body (the "basidiocarp"), on whose gills or in whose pores are borne club-shaped dikaryotic cells (the "basidia"), whose nuclei fuse. On their outer surface, the then diploid basidia produce (via meiosis) haploid cells ("basidiospores"), which after being released, germinate into new, haploid hyphae.

In club fungi like rusts, the original dikaryotic cell is typically a resistant "teliospore", which eventually germinates directly into the basidium (there is no "mushroom" or other basidiocarp). The nuceli of the basidium fuse, and this diploid basidium forms (via meiosis, sometimes followed by mitosis) the haploid basidiospores.

Eukaryotes (Domain Eukaryota)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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