Fungi
Reading: Campbell, Chapter 31.
Saccharomyces
Saccharomyces is the common yeast often used in making wine, beer, and bread.
The term "yeast" simply refers to fungi that grow as single, roundish cells and don't form hyphae. Yeast is not a taxonomic group; it's a description of a body type.
These eukaryotic cells contain a nucleus (stained dark in this image)
and a central vacuole (light staining), as well as other internal membrane-bound
structures.
Aspergillus
Aspergillus is a typical mold. A mold is a fungus with a body composed of thin, stringy hyphae. The whole body of connected hyphae is called a mycelium. The mycelium is haploid, except for reproductive structures.
Various species of Aspergillus are used commercially, for example in the production of citric acid or digesting the starch in rice as a step in making sake. Some species of Aspergillus occasionally act as human pathogens.
Aspergillus is an ascomycete (phylum Ascomycota).
This electron micrograph shows a spore-producing structure (called a conidiophore), with numerous spores being produced.
Molds can spread rapidly because their thin hyphae penetrate into new food sources (rotting fruit, for example) and can grow very rapidly. Molds can also produce huge numbers of asexual spores via mitosis.
Molds also reproduce sexually, but this is much less common than asexual
spore formation.
Rhizopus
Rhizopus is another common mold. Like Aspergillus, it typically makes a huge number of asexual spores and undergoes sexual reproduction less frequently.
This image shows sporangia, which produce spores. These sporangia could be produced either sexually or asexually.
During sexual reproduction, two haploid hyphae from different parents perform plasmogamy, joining together, producing a heterokaryotic cell. This multinucleate heterokaryotic cell forms a zygosporangium.
Pairs of nuclei (one from
each parent) perform karyogamy in the zygosporangium,
fusing to form a diploid nucleus. These diploid nuclei are zygotes;
they immediately undergo meiosis to begin producing haploid sexual
spores. A sporangium sprouts out of the zygosporangium to release
these spores.
Coprinus
Coprinus is in the phylum Basidiomycota, the phylum that makes mushrooms. Like many mushroom-forming fungi, Coprinus may have very large haploid mycelia underground, while occasionally forming heterokaryotic hyphae that grow into mushrooms above ground. The main body of the mushroom is heterokaryotic. In this case, each cell has two different nuclei in each cell; this kind of heterokaryotic cell is called dikaryotic.
The main umbrella-shaped part is sometimes called the cap. Under the cap
there are gills. (The gills have nothing to do with gas exchange; they're
called gills because they look like fish gills.) Some of the dikaryotic
cells on the gills undergo karyogamy, fusing their two different haploid
nuclei to make a diploid zygote. As with all fungi, the zygote then performs
meiosis, making four haploid cells. These haploid cells turn into spores.
These images show the gills in a cross-section of the mushroom at different
magnifications.
This magnified view gives you a better view of the spores.
Peziza
Peziza is a cup fungus in the phylum Ascomycotes. Like a mushroom (phylum Basidiomycota), Peziza produces above-ground heterokarkyotic reproductive structures that come from an underground mycelium.
One way that Peziza differs from mushrooms is that it produces spores
on top of its cup, not underneath like a mushroom.
In cross-section, Peziza shows a mycelium constructed of loose hyphae underneath, with tightly packed spore-producing structures (called asci) on top.
In this image, each ascus is a long, narrow cell with eight spores inside. The ascus begins as a single dikarkyotic cell.
The two nuclei fuse, forming a single diploid nucleus; this is called karyogamy. The diploid nucleus is called a zygote.
The zygote undergoes meiosis, producing four haploid nuclei.
Each of these haploid nuclei divides once (via mitosis). The end result is eight haploid nuclei in one ascus; these nuclei form new walls and turn into spores.
In this image, you can see spores at different stages of maturation.
Lichens
A lichen is two different organisms living in a close symbiotic association.
One partner is a fungus, which forms a tough, leathery coating that provides a protected space inside. The fungal cells can tolerate harsh, dry conditions, but they cannot produce food on their own.
The other partner is an alga -- a unicellular photosynthetic organism.
The algae thrive in the protected environment created by the fungal mycelium.
The algae perform photosynthesis, making the sugars that can be used
as energy by both the algae and the fungus.
This page updated September 17, 2011