The Eden Organism

Higher Algae

The basic ideas for this lecture are indicated in the table below. Many slides of examples are shown in the lecture. A few simple life cycles in the green algae were a primary focus. These represent the "root" of Kingdom Plantae, so we might think of them as the "Eden" organisms. There is a current hypothesis pointing to Coleochaete as an example of what might have been the common ancestor shared between the algae and true plants.

The life cycle of Chlamydomonas

This organism consists of single cells.

• The haploid (gametophyte stage) consists of motile or non-motile cells that can interconvert and divide by mitosis to reproduce asexually.
• Motile cells of opposite mating type are isogametes.
• The zygote becomes the resting spore (=sporocyte).
• Meiosis produces more haploid cells.
• The life cycle is zygotic (or possibly zygotic AND gametic--depending on interpretation).

The life cycle of Oedogonium

This organism consists of filaments.

• The haploid (gametophyte stage) filaments are heterothallic or homothallic.
• Asexual reproduction is via multiflagellate (stephanokont) zoospores made by mitosis.
• Oogonium is the female gametangium--egg is gamete.
• Antheridium is the male gametangium--stephanokont sperm is gamete.
• The alga is oogamous!
• They can be homothallic or heterthallic.
• The zygote becomes the resting spore (=sporocyte).
• Meiosis produces haploid stephanokont zoospores.
• The life history is zygotic (haplontic).

The life cycle of Ulva

This organism consists of sheets of parenchyma.

• The haploids (gametophytes) are heterothallic but isomorphic.
• Asexual reproduction is via haploid zoospores made by mitosis.
• Oogonium is the female gametangium--slightly larger biflagellate gamete.
• Antheridium is the male gametangium--slightly smaller biflagellate gamete.
• The alga is slightly anisogamous.
• The alga is heterothallic.
• The quadriflagellate zygote grows into a diploid (sporophyte) isomorph.
• Asexual reproduction is via diploid zoospores made by mitosis.
• A zoosporangium holds zoosporocyte
• Zoosporocyte undergoes meiosis to produce haploid quadriflagellate zoospores.
• The life history is sporic (diplohaplontic).

The life cycle of Codium

This organism consists of a multinucleate siphonaceousous thallus.

• The "dead man's fingers" may be a diploid sporophyte
• The surface of the ropy thallus is surrounded in siphonaceous filaments
• The filaments end in utricles which contain sporocytes
• Sporophytes are either heterothallic or homothallic depending on species
• Sporocytes undergo meiosis to make spores
• Spores become biflagellate strongly anisogamous gametes
• Gametes unite to form quadriflagellate zygote
• Zygote settles and grows into new sporophyte
• The life history is gametic (diplontic).

This page © Ross E. Koning 1994.

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