Asexual Reproduction Cycle of Green Algae Labeled

All 12 stages and parts on the diagram

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Asexual cycle 5

Vegetative cell The full-grown swimming cell. It is haploid.
Flagella Two whips at the front end. The cell swims with them.
Cell division (mitosis) The cell stops swimming and divides inside its wall. It may divide again, into four or eight.
Mother cell wall The old wall of the cell that divided. The daughter cells break out of it.
Zoospores (daughter cells) The daughter cells: small copies of the parent. Each grows to full size.

Sexual cycle 7

Plus gamete A sex cell of the plus mating type. It looks just like a minus gamete.
Minus gamete A sex cell of the minus mating type. It fuses with a plus gamete.
Gametes fusing A plus and a minus gamete join into one cell.
Zygote The one cell made when two gametes fuse. It is diploid.
Zygospore The zygote inside a thick, hard wall. It rests through hard conditions.
Meiosis The zygote divides into four haploid cells.
Four haploid cells Four haploid cells swim free. Each grows into an ordinary swimming cell.

How this list was compiled: Every name and statement was checked on 2026-10-03 against sources read that day; a name is kept only when two of them use it. (1) Wikipedia, text only: Chlamydomonas reinhardtii, https://en.wikipedia.org/wiki/Chlamydomonas_reinhardtii (vegetative cells are haploid; cells may divide by mitosis two or three times before the old cell wall lets the daughter cells out, so one mother cell gives 4 or 8 daughter cells; starved of nitrogen, vegetative cells turn into haploid gametes of two mating types, identical in appearance, mt(+) and mt(-), which fuse into a diploid zygote; the zygote has no flagella and is a dormant form; after meiosis the zygote frees four flagellated haploid cells, which restart the vegetative life cycle), and Chlamydomonas, https://en.wikipedia.org/wiki/Chlamydomonas (two anterior flagella; asexual reproduction by zoospores). (2) Chlamydomonas Resource Center, University of Minnesota, About Chlamydomonas, https://www.chlamycollection.org/resources/about-chlamydomonas/ (two anterior flagella with which the cells swim; cells are haploid; deprived of nitrogen, haploid cells turn into isogamous gametes; the two mating types are designated plus and minus; gametes of opposite mating types fuse into a diploid zygospore, whose hard outer wall shields it from bad conditions; when conditions improve the diploid zygote goes through meiosis and frees four haploid cells that restart the vegetative cycle). (3) John N. Martin, Botany for Agricultural Students, 1919, public domain, https://archive.org/details/botanyforagricul00mart (Chlamydomonas, pages 303 and 304: reproduction by zoospores and gametes; in forming zoospores the plant becomes quiescent and the protoplast divides into two or more ciliated cells which are miniatures of the parent; these daughter cells or zoospores escape from the mother cell and enlarge to the parent size; gametes fuse in pairs to form resting zygospores; the zygospore is a well-protected spore, dormant until favorable conditions return; the gametes are alike, isogametes). (4) The summaries of three journal papers, read in Europe PMC: Journal of General Physiology 1954, PMID 13174779 (with no nitrogen source, vegetative cells turn into gametes), Proceedings of the National Academy of Sciences 1991, PMID 1871143 (gametes of opposite mating type; the young diploid zygote becomes a dormant zygospore able to complete meiosis) and Cell 2008, PMID 18510927 (gametes of plus and minus mating types; gametic fusion; zygote; vegetative cells; meiosis); and Gilbert M. Smith, Manual of Phycology, 1951, one passage read by full-text search on archive.org, https://archive.org/details/manualofphycolog0000smit (meiosis generally takes place during the germination of the zygote; in Chlamydomonas the zygote usually produces four zoospores). Licence: own work

The drawing: Drawn for this site on 2026-10-03 from plain curves; own work, nothing traced. Licence: own work

How it works

The asexual reproduction cycle of green algae is drawn here for Chlamydomonas, a single cell with two flagella. The first loop shows one cell dividing into zoospores. The second shows two gametes fusing into a zygospore.

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The asexual loop

A full-grown cell stops swimming and divides inside its own wall, by mitosis. The daughter cells, called zoospores, are small copies of the parent. They leave the old wall and grow to full size.

The sexual loop

Short of nitrogen, cells turn into gametes of two mating types, plus and minus. A plus and a minus gamete fuse into one diploid cell, the zygote. It builds a thick wall and rests as a zygospore. When conditions improve, the zygote divides by meiosis and sets free four haploid cells, which grow into ordinary swimming cells.

Every swimming cell in both loops is haploid. Only the zygote is diploid.
Plus and minus gametes look the same. The signs beside them are on the drawing only to tell them apart.

Questions and answers

How does Chlamydomonas reproduce asexually?

A cell divides by mitosis inside its own wall, once or more, into two, four or eight daughter cells called zoospores. They leave the old wall, and each grows into a full-size cell.

What are zoospores in green algae?

In Chlamydomonas they are the daughter cells of asexual reproduction: small swimming copies of the parent, each with two flagella.

What is the difference between a zygote and a zygospore?

The zygote is the single diploid cell made when two gametes fuse. Once it has built a thick, hard wall it is called a zygospore, a resting stage that lasts through hard conditions.

Is Chlamydomonas haploid or diploid?

Haploid for most of its life: the swimming cells, the zoospores and the gametes. Only the zygote is diploid, and meiosis turns it back into four haploid cells.

What are plus and minus gametes?

The gametes of the two mating types of Chlamydomonas. They look the same and are named plus and minus. Gametes of opposite types fuse.