Chlamydomonas shows all the three modes of sexual reproduction but most of the species of Chlamydomonas show isogamy.

The common mode of sexual reproduction in Chlamydomonas is isogamous.

Explanation:
Chlamydomonas is a genus of green algae that exhibits both sexual and asexual reproduction. Sexual reproduction in Chlamydomonas occurs through the fusion of gametes.

Isogamous Reproduction:
Isogamous reproduction is a type of sexual reproduction in which the gametes produced by the two individuals are morphologically similar or identical. In Chlamydomonas, the gametes produced by both male and female individuals are morphologically identical and are called isogametes.

Key Points:
- In isogamous reproduction, the gametes are similar or identical in size, shape, and structure.
- Both male and female individuals in Chlamydomonas produce isogametes.
- The isogametes in Chlamydomonas are flagellated and motile.
- The process of sexual reproduction in Chlamydomonas begins with the fusion of two isogametes, one from a male individual and one from a female individual.
- The fusion of the isogametes is called syngamy, resulting in the formation of a diploid zygote.
- The zygote undergoes meiosis to produce haploid cells called zoospores, which can develop into new individuals.

Other Modes of Sexual Reproduction:
Although isogamous reproduction is the common mode of sexual reproduction in Chlamydomonas, there are other modes of sexual reproduction observed in different organisms:

- Anisogamous reproduction: In anisogamous reproduction, the gametes produced by the two individuals are different in size, shape, or structure. This type of reproduction is observed in many multicellular organisms, such as plants and animals.

- Oogamous reproduction: In oogamous reproduction, the gametes produced by the two individuals are highly differentiated. One gamete, called the egg or ovum, is large and non-motile, while the other gamete, called the sperm, is small and motile. Oogamous reproduction is observed in many higher plants and animals.

- Hologamous reproduction: Hologamous reproduction is a type of sexual reproduction where the gametes are morphologically similar, but they differ in their motility. This type of reproduction is observed in some algae and fungi.

In conclusion, the common mode of sexual reproduction in Chlamydomonas is isogamous, where both male and female individuals produce morphologically similar and motile gametes called isogametes.