In this article we will discuss about the asexual and sexual methods of reproduction that occur in the life cycle of chlamydomonas.
1. Asexual Reproduction:
It takes place by following methods:
(A) By zoospores- The zoospore formation takes place during favourable conditions. The zoospore formation takes place as follows:
The protoplast contracts and gets separated from the cell wall. The parent cell loses flagella or in some species of Chlamydomonas flagella are absorbed. The contractile vacuoles and the neuro-motor apparatus disappear. The protoplasm divides longitudinally by simple mitotic division forming two daughter protoplasts.
The second longitudinal division of protoplasm takes place at right angle to the first, thus making four daughter chloroplasts. Sometimes the protoplasm may further divide to make 8-16-32 daughter protoplasts. The pyrenoids and initials of neuro-motor apparatus also divide. The contractile vacuoles also develop in daughter protoplasts. Each daughter cell develops cell wall, flagella and transforms into zoospore (Fig. 6).
The zoospores are liberated from the parent cell or zoosporangium by gelatinization or rupture of the cell wall. The zoospores are identical to the parent cell in structure but smaller in size. The zoospores simply enlarge to become mature Chlamydomonas. Under favourable conditions the formation of zoospores can take place every 25 hours.
(ii) By Aplanospores:
The aplanospores are formed slightly under unfavorable conditions e.g., in C. caudata. The parent cell loses flagella.
The protoplast rounds off and secretes a thin wall outside but does not develop Fig. 7. (A) Parent cell, (B) Aplanospore formation, (C) Hypnos pore flagella. These non-motile structures are called aplanospores. On approach of favourable conditions aplanospores may germinate either directly or divide to produce zoospores (Fig. 7 A, B).
(iii) By Hypnospores:
In extreme unfavorable conditions the protoplast develops thick wall and the structure developed is called Hypnos pore e.g., in C. nivalis. The hypnospores also germinate like aplanospores on approach of favourable conditions. (Fig. 7 C).
(iv) Palmella Stage:
The palmella stage is formed under unfavorable conditions as shortage of water, excess of salts etc. The protoplast of parent cell divides to make many daughter protoplasts but they do not form zoospores. The parent cell wall gelatinizes to make mucilaginous sheath around daughter protoplasts. The daughter protoplasts also develop gelatinous wall around themselves but do not develop flagella.
These protoplast segments are called palmellospores. The division and red visions of these protoplast ultimately forms amorphous colony with indefinite number of spores and it is called palmella stage (Fig. 8). When favourable conditions return the gelatinous wall is dissolved, palmellospores develop flagella, and the spores ire released to make new thalli.
2. Sexual Reproduction:
The sexual reproduction in Chlamydomonas can be isogamous, anisogamous or oogamous. he thallus can be homothallic i.e., both types of gametes are produced in same thallus e.g., C. mogama and C. media or can be heterothallic i.e., (+) and (-) gametes come from different parents, he gametes may be naked and called gymnogametes e.g., C. debaryana or covered by cell wall id called calyptogametes e.g., C. media.
Most of the Chlamydomonas species are isogamous in nature. In isogamous reproduction the fusion of gametes, which are similar in size, shape and structure, take place. These gametes are morphologically similar but physiologically dissimilar.
In many isogamous species the vegetative cells may directly function as gametes without undergoing any division e.g., in C. snowiae, this fusion is called as hologamy. The thalli shed their walls and function as gametes.
The two gametes come close to each other by their anterior ends and later fusion proceeds to lateral sides (Fig. 9A-D). The fusion product is quadri flagellate and bi-nucleate structure with two pyrenoids and two eye spots. The quadri flagellate zygote remains motile for several hours to few days. (Fig. 9 E, F).
In C. eugametos, the vegetative cells do not shed their walls, after union the contents of one gamete enter into another gamete as such. According to Chapman (1964) the isogamous reproduction takes place by production of 8, 16 or 32 bi-flagellated gametes. The process takes place as follows (Fig. 10). The vegetative thallus functioning as gametangium comes to rest and loses its flagella.
The protoplast withdraws itself from the cell wall. The protoplast divides by repeated longitudinal mitotic divisions to produce 8-16-32 or 64 daughter protoplasts. Each daughter protoplast develops a pair of flagella and transforms into gamete. The gametes are liberated by breaking the wall of gametangium. The flagella of gametes are covered by agglutins and secrete a hormone called gamone.
These chemical substances are involved in the recognition of gametes of the opposite strains. In heterothallic species (+) and (-) strain gametes cluster together and this phenomenon is called clumping. The gametes of opposite strain fuse by anterior end i.e., apical fusion or laterally i.e., lateral fusion (Fig. 10). The paired gametes move away from the clump.
The wall at the place of contact dissolves and fertilization takes place in two steps—plasmogamy and karyogamy. In plasmogamy the fusion of cytoplasm and in karyogamy the fusion of nuclei takes place. After fertilization a quadriflagellate zygote is formed. The zygote later on loses flagella and gets covered by a thick wall and is now called zygospore.
In anisogamous reproduction the gametes are unequal in size. The male gametes or microgametes are smaller, the female gametes or macrogainetes are larger e.g., in C. braunii and C. suboogama. The macrogametes are formed in female gametangium in which the protoplast divides to make 2 to 4 gametes only (Fig. 11 A, C).
The microgametes are formed in male gametangium where the protoplast divides to make 8-16 gametes (Fig. 11 B, D). The microgametes are more active than macrogametes. The microgametes come close to the macrogamete, the protoplast of microgamete enters into macrogamete and after fusion a diploid zygote is formed. The zygote secretes a thick wall and transforms into zygospore (Fig. 11 E-H).
The oogamous sexual reproduction takes place in C. coccifera and C. ooganum. The vegetative thallus functioning as female cell withdraws its flagella and directly functions as non-motile macrogamete or egg. The female gamete contains many pyrenoids (Fig. 12A, B).
The microgametes are formed by four divisions of protoplast as in case of anisogamous reproduction (Fig. 12 C, D). The microgamete reaches the female gamete and unites by anterior ends. The contact wall between the two dissolves. After plasmogamy and karyogamy a diploid zygote is formed (Fig. 12 E-G). The zygote secretes a thick wall and transforms into zygospore.
The zygote is resting diploid spore. The zygote secretes a thick wall which is smooth or ornamented. The zygote accumulates large amount of oils and starch. The zygospores are red in colour due to the presence of haematochrome. the zygospore survives long period of unfavorable conditions and germinates on approach of favourable season.
When the resting period is over and the favourable conditions reappear the zygospore germinates. Its diploid nucleus divides by meiosis to make four haploid nuclei. The four daughter protoplasts, each with one haploid nucleus, form four haploid zoospores or meiozoospores.
Each zoospore contains neuro-motor apparatus, eye spot, two flagella and contractile vacuoles. In 4 zoospores two may be of (+) type and two (-) type in heterothallic forms. The number of meiospores per zygospore are 8 in C. reinhardtii or 16-32 in C. inter-media (Fig,13 A-D, 14, 15).