Reproduction in Euglena
In E. viridis no sexual reproduction occurs. It reproduces by asexual binary and multiple fissions and undergoes encystment. Under favorable conditions euglenas reproduce by longitudinal binary fission. The longitudinal binary fission is always symmetrogenic, that is, the parental Euglena divides into two daughter individuals, where one is the plane mirror image of the other.
A. Binary Fission:
- The nucleus divides into two mitotically followed by division of cytoplasm. In prophase stage all the nuclei (endosomes) fuse together and each chromosome splits into two daughter chromosomes or chromatids. In metaphase all the chromatids are arranged in the equator. No spindle is formed at the anaphase, but the chromatids are separated and moved towards the opposite poles. In telophase due to the constriction of the nuclear membrane the nucleus is finally separated into two daughter nuclei. Following the nuclear division all the anterior extra-nuclear organelles such as the blepharoplasts, reservoir, cytophraynx and stigma, etc. are all duplicated.
- The division of nucleus (karyokinesis) is immediately followed by division of cytoplasm (cytokinesis) where due to the longitudinal splitting of the cytoplasm the Euglena divides into two daughter euglenae.
- In some cases the stigma breaks into component granules. Ordinarily the original flagellum of the parent is retained by one daughter euglena and the other develops a new one. On the other hand, some observers have reported complete disappearance of the entire locomotory apparatus during division, and each daughter cell reconstructs a new set.
B. Multiple Fission:
Cases of multiple fission, though rare, have also been reported. Multiple fission takes place in encysted condition. Euglena very readily encysts forming both thick and thin walled cysts within which it divides into several (16-32) daughter englenas. Sometimes the flagellate loses its flagellum and rounds up into an alga-like cell in which metabolisms continues and reproduction occurs by fission, thus, forming extensive green scums on the surface of ponds. In this condition, they are said to assume the Palmella state. Such a palmella stage is of regular occurrence in some species.
Under certain conditions ordinary protective cysts are also formed. Encystment is stimulated by lack of food, lack of oxygen, drying, heat (as in strongly illuminated cultures) and fouling of the medium. The cyst is composed of a special carbohydrate and is of yellowish brown colour. The cysts are generally rounded, their walls being made of two or three concentric layers. The cysts are usually small, their total width being equal to the diameter of the animal, it may be larger sometimes. Thin and stalked cysts have also been reported in some species and in others each may be provided with an operculum. The organisms may lie in the centre of the cyst or towards one side (excentrally). The cysts are protective structures that help the organisms to withstand unfavorable circumstances and also help their dispersal. On the return of favorable conditions the cysts dissolve and the organisms come out and begin normal life.