Reproduction of Algae | Vegetative, Asexual & Sexual reproduction

Reproduction of Algae | Vegetative reproduction, Asexual reproduction & Sexual reproduction

1. REPRODUCTION OF ALGAE Algae reproduce by various methods, as follows; Vegetative Asexual Sexual

2. VEGETATIVE REPRODUCTION The type of reproduction, in which the whole new plant is produced from an organ (or vegetative part of the plant) which is not involved in sexual reproduction is called Vegetative reproduction. Vegetative reproduction is a form of asexual reproduction in which specialized organs formed by the parent, become detached and generate new individuals. In algae, the vegetative reproduction is widespread and commonly seen among filamentous members.

3. TYPES OF VEGETATIVE REPRODUCTION Fragmentation Hormogonia Budding Propagules / Bulbils Akinetes Cell division Fission Abscissed Pinnules / Proliferous shoot Amylum stars / Protonema like outgrowth

4. FRAGMENTATION : In this process, the fragments or pieces of the parent become detached and grow into new individuals. Eg ; Hormidium , Nostoc HORMOGONIA : In cyanophyta , the short lengths or small fragments of trichomes are cut off by the occurrence of simple breaks in the filaments. Thes are called are Hormogonia. They may also form after the formation of the intercalary cells called necridia or seperation disc. Eg ; Oscillatoria , Nostac

5. BUDDING : A lateral bud is formed by the vesicles by the formation of a cell wall in some genera of green algae. This bud seperates and develops into another vesicle with its own rhizoid. Eg ; Protosiphon PROPAGULES / BULBILS : Some wedge-shaped modified branches develop in some genera, as in Sphacelaria . They bear two or more short processes called propagules or bulbils . These are rich in food material, and on being detached they develop into new plant.

6. AKINETES : The formation of thick walled cells containing large quantity of food reserves is very common in sevral algae. These cells are called Akinetes. They can withsatnd unfavourable conditions and thus help in perennation . Each akinete can germinate into a new plant. Eg ; Anabaena CELL DIVISION : In desmids and diatoms, cell division is a common method of vegetative reproduction. In desmids, cell division occurs when a set size is attained by the algae. In diatoms, vegetative or mitotic cell division starts after a period of increase in cell volume, thus resulting in a progressive reduction cell size.

7. FISSION : In unicellular flagellates, the vegetative reproduction takes place by longitudinal fission of the cell along with the simultaneous reformation of organelles which have not divided, Fission usually starts at the anterior end progresses downward. Eg ; Euglena . ABSCISSED PINNULES : Abscission of older pinnules in Bryopsis and proliferous shoots in Caulerpa are also the means of vegetative reproduction. These abscissed pinnules or shoots germinate into new plant.

8. AMYLUM STARS : These are found in Chara . Star like aggregates of cells developing on the lower nodes are called amylum stars . They are filled with starch. Sometimes, Protonema like outgrowths also grow on the lower nodes in Chara . Both these vegetative structure give rise to new plant.

9. ASEXUAL REPRODUCTION Asexual is a type of reproduction from one individual, without the fusion of sex cells from two different parents. Therefore, the asexual reproduction may also be defined as “any form of reproduction not depending on a sexual process or on a modified sexual process”. In some aspects, asexual reproduction is very similar to certain forms of vegetative reproduction, particularly in flagellate algal groups. Zoospores. Non motile asexual spores.

10. ZOOSPORES – The motile asexual spore The zoospores are unicellular, flagellated and motile structures, each with a small amount of protoplast. These are known from Chlorophyceae , Prasinophyceae , Xanthophyceae , Chrysophyceae , Dinophyceae , Prymnesiophyceae , Eustigmatophyceae and Phaeophyceae . The zoospores are usually biflagellate structures devoid of a cell wall. In general, the production of zoospores, as of other asexual spores, is preceded by mitotic divisionof the nuceli followed by partion of protoplast. Resulting into the formation of 2, 4, 8 or more zoospores formed by longitudinal cleavage of the parent cell wall.

11. The entire cell thus functions as a sporangium called zoosporangium, from which zoospores escape by rupture or lysis of the parental wall. Zoospores swim for some time, settle down on some sustainable substratum, attain a cell wall and each develops into a new plant. In many members of Ectocarpales , zoospores from plurilocular sporangia regenarate the sporophyte generation, as in Ectocarpous , Laminariocolax and Giffordia .

12. NON-MOTILE ASEXUAL SPORES Aplanospores : The protoplast released from the algal cell as non-motile and non-flagellate spore is called an aplanospores . Eg ; Ulothrix , Vaucheria . Autospores : Such aplanospores which the same morphological features as that of parent cell are called autospores as in Chlorerlla . Hypnospores : Aplanospores with very thick wall are called hypnospores , as in Chlamydomonas . Tetraspores : In some Rhodophyta and Phaeophyta instead of mitosis, the spores are also formed after meiosis, and these are called tetraspores .

13. Exospores : In Chamaesiphon and some other epiphytic blue-green the cell membrane bursts at the apex, exposing the protoplast, which soon becomes fragmented into spherical spores called exospores . Endospores : In Dermocarpa calvata and some other blue-green algae, the successive divisions of cell contents in three planes result in the formation of endospores . Monospores : In Tilopteris , chantransia stage of Batrachospernum and some other brown algae and red algae non-motile spore developing singly in a sporangium is called monospores . Carpospores: These are non-motile spores formed by the divisions of the zygote in some red algae, Eg ; Batrachospernum , Polysiphonia . They are formed in the carposporangium of carposporophyte .

14. Neultral Spores: The vegetative cells in the some algae transform directly into the spores called neutral spores. Bispores : In some red algae, two spores develop in a sporangium. These are called bispores developing in a bisporangium . Polyspores : In some red algae more than four spores are formed in a sporangium after reduction division. These haploid spores are called polyspores or paraspores . Statospores : Smooth walled or partially ornamented thick-walled internal cysts or statospores are found in some members of Bacillariophyceae , Chrysophyceae . Cysts : Cysts are the resulting stages of many algae, in Vaucheria , amoeba like multi-nucleate cysts develop, which, on liberation, germinate into new thalli .

15. SEXUAL REPRODUCTION Sexual reproduction is a type of reproduction which involves the fusion of two cells and their nuclei from two parent individuals, so that the offspring receives the genetic material from both parents. Fusion actually means the joining together of two gametes to form a zygote. A gamete is a haploid sex cells, whose functions is to join with a gamete of opposite sex to form a diploid zygote. Zygote is, therefore, a diploid cell which is produced by the fusion of two haploid gametes.

16. A fertilized ovum is a zygote, and it develops first into an embryo and then into a sporophyte . Blue-green algae and bacteria lack sexual reproduction. In many genera of Chrysophyta and Bacillariophyta , the sexual reproduction has been rarely observed, although it is probably a feature of the life history of most of the species. In algae, the sexual reproductive units may vary in their morphology and behaviour . Due to this, the sexual reproduction in algae may be of three different types, viz. isogamous , anisogamous and oogamous .

17. ISOGAMY When two fusing gametes of the opposite sex are morophologically identical and have same size, they are called isogametes and their process of fusion is called isogamy .

18. ANISOGAMY When two fusing gametes of the opposite sex are morophologically dissimilar and have different size, they are called anisogametes and their process of fusion is called anisogamy .

19. OOGAMY When two fusing gametes are extremely dimorphic, the larger, non-motile and called egg, and the smaller, motile by flagella and sperm or male gamete, the gametes are called oogamous, their process of fusion is called oogamy.