Ectocarpus Sexual Reproduction

BSc (Hons.) PART 1 Prepared by: Paper I CRYPTOGAMS Dr. JYOTI PANDEY

Majority of the species are ISOGAMOUS and homothallic. It is reported in E. globifer. Here the fusing gametes are similar in every respect looking alike and behaving alike. Fusion occurs between isogametes coming from the same plant or even the same gametangium.

The sexual reproduction is also of anisogamous type. Oogamy is absent in Ectocarpus. Anisogamy may be of two types: Morphological Anisogamy (E. secundus) and Physiological Anisogamy (E. siliculosus). In physiological anisogamy, both the uniting gametes are morphologically similar, but in morphological anisogamy the female gamete is larger than the male gamete.

Physiological anisogamy occurs in E. siliculosus, which is a dioecious species. Fusion in this species occurs between gametes from different plants. The fusing gametes are morphologically identical, but differ in their sexual behavior. One is less active and often called the female gamete. It becomes passive and motion-less after a short time. The more active male gametes cluster around and fix themselves to the body of the female gamete by forwardly directed flagella. Soon the anchoring flagellum of one of these gametes contract and its body is brought in contact with female gamete. Finally the two bodies fuse nucleus to nucleus and cytoplasm to cytoplasm to form the zygote

PLURILOCULAR GAMETANGIUM

The meiozoospores/gametes are produced inside the plurilocular gametangia borne on haploid plants (gametophytes).

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They are large, elongated, sessile or short-stalked multicellular structures. Morphologically plurilocular gametangia are similar in structure and development to plurilocular sporangia, but are produced by haploid or gametophytic plants.

These gametophytic plants in certain species are physiologically of two types, but morphologically similar. The plurilocular gametangia produce haploid gametes. During their development, the terminal cell of the lateral branchlet gets inflated, followed by repeated transverse divisions to produce a vertical row of flat cells. This is followed by longitudinal and transverse divisions, resulting in the formation of several hundred small cubical cells, arranged in 20-40 transverse tiers.

GAMETE

Each cell gives rise to one, sometimes two, biflagellate pyriform gametes, which in structure are similar to zoospores but slightly smaller in size than them. The gametes are liberated from the gametangia, following the same procedure as in the zoospore liberation from the plurilocular sporangia.

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FERTILIZATION

Fertilisation is external in brown algae, involving fusion of naked gametes that have been released into the surrounding sea water. During fertilization many male gametes encircle the female gamete and get entangled to it by the anterior large flagellum. This stage is called clump formation.

Out of the many, only one male gamete fuses with the female gamete, and the remaining gametes go astray and gradually get destroyed.

The uniting gametes then form the zygote through plasmogamy and karyogamy.

Germination of zygote

The zygote undergoes germination without going into a resting stage. There is no ZYGOTIC meiosis. On germination, it develops into a sporophytic (2n) plant, which is morphologically similar to the haploid or gametophytic plant. The resultant sporophyte/diploid thallus produces in some species (E. siliculosus) both plurilocular and unilocular sporangium. But in some species (E. reptans) either of the two sporangia is produced.

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Life Cycle: This typical life cycle of Ectocarpus exhibits morphologically identical filaments representing sporophyte and gametophyte— isomorphic alternation of generations.

The filament produced by the germination of haploid zoospore bears plurilocular reproductive structure which produces gametes, is the gametophyte, and its cells are haploid.

While the filament/thallus formed by the direct germination of the zygote bearing unilocular sporangia and plurilocular sporangia is the sporophyte having diploid cells.

Reduction division takes place during the production of zoospores in the unilocular sporangia which behave as zoosporangia. So, SPOROGENIC meiosis takes place.

Thus, the diploid thallus is concerned with asexual reproduction and haploid thallus with sexual reproduction.

There is alternation of two distinct vegetative individuals/ thalli (gametophye and sporophyte) having not only distinct chromosome number but also distinct functions. So, life cycle is DIPLOHAPLONTIC.

………………………………………………………………….End of Ectocarpus

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