Ectocarpus: Classification, Structure and Reproduction

Table of Contents

Ectocarpus is a filamentous brown alga belonging to the family Ectocarpaceae. It has a small mature thallus and a complete life cycle; for this reason, it serves as a model organism for the study of genomics of multicellular organisms. It is a marine alga that is found all over the world, along the shorelines in temperate regions.

Classification

Habitat

The species of Ectocarpus can be found growing as epiphytes on other organisms or on rocky substrates along a shoreline. They are commonly found attached to the substrate with the help of rhizoids, but some are also found floating on water.

Structure of Ectocarpus

• The two most commonly studied species of the genus include Ectocarpus siliculosus and Ectocarpus fasciculatus.
• The thallus of Ectocarpus is profusely branched with uniseriate heterotrichous filaments, such that it consists of both prostrate and erect branches.
• The erect branches are arranged end-to-end, such that they are polysiphonous in nature. Sometimes, the erect branches may grow vertically to form more than one row of cells.
• The prostrate branches grow apically in contrast with the erect branches that have an intercalary growth.
• The prostrate branches have irregularly branched filaments and remain attached to the substratum with the help of rhizoids.
• The cells of the filament are rectangular or cylindrical in shape and uninucleate in nature.
• The cell wall is thick and is composed of pectin and cellulose.
• The presence of pigment fucoxanthin gives the alga its characteristic brown colour.

Reproduction

Ectocarpus undergoes both sexual and asexual reproduction.

Asexual Reproduction

Asexual reproduction in Ectocarpus takes place by biflagellated zoospores that are produced in both unilocular and plurilocular sporangia. While unilocular sporangia give rise to haploid zoospores, plurilocular sporangia form diploid zoospores.

1. Unilocular Sporangia: Unilocular sporangia are formed on the tips of Ectocarpus branchlets. The tip gradually increases in size and becomes elliptical in shape, functioning as a sporangial initial. The cell divides by meiotic and equational divisions to form 32-64 haploid nuclei.

There is no wall formation after the division of nuclei. But a protoplast is formed around each haploid nucleus that transforms into biflagellated haploid zoospores. The zoospores are uninucleate and pyriform and have two flagella that are unequal and laterally inserted.

The zoospores are released from the sporangium via a terminal pore on the sporangia. They swim in the water, retract their flagella, and get attached to a substratum. The zoospores then give rise to a new gametophytic plant that bears plurilocular gametangia.

2. Plurilocular Sporangia: Similar to unilocular sporangia, plurilocular sporangia are also borne as terminal cells on branchlets of sporophytic plants. The sporangial initial enlarges to become spherical in shape. It is divided by transverse divisions to arrange itself in a layer of 5-12 cells. It then undergoes repeated transverse and vertical divisions to form cubical cells arranged in layers of 20-40 cells. This multicellular structure is known as the plurilocular sporangia.

Each diploid unicellular cell of the sporangia transforms into biflagellated zoospores that are similar to haploid zoospores produced in unilocular sporangia. The zoospores are released from sporangia, which germinate into diploid sporophytic thallus that bears unilocular and plurilocular sporangia after maturation.

The plurilocular sporangia plays no role in alternation of generation.

Sexual Reproduction

Sexual reproduction in Ectocarpus is either isogamous or anisogamous. Most of the species are isogamous and homothallic, while some are anisogamous and heterothallic. The gametes are produced in plurilocular gametangia. These gametangia develop in a similar way to plurilocular sporangia.

The gametangia are multicellular, sessile, with a short stalk and elongated conical structure. The gametangia develop as a terminal cell on a lateral branch. It grows in size and divides by transverse meiotic divisions to produce a vertical row of flat cells.

These rows of cells divide by repeated transverse and longitudinal divisions to form hundreds of small cubical cells that get arranged in transverse rows of 24-40 layers. Each cell transforms into a biflagellate, pyriform and haploid gamete. The gametes are very similar to zoospores produced in asexual reproduction, the only exception being flagella, which are relatively smaller in size. The gametes release through an apical pore from the gametangium.

The gametes fuse to form a diploid zygote. The zygote does not undergo meiotic divisions but rather develops into a diploid sporophyte that bears unilocular and plurilocular sporangia.

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