Cell Division and Reproduction

Asexual Reproduction

• Asexual reproduction occurs in unicellular forms, resulting in reproduction, and in multicellular forms, leading to growth.
• It takes place under favorable conditions.
• There are three types of asexual reproduction: mitosis, fragmentation, and akinetes.

Mitosis

• Mitosis results in two daughter cells, which usually separate immediately.
• Each daughter cell grows into a new individual.

Fragmentation

• Fragmentation occurs in multicellular forms with a filamentous thallus.
• It involves breaking of the filament into small portions of living cells called fragments.
• Each fragment grows into a new individual under favorable conditions.

Akinetes

• Akinetes are single-celled resting fragments, specially modified to function as a means of perennation and multiplication.
• They are formed either singly or in chains in the vegetative filament.
• Akinetes are gorged with reserve food materials and have a highly resistant cell wall.
• Each akinete grows into a new filament under suitable conditions.

Sexual Reproduction

• Sexual reproduction is oogamous.
• When zygotes are formed, meiosis occurs, followed by ordinary mitosis.
• Three of the resulting nuclei diminish, and the remaining one gives a motile cell that emerges from the zygote wall and swims for a short time.
• This cell ceases to move and undergoes successive divisions to form a new colony.

Order II: Chlorococcales

• This order is represented by three families: Chlorococcaceae, Protosiphonaceae, and Hydrodictyaceae.
• Hydrodictyaceae includes Pediastrum and Hydrodictyon.

Pediastrum

• Pediastrum is a planktonic, flat colony whose peripheral cells are provided with one or two protrusions.
• Asexual reproduction is accomplished by the formation of autocoenobia.
• Certain cells of the coenobium divide, forming zoospores that swim for a time and then aggregate and arrange themselves in a manner similar to that of the mother colony.

Sexual Reproduction in Pediastrum

• Sexual reproduction in Pediastrum is isogamous.
• Isogametes are produced in certain cells and fuse to produce a diploid zygote.
• The zygote germinates, producing zygospores that swim shortly, rest, and each give rise to a polyhedral cell that is responsible for the production of autocoenobium.

Hydrodictyon

• Hydrodictyon has a net-like structure, with cylindrical cells that have many nuclei.
• The chloroplast is reticulate and has a number of pyrenoids.
• Asexual reproduction is mainly through autocoenobia formation, as in Pediastrum.

Types of Sexual Reproduction

• Isogamous: both male and female gametes are morphologically similar but differ physiologically; it is considered primitive.
• Heterogamous: both male and female gametes differ morphologically and physiologically.
• Anisogamous: both fusing gametes are motile, but the male gametes are smaller in size.
• Oogamous: the most advanced type of sexual reproduction, common among higher algae; male gametes are motile and numerous, while female gametes are non-motile and larger in size.

Life Cycles in Algae

• There are three different types of life cycles, depending on when meiosis occurs, the type of cells produced, and if there is more than one free-living stage present in the life-cycle.