Reproduction in Algae | Vegetative Reproduction | Sexual Reproduction | Asexual Reproduction | Types of Sexual Reproduction

 Reproduction in Algae

 

Algae are photosynthetic organisms that are thalloid. It is mostly found near water bodies. Algae can be found in unicellular form or can be multicellular. It is a very important marine photosynthetic organism. Algae can be classified based on the mode of their reproduction. There are three main types of reproduction in algae:

1. Vegetative reproduction
2. Asexual reproduction 
3. Sexual reproduction

 

1) Vegetative Reproduction: 

• Vegetative reproduction is the most common and simple type of reproduction of algae in which any part of the body can reproduce.
• In this type of reproduction, there is no formation of specialized cell structures or organs. 
• It can also be said as fragmentation because when any part of an algae breaks it seems like fragments. 
• Sometimes, vegetative reproduction or fragmentation is not considered the type of reproduction because it occurs accidentally. 
• In this type of reproduction, any part of the body of algae breaks (accidentally), and then this broken part starts to divide. 
• Fragmentation is not age-dependent. It can be seen at any age of algae.
• As algae are thalloid organisms, not a specific part of algae shows this kind of reproduction. The whole body of algae shows the vegetative mode of reproduction.
• As it occurs mostly accidentally and no specific part of algae is involved in this reproduction, most of the scientists did not consider it as the type of reproduction. According to them, in vegetative reproduction, just simple cell division takes place in which cells divide and form new identical cells.
• Vegetative reproduction may take place in many ways like simple binary fission (mostly observed in unicellular algae), Production of Hormogia (a small motile filament that can generate a new filament), and Formation of Propagules.

 

2) Asexual Reproduction: 

• In the asexual mode of reproduction, the formation of specialized cells or organs can be seen. 
• These special cells or organs give rise to the new body. 
• Spore formation is the most common example of it. 
• Spores can be attached to the vegetative part of algae or can also be produced in a special type of cell structure called sporangia. 

Based on their motility, spores can be divided into two types:

1. Palnospores ( motile spores)
2. Akinetes ( non-motile spores)

 

Planospores: 

• Planospores are motile spores.
• They have flagella in their structure and with the help of these flagella, they can move and motility.
• These types of spores can also be classified based on the number of flagella they have. 
• These flagella can vary from one to eight in number. 
• The zoospores are the most common of them and a planospore has two flagella in its structure.
• Zoospores are produced in a specialized vegetative cell called zoosporangia by a process called zoosporogenesis. 
• Spores can germinate and can be converted into new individuals. 

Akinetes:

• Akinetes are non-motile spores because they have no flagella in their cell structure. 
• They mostly have a thick wall around them which makes them more resistive than Planospores. 
• They can withstand adverse environmental conditions. 
• Reserve food is also present in abundant quantity in akinetes. 
• Under favorable conditions, these spores give rise to a new individual. 

Akinetes can be further divided into six types:

1. Aplanospores: They are similar to zoospores. The only difference is that they lack flagella. They remain immotile throughout life. 
2. Hypnospores: They are thick-walled, non-motile spores. They are also called resting cysts because they undergo a resting period for a long time.
3. Statospores: Spores with ornamented walls produced for overwintering. They are not actively released from the fruiting body of algae. 
4. Autospores: Another example of non-motile spores is Aitospores. An example of autospores is Chlorella. 
5. Tetraspores: These spores are produced by the tetra sporophyte and it is characteristic of red algae. 
6. Auxospores: These spores are formed in diatom algae and they give rise to enlarged vegetative cells. 

 

3) Sexual Reproduction:

 

• Sexual reproduction is the fusion of gametes that comes from both male and female sides of algae.
• One gamete comes from the male sex organ called antheridium. 
• This gamete is usually small and motile. 
• The other gamete comes from the female part called archegonium. 
• This gamete is usually large and non-motile. 

Based on the formation of these gametes, sexual reproduction can be divided into: 

1. Monoecious algae: Male and female gametes are formed on the same thallus. 
2. Dioecious algae: Male and female gametes are formed on different thalli. 

Types of Sexual Reproduction.

Sexual reproduction can be seen in many ways. Some of them are discussed here:

Isogamous: 

• This is the most primitive type of sexual reproduction. 
• In this type, both male and female gametes are of the same size, shape, and structure. 
• We can’t differentiate them and because of this, we can’t be called male or female gametes. 
• Because of this reason, we call as plus (+) or minus (-) gametes. 

Conjugation: 

• Conjugation is another type of sexual reproduction in which two different organisms come close to each other and exchange their genetic materials.
• The exchange of genetic material is through a tube called a sex pill. 
• After the exchanging of materials, both organisms get separated from each other.
• In this type, one organism has an extra genetic material that is specified for the conjugation process. This organism is called a donor because it donates that genetic material. 
• The second organism that receives this material is called a receiver because it receives it. 
• The receiver organism then produces a new individual after cell division. 

Autogamous: 

• Autogamy is also known as the self-fertilization of algae.
• In the autogamous type of sexual reproduction, a single individual of algae produces two gametes.
• The fusion of these two gametes can lead to the formation of a zygote.
• This type of sexual reproduction is seen mostly in red algae, green algae, and brown algae.   

Oogamous: 

• Oogamous is the most advanced type of reproduction in algae. 
• In this type, there is the fusion of a large female part which is non-motile (egg), with a smaller motile male part (sperm). 
• The eggs are formed within a structure called oogonium. 
• The sperms are also formed in a special structure called antheridium. 
• The egg contains well-developed chloroplasts and many reserve materials. 
• The sperms have less developed chloroplasts and few reserve materials.
• Sometimes chemotaxis is observed, in which mature egg cells release some chemical substance that attracts sperms towards them. 
• This results in the fertilization of the egg with sperm which ends with the formation of a zygote. 
•  After fertilization, the egg forms a thick wall around it which makes it more resistant. 
• The fertilized egg (or zygote) also has some food in the stored form which helps to survive for a long time. 
• The zygote in this form is called a zygospore or oospore. 
• Under favorable conditions, the zygote germinates to give rise to a new individual body.