Amoeba is well-known due to its ability for ever-changing shape. Its ability to change shape is due to the formation of pseudopodia (false feet), which are common in eukaryotic microbes.
Amoeba proteus does not reproduce sexually by mating, that is, the fusion of sex cells or gametes doesn’t occur. Their reproduction is essentially of asexual type and takes place by various methods such as binary fission, multiple mission, and sporulation.
They don’t always reproduce sexually because their type of environment is best suitable for reproducing asexually. But, there are amoeba species like Dictyostelium discoideum that can reproduce sexually if certain conditions are met.
Amoebas are very simple, and most primitive animalcules that are still living to date. These are tiny, single-celled organisms that live in moist conditions, such as fresh and saltwater, soil, and within animals.
A majority of the commonly found Amoeba species are said to asexually reproduce approximately every two days and, their reproduction rate depends on the species and environmental conditions.
Here in this post, we will talk about the reproduction of Amoeba with reference to the well-known and commonly studied species type, Amoeba proteus. It can be easily found on decaying bottom vegetation of freshwater streams and ponds and can be well-cultured for studying its reproduction mechanisms.
How Does Amoeba Reproduce? – Reproduction in Amoeba is of 4 types
1. Binary Fission
Binary fission is a process of asexual reproduction that occurs when the cell divides into two daughter cells. It is the most common mode of reproduction employed by Amoeba.
During the initiation phase of this binary fission, it is seen that the Amoeba prepares itself by withdrawing its pseudopodia and form a spherical shape.
And so, when the favourable conditions of water, temperature, and food availability are met, Amoeba divides by simple binary fission.
The binary fission division first involves the nuclear division which includes the duplication of the genetic contents of a parent cell by the process of replication, followed by karyokinesis (nuclear division). Then, the cytokinesis (cytoplasmic division) takes place that divides the parent cell into two equal daughter cells.
Mitosis cell division is observed in the nucleus, and the cytoplasm divides at the center of the cell and separates forming two daughter cells.
Due to the mitotic cell division, the two resulting daughter cells are also known as the identical clones of the parent cell.
2. Multiple Fission
This theory of reproduction in Amoeba was proposed by Scheel in 1899 and Carter in 1915.
Multiple Fission is usually performed under inactive conditions, that is when the favourable conditions of water, temperature, and food availability are not met.
While Binary fission means the formation of two daughter cells, Multiple fission means the formation of more than two daughter cells i.e. the production of multiple daughter cells.
One parent cell can lead to the formation of multiple daughter cells. These multiple daughter cells can divide by binary fission one after the other and are all clone of the parent cell.
Under inactive and adverse environmental conditions, like that during the shortage of food, Amoeba stops its activities and will retract its pseudopodia forming a spherical shape.
It will totally stop its movements and will protect itself by forming a coat around its body.
The Amoeba becomes rounded and gets embedded in an extensive, three-layered thick, and protective mucilaginous coat covering its whole body called cyst. This is also called the Encystment of Amoeba.
Inside the cyst, the nucleus repeatedly divides using mitosis cell division through fission to form several daughter nuclei, which arrange themselves near the periphery.
These newly formed numerous daughter individuals remain embedded within the mucilaginous mass of the cyst covering.
Inside the cyst, each daughter nucleus will become enveloped by a small amount of cytoplasm, thus forming a daughter Amoeba, called amoebula or pseudopodiospore.
Later, when the favourable conditions of water, temperature, and food availability are met, these daughter individuals called amoebula will soon escape from the cyst and will form very fine pseudopodia.
These amoebula will now grow into adult Amoeba and will lead an independent life.
This theory of sporulation in Amoeba is proposed by Taylor, and it states the formation of nearly asleep and inactive forms of amoeba as spores via sporulation.
Spores can preserve the amoeba’s genetic material when the conditions are harsh, adverse, and difficult for the normal form of an amoeba to lead life.
It is seen that during unfavourable conditions Amoeba proteus multiplies by sporulation without encystment.
Sporulation begins with the breakdown of the nuclear membrane in the parent cell’s cytoplasm into several small nuclear fragments called chromatin blocks.
Each of these blocks develops a nuclear membrane, and becomes surrounded by a little cytoplasm and also develops a spore case around it.
In simple words, each and every Chromatin block in the cytoplasm of the parent cell obtains a nuclear membrane and becomes a small daughter nucleus.
With the disintegration of the parent body, about 200 to 300 of such small spores are liberated, and each of these spores hatches into a small amoebula when the favourable conditions are met.
These amoebula will now grow into adult Amoeba and will lead an independent life.
The formation of such spores that is due to sporulation provides a multilayered structure to the amoeba that can be maintained for a long time, maybe for months of harsh conditions.
Spores are designed to protect an amoeba from extreme dryness, heat, and intense radiation for a long time, relative to the normal life span of the microorganism.
This is a type of fusion that occurs between two amoeba individuals and is often regarded as the sexual type of reproduction.
Although some observers have described the temporary fusion technique (also called conjugation), but it has not been confirmed by other workers yet.
Conjugation is described as a type of sexual process in which two amoeba individuals unite with a temporary fusion between them. This temporary fusion causes the exchange of nuclear material between the two individuals.
During this process, either the complete transfer of one individual’s nuclear contents to that of the other individual occurs, or the fusion of the two individual’s nuclear contents to form one individual Amoeba takes place. Either one can take place, but what takes place is not yet clear.
Within a given population of Amoeba proteus organisms, the various forms that may engage in conjugation are known as mating types.
It is well-said that this type of temporary union enables the two amoebae individuals to lead a more active and vigorous life. This phenomenon is often termed as rejuvenation.
The process of fission in Amoeba to form two daughter cells
The process of fission to form two form daughter cells in both binary and multiple fission processes include Mitosis (Stages: Prophase, Metaphase, Anaphase, and Telophase) and, later Karyokinesis and then Cytokinesis occurs.
The stages are as followed:
1. Prophase Stage: In this stage, all the nucleoli (endosomes) fuse together into a single nuclear body and each chromosome splits longitudinally into two daughter chromosomes or chromatids.
2. Metaphase Stage: In this stage, all the paired chromatids come to lie in a longitudinal plane. The microtubules are present in the nucleus but they do not form any spindle fibres.
3. Anaphase Stage: In this stage, the paired chromosomes separate and move towards their respective poles with mutual repulsion.
4. Telophase Stage: In this stage, the nuclear membrane begins to constrict longitudinally and gets deepened.
5. Karyokinesis: In this stage, the nuclear membrane fully deepens and finally separates the nucleus into two daughter nuclei.
6. Duplication of cell organelles: Organelles such as the mitochondria, contractile vacuole, endoplasmic reticulums, etc. duplicates.
7. Cytokinesis: In this stage, a longitudinal wrinkle appears in the cytoplasm of the cell with the two daughter nuclei in it. This eventually deepens and finally divides the cell into two daughter cells with one daughter nuclei and one previously duplicated cell organelles in each cell.
Sexual reproduction in Amoeba (With reference to Dictyostelium discoideum)
Just like all other amoebas that perform asexual reproduction, Dictyostelium discoideum is a soil living amoeba that also performs asexual reproduction all of the time.
But, it is seen that Dictyostelium discoideum is still capable of sexual reproduction if certain conditions are met.
There are 3 different mating types in Dictyostelium discoideum. These are Dictyostelium discoideum Type-I, Dictyostelium discoideum Type-II, and Dictyostelium discoideum Type-III. And it is also very important to note that each type can only mate with the two different types and not with its own.
When Dictyostelium discoideum is properly incubated with their bacterial food supply in a dark and wet environment, and with other better environmental conditions, it is seen that they will show Heterothallic sexual development processes.
In heterothallic mating, two amoebae of different mating types fuse together thus forming a giant zygote cell. This giant cell will then engulf all other neighbouring Dictyostelium discoideum cells in aggregate by releasing cAMP.
After ingesting all of its neighbors as an aggregate, the giant cell will encyst itself by encasing the whole aggregate in a thick, cellulose wall to protect it. This is known as a macrocyst.
Inside the macrocyst, the giant cell will undergo meiosis and mitosis division a number of times under the protection of the cyst layers. The giant cell divides first through meiosis, then through mitosis to produce many haploid amoebae.
When appropriate environmental conditions are met, the macrocyst will burst, releasing new Dictyostelium discoideum individuals. Inside each macrocyst, there can be more than 200 developed Dictyostelium discoideum individuals.
Since this process involves meiosis with the fusion of two different amoebae, the resulting daughter cells will be genetically distinct from the parent cells.