Natural Cloning

The Difference Between Natural and Artificial Cloning

While artificial and natural cloning both lead to the same result, with the formation of a clone, meaning an organism with an identical or almost identical, genome to another organism, the circumstances of this creation differ greatly between the two. The main difference between the two is that natural cloning does not involve human intervention, whereas artificial cloning is a genetic engineering technique.

Artificial and Natural Cloning Definition

Examples of Natural Cloning

Natural cloning occurs through many mechanisms within nature, ranging from single-celled organisms to complex, multicellular organisms. Some of these mechanisms are outlined below and then explored in further detail. While this article does split the mechanisms into plants and animals, it is worth remembering that there are overlaps between the two in the methodologies used.

• Binary Fission
• Budding
• Fragmentation
• Parthenogenesis
• Vegetative Propagation
• Spore Production
• Monozygotic Twin Formation

Natural Cloning in Animals

Binary Fission

Binary fission is a form of asexual reproduction in which the parent organism splits itself into two daughter cells. It is the primary reproduction method of prokaryotes such as bacteria and archaea, however, it also occurs in flatworms and protists.

To begin the process, the cell gradually enlarges to have enough membrane to contain the cellular contents necessary for the two daughter cells. The cell then duplicates its genetic material before attaching each copy to opposing ends of the parent cell. Finally, the cell pinches itself off into two new cells by forming a new cell wall across the middle of the parent cell and then pulling the cell apart into the two new daughter cells.

Budding

Budding is a similar form of reproduction to binary fission, with one key difference. Instead of duplicating the DNA and then splitting the parent cell into two identically sized daughter cells, thereby eliminating the parent cell, the new cell grows as a small projection from the parent cell. It is a reproductive method used primarily by members of the animal genus Hydra, along with yeasts (a type of unicellular fungus). The genetic material of the organism is duplicated and then placed into the small outcropping of the cell membrane, which over time matures into a new cell and eventually detaches from the main body, forming a new smaller cell, and leaving the parent cell behind to reproduce again. This process is shown below in figure 1.

Figure 1. The yeast Saccharomyces cerevisiae reproducing via budding.

Fragmentation

Fragmentation is, as the name suggests, the formation of clonal organisms from smaller fragments of the parent organism. This may occur either intentionally, through the formation of sections of the organism designed to be intentionally shed or broken off, to later develop into new clones or unintentionally as a result of trauma to the organism. These fragments then regrow into two new whole organisms which are genetically identical to the organism the fragments come from, thereby creating clones. Fragmentation occurs in many organisms, including starfish, some worms, sponges and fungi.

Parthenogenesis

Parthenogenesis is a form of reproduction in which a gamete develops into a new organism without undergoing fertilisation. This usually occurs with female gametes, however, does also occur with male gametes. There are many subtypes of parthenogenesis, not all of which result in clonal organisms. Parthenogenesis occurs in plants, insects and many types of animals. Organisms that use this form of reproduction may do so in an obligate manner (because they don't have any other way), or a facultative manner, where the reproductive method used changes depending on environmental conditions.

Monozygotic Twin Formation

Twins can occur in several ways; however, only monozygotic twins are genetically identical and therefore technically clones. Monozygotic twins occur when an egg cell is fertilised during sexual reproduction, but instead of forming a single offspring as normal it splits into multiple offspring at the blastocyst stage, hence the term monozygotic, meaning multiple offspring from one zygote. This is not limited to twins and can result in more than two identical offspring; however, this is a rare occurrence on top of the splitting of a zygote, which already only occurs in around 3 in 1000 births.

Natural Cloning in Plants

There are two main cloning methods that are used by plants, vegetative propagation and sporulation, however, sporulation is also used by organisms other than plants. Vegetative propagation is also closely related to the fragmentation outlined above in natural animal cloning.

Vegetative Propagation

Many plants can regrow whole new organisms from small sections of plant tissue in a process known as vegetative propagation. Plants contain tissue known as meristems, which contain a number of totipotent cells.

Plant species that possess larger amounts of meristematic tissue have an easier time reproducing through vegetative propagation. Plants may produce structures intentionally designed to take advantage of vegetative propagation, such as plantlets or runners (Figure 2), but clones can also arise through other parts of the plant that are removed, an ability that is also utilised to artificially clone plants!

Fig. 2 - An illustration of the formation of new plants from plant runners, with the initial parent at the right and the new plants to the left, at different growth stages

Sporulation

Spores are small, unicellular reproductive cells that may be used during asexual and sexual reproduction. They are generally more robust than the parent cell, allowing the population to survive environmental stresses that otherwise may not be survivable. They are not gametes because spores do not require fusion with another cell to create a new organism as spores formed during sexual reproduction are already fertilised by gamete fusion before formation.

Spores used in asexual reproduction are formed through mitosis, and when exposed to suitable environmental conditions develop into new organisms identical to the parent. This strategy is used in bacteria, plants, fungi, protozoa and algae.

Figure 3: Fern spores contained in structures known as sporangia, which when released create clones of the original fern.