Asexual Reproduction in Animals

Introduction

The development of new individuals from a lone parent is known as asexual reproduction. This type of reproduction is seen in single-celled organisms. There is no gamete fusion in this process, and a single parent cell splits into two or even more daughter cells. Clones are genetically  identical offsprings of any organism.

Characteristics of Asexual Reproduction in Animals 

The following are the important characteristics of asexual reproduction in animals:

• There is no gamete production or fertilisation
• There is only one parent involved
• The procedure takes relatively less time
• There is no variety in the children generated; they are  replicas of the parent
• The offspring develops at a rapid rate

Types of Asexual Reproduction in Animals

Fission, fragmentation, budding, and agamogenesis are the different types of asexual reproduction in animals. Spore development is observed in plants, algae, and fungi.

Fission

A parent splits into two or more individuals of almost equal size in fission. Single-celled creatures such as bacteria, archaea, and unicellular eukaryotes like protists, as well as some fungi, use this kind of reproduction. When a single cell splits into two daughter cells, it is called a binary fission, this division occurs in favourable conditions. Amoeba undergoes binary fission . But when a single cell splits into many daughter cells, it is called a multiple fission, this division occurs in unfavourable conditions. Plasmodium shows multiple fission.

Fragmentation

In this process, the body is divided into many fragments, which eventually evolve into whole creatures. This reproduction method is frequently seen in worms, fungi, and plants, in addition to starfish. Many of these species can reproduce sexually as well. Most lichens, formed when a fungus and photosynthetic algae or cyanobacteria develop a symbiotic association, reproduce by fragmentation. This ensures that both symbionts are present in the new individuals.

Budding

In this process, a bud-like outgrowth develops on the parent body which resembles the parent organism. Through budding, organisms reproduce by splitting off new individuals from existing ones, resulting in genetically similar parent and daughter organisms. The bud may remain linked to the parent, or it may break loose. Budding occurs in eukaryotic species such as single-celled yeast and multicellular Hydra.

Agamogenesis

Any reproduction process that does not include a male gamete is known as agamogenesis. 

Parthenogenesis and apomixis are two types  of agamogenesis. Parthenogenesis is the process of asexual reproduction observed in animals where female gametes develop into embryos and  grow, without being fertilised. Aphids, rotifers, nematodes, and other invertebrates, and also numerous plants and reptiles like the Komodo dragon, naturally undergo parthenogenesis. Apomixis is asexual reproduction in which any cell form the embryo sac inside the ovary of the pistil develops into embryo (no fusion of gametes) and eventually the ovary transforms into fruit. 

Animals That Also Reproduce Sexually

Sharks

These sea predators were originally supposed to reproduce only through sexual reproduction. However, scientists have documented multiple captive female zebra sharks and hammerhead sharks breeding. Through DNA tests, it was discovered that its genome was similar to that of the mother, ruling out the already improbable notion that they had been keeping male sperm for many years. This type of asexual reproduction in animals is unlikely to be found in the wild since it limits the genetic diversity accessible to children, potentially leading to inbreeding within a few generations. Nonetheless, this is certainly a useful tendency to have in situations of reproductive shortage.

Komodo Dragons

The Komodo dragon is the world’s largest asexually reproducing vertebrate, measuring around 10 feet long and weighing 300 pounds. It was not known that they could breed asexually for a long time until two lone females at London’s Chester Zoo became pregnant in 2006, causing a stir in the scientific community. 

A clever approach is used to achieve this type of asexual reproduction. The female dragons normally produce four “pre-egg” cells when they reproduce sexually. The egg originates from one of these, while the other three are sucked back into the system. However, when a female generates asexually, either of these wasted eggs can act as a surrogate sperm, supplying genetic material to the genuine egg.

Whiptail lizards

The whiptail lizard is a female-only species that reproduces by generating an egg through parthenogenesis. The whiptail is a lizard family that is mainly found in the Americas. At first appearance, they appear to be no different from any other reptile. After years of meticulous research, we now know that certain species are entirely female and have the incredible capacity to breed asexually. This is not just a clever ploy or a backup plan; it’s their primary method of reproduction.

Aphids

Winged and non-winged female aphids can have genetically similar offspring. The aphid is a soft-bodied insect that feeds on the sap of the plants’ stems and leaves. To generate the next generation, many aphid species rotate among asexual reproduction in the summertime and sexual reproduction in the autumn and winter. Since, the asexual offspring are genetically identical to their mother, they are all born female. Male or female sexually generated offspring are possible.

Asexual Reproduction in Sponges

The pinacoderm, or outer dermal layer, comprises flat pinacocytes, and the choanoderm, or inner gastral layer, of flagellated collar cells or choanocytes, whose flagella beat and keep water flowing, making up the body wall of their sponges.

Porifera have calcareous spicules, siliceous spicules, or sponging threads as an endoskeleton.

Depending on the endoskeletal elements, they are categorised into three categories:

• Calcarea or Calcispongiae is a class of bacteria

• Hexactinellida or Hyalospongiae is a Hexactinellida or Hyalospongiae class

• Desmospongiae is a class of bacteria

They have two pores: countless small Ostia and one or a few significant osculates that operate as water stream inlets and outputs. Their canal system is unusual, with a central cavity known as the perigastric cavity or spongocoel.

The majority of sponges are either bisexual, hermaphrodite, or monoecious. Internal fertilisation takes place, and the development occurs in a free-swimming amphiblastula larva or parenchymal larvae.

Fragmentation, external budding, and gemmules are all examples of asexual reproduction. The internal bud of the freshwater sponge Spongilla is known as Gemmule. It comprises a mass of archaeocytes encased in an Amphi disc spicule coat. It aids in the process of perennation. These behave as sponge spores, and their creation is known as gemmation. Gemmules can also be found in marine sponges such as Ficulina, Tethya, and others.

Conclusion

In multicellular creatures, particularly mammals, a total lack of sexual reproduction is uncommon. It is unclear why the ability to procreate sexually is so widespread among them. 

According to current hypotheses, asexual reproduction in animals may provide short-term benefits when population increase is vital or in stable circumstances. In contrast, sexual reproduction provides a net benefit by allowing for faster development of genetic diversity and adaptation to environmental changes.

Few animals have entirely abandoned sexual reproduction in their life cycles, which could be due to developmental restrictions. Meiosis is maintained in almost all asexual mechanisms of reproduction, in either a modified form or as an alternative pathway. The lack of meiosis, as well as the protective recombinational healing of DNA damage provided by meiosis, is another barrier to moving from sexual to asexual reproduction.