Difference Between Sexual and Asexual Reproduction

Main Difference – Sexual vs Asexual Reproduction

Sexual and asexual reproduction are two mechanisms that produce offsprings of the living organisms. During sexual reproduction, two types of gametes, known as male and female gametes, are formed inside the male and female reproductive organs, respectively. Diploid germs cells produce haploid gametes by the cell division process called meiosis. During asexual reproduction, diploid somatic cells are divided by mitosis, in order to produce new diploid daughter cells. The main difference between sexual and asexual reproduction is that sexual reproduction utilizes meiosis in the cell division and fusion of haploid gametes in order to produce the diploid zygote whereas asexual reproduction utilizes mitosis as their cell division mechanism, maintaining a uniform ploidy throughout all cell generations.   

This article explains, 

1. What is Sexual Reproduction
      – Definition, Characteristics, Types, Examples
2. What is Asexual Reproduction
      – Definition, Characteristics, Types, Examples
3. What is the difference between Sexual and Asexual Reproduction

What is Sexual Reproduction

Sexual reproduction is the fusion of two morphologically distinct types of gametes, which are called male and female gametes, in order to form a diploid zygote. Male gamete is small and is known as the sperm. Female gamete is large and is known as the ovum or egg. Each gamete is haploid and is formed through a process called meiosis. Meiosis only occurs in eukaryotes. During meiosis, chromosomal crossing over occurs at synapsis via points called chiasmata. The recombination of non-sister chromatids leads to the genetic variation in the producing gametes. Genetic variation promotes the evolution by producing new traits. Two rounds of cell divisions occur during meiosis, producing four haploid gametes from a single diploid germ cell.

Fertilization is the event where the two gametes are fused to form the diploid zygote. A human somatic cell contains 46 chromosomes which can be divided into two homologous sets; one bears a maternal origin and the other bears a paternal origin. By the law of independent assortment, one set, which containing 23 chromosomes, bearing both maternal and paternal origins separates into one gamete. Independent assortment of chromosomes in the genome while the formation of gametes also promotes the genetic variation during sexual reproduction. During fertilization, the fusion of a sperm with an ovum regenerates the diploid status, consisting of 46 chromosomes in the zygote. Sexual cycle of eukaryotes is shown in figure 1.

Figure 1: Sexual Cycle

Finding a mate for the sexual reproduction is known as sexual selection, which promotes the natural selection in evolution.

Types of Sexual Reproduction

Sexual Reproduction of Bacteria and Archaea

Prokaryotes usually reproduce by asexual reproduction. But, lateral gene transfer, which occurs during conjugation, transformation and transduction are considered as sexual reproduction mechanisms.

Sexual Reproduction of Fungi

In fungi, resting spores are produced by sexual reproduction. These spores are used to survive during harsh conditions. Three phases can be identified in the sexual reproduction of fungi: plasmogamy, karyogamy, and meiosis. During plasmogamy, the two parent cells are fused by their cytoplasm. The two nuclei of those fused cells are then fused during karyogamy. Finally, during meiosis, haploid gametes are produced, which are then developed into spores. A fungus emitting spores is shown in figure 2.

Figure 2: Puffballs Emitting Spores

Sexual Reproduction of Plants

Bryophytes like liverworts, mosses, and hornworts consist of motile sperms with flagella. Hence, they need water for reproduction. The life cycle of these plants consists of a haploid spore, which grows into the dominate form of the life cycle. The haploid dominate is known as the gametophyte, which is a photosynthesizing multicellular body, consisting of leaf-like structures. This multicellular body consists of antheridia, producing haploid gametes by mitosis. Fertilization of gametes produces a diploid zygote. The zygote divides by the mitotic division, producing the sporophyte. Spore capsules are produced in the sporophyte. They produce spores by meiosis.

In ferns, diploid sporophyte produces spores. Spores germinate to produce gametophytes, which produces sperms and eggs. Sperms swim through a film of water in order to fertilize the egg. Produced zygote grows into a new sporophyte.         

Flowers are the reproductive organs of the flowering plants. Pollen grains, which contains the male gametophyte are produced in the anther. Female gametophyte is located in the ovary. Fertilized zygote is developed into a fruit containing seeds. A syrphid fly, pollinating a flower is shown in figure 3. 

Figure 3: Pollination of Flowers by Insects

Sexual Reproduction of Animals

In insects, males produce spermatozoa and females produce ova. Fertilization produces the zygote. Higher animals like mammals consist of complex reproductive organs in order to produce gametes, fertilize the gametes and develop the zygote into a new birth.

What is Asexual Reproduction

Asexual reproduction is the production of offspring from a single organism, inheriting identical genes only form that parent. Hence, no gametes are formed and no fertilization is involved in the formation of a new organism. Asexual reproduction is mostly found in lower life forms like bacteria and archaea. Asexual reproduction can be observed in fungi and plants as well. Asexual reproduction can form generations rapidly compared to sexual reproduction.

Types of Asexual Reproduction

Various types of asexual reproduction mechanisms can be identified like fission, budding, vegetative propagation, sporogenesis, fragmentation, and agamogenesis.

Fission

Two types of fission can be identified: binary fission and multiple fission. Parent organism is replaced by two daughter organisms in binary fission. Bacteria and archaea mostly show binary fission. Multiple fission occurs in protists. The nucleus is divided several times to produce multiple daughter cells.

Budding

Some fungi like baker’s yeast produce protrusions to produce a daughter cell from the mother cell. Hydra also asexually reproduces by budding. Growing into a mature individual breaks away the daughter organism from the mother organism.  

Vegetative Propagation

During vegetative propagation, plants asexually reproduce without forming seeds or spores. Formation of plantlets on leaves of Kalanchoe, formation of new plants from rhizomes or stolon in strawberry, and formation of bulbs in tulip or tubers in dahlia are examples of vegetative propagation. Vegetative plantlets in Kalanchoe are shown in figure 4.  

Figure 4: Kalanchoe plantlets on the leave

Sporogenesis

Plants and algae produce spores during their asexual reproduction by a process called sporic meiosis. The germination of the spores produces haploid gametophyte. Gametophyte produces gametes by mitosis. Fertilization of gametes produces the zygote, which ultimately forms the sporophyte. 

Fragmentation

The formation of a new organism from a fragment of the parent organism is called fragmentation. Each fragment is capable of developing into a new organism. Planarians, annelids and starfish show fragmentation. Some plants like liverworts contain structures like gemma, which are specialized to reproduce via fragmentation. A starfish, regenerating its legs by fragmentation is shown in figure 5.

Figure 5: Starfish Regenerating its Legs

Agamogenesis

Any form of reproduction which does not involve male gametes is known as agamogenesis. Parthenogenesis and apomixis are examples for agamogenesis. In parthenogenesis, unfertilized eggs are developed into new individuals. Rotifers, aphids, water fleas, some ants, bees, stick insects, amphibians, and reptiles exhibit parthenogenesis. Formation of a new sporophyte without fertilization in plants is called as apomixis. The formation of seeds without fertilization is a common example for apomixis. An aphid, giving birth to a live young by parthenogenesis is shown in figure 6.

Figure 6: Parthenogenesis in aphid

Difference Between Sexual and Asexual Reproduction

Type of Organisms

Sexual Reproduction: Sexual reproduction is found in almost all the animals, plants and other life forms including fungi, bacteria, and protists.

Asexual Reproduction: Asexual reproduction is found in lower animals and plants, fungi, protozoans, and bacteria.

Number of Parents

Sexual Reproduction: Sexual reproduction is a bi-parental process.

Asexual Reproduction: Asexual reproduction is a uni-parental process.

Formation of Gametes

Sexual Reproduction: Male and female gametes are formed during sexual reproduction.

Asexual Reproduction: Gametes are not formed during asexual reproduction.

Reproductive Units

Sexual Reproduction: Germ cells act as reproductive units during sexual reproduction.

Asexual Reproduction: Somatic cells act as reproductive units during asexual reproduction.

Fertilization

Sexual Reproduction: Fertilization of male and female gametes occurs in order to obtain the zygote.

Asexual Reproduction: No fertilization occurs during asexual reproduction.

Ploidy

Sexual Reproduction: During meiosis, haploid gametes are produced from diploid germ cells. Fusion of gametes regenerates the diploid zygote.

Asexual Reproduction: Chromosomes are diploid throughout the process.

Mitosis/Meiosis

Sexual Reproduction: Meiosis is involved in cell division and mitosis continues the process during sexual reproduction.

Asexual Reproduction: Mitosis, fission, budding and regeneration are involved in the cell division during asexual reproduction.

Type

Sexual Reproduction: Meiosis, syngamy, and conjugation are involved in the sexual reproduction.

Asexual Reproduction: Budding, vegetative reproduction, fragmentation and spore production are the types of asexual reproduction.

Genetic Variation

Sexual Reproduction: Chromosomal crossing over lets the genetic recombination to occur, introducing genetic variations to the offspring.

Asexual Reproduction: Daughter cells are genetically identical to their parents due to the involvement of mitosis during the cell division.

Contribution to Evolution

Sexual Reproduction: Genetic variation among the offspring during sexual reproduction allows the evolution to proceed.

Asexual Reproduction: Asexual reproduction allows the continuity of genetic information through the progeny.

Efficiency of the Process

Sexual Reproduction: Sexual reproduction produces their offspring less rapidly. 

Asexual Reproduction: Asexual reproduction is involved in the rapid production of offspring in a short time period.

Progeny

Sexual Reproduction: Progeny of sexual reproduction is very healthy.

Asexual Reproduction: Progeny of asexual reproduction is healthy or little healthy.

Lifespan

Sexual Reproduction: The cells undergoing sexual reproduction are mortal.

Asexual Reproduction: The cells undergoing asexual reproduction are considered immortal.

Reproductive Organs

Sexual Reproduction: Prominent male and female reproductive organs are required for sexual reproduction.

Asexual Reproduction: Reproductive organs are not required for asexual reproduction.

Conclusion

Sexual and asexual reproduction are the two major forms of reproductions found in organisms. Sexual reproduction involves the production of haploid gametes by meiosis, which is followed by the fertilization of two morphologically distinct gamete in order to regenerate the diploid zygote. However, during asexual reproduction, a single parent is involved in the production of the offspring. Cell division in the asexual reproduction occurs through mitosis, maintaining a uniform ploidy throughout all the cell generations. Sexual reproduction is found in almost all the living forms including bacteria. Bacterial sexual reproduction occurs through conjugation. Asexual reproduction is mostly found in lower life forms like bacteria and archaea. Asexual reproduction can occur through fission, budding, vegetative propagation, sporogenesis, fragmentation and agamogenesis. The most important feature found in sexual reproduction of organisms is the contribution to the evolution. Genetic variations are introduced into the offspring by independent assortment of chromosomes and chromosomal cross over occurred during synapsis. These are the differences between sexual and asexual reproduction.

 Reference:
1. “Sexual reproduction.” Wikipedia. Wikimedia Foundation, 21 Mar. 2017. Web. 21 Mar. 2017.
2. “Asexual reproduction.” Wikipedia. Wikimedia Foundation, 17 Mar. 2017. Web. 21 Mar. 2017.

Image Courtesy:
1. “Sexual cycle” By Traced by User:Stannered – en:Image:Sexual cycle.png (CC BY-SA 3.0) via Commons Wikimedia
2. “Puffballs emitting spores” By Lesmalvern – Own work (CC BY-SA 4.0) via Commons Wikimedia
3. “Eristalinus October 2007-6” By Alvesgaspar – Own work (CC BY-SA 3.0) via Commons Wikimedia
4. “Bryophyllum daigremontianum nahaufnahme2” By Photographer: CrazyD, 26 Octobre 2005 – Own work (CC BY-SA 3.0) via Commons Wikimedia
5. “Sea star regenerating legs” By Brocken Inaglory (CC BY-SA 3.0) via Commons Wikimedia
6. “Aphid-giving-birth” By MedievalRich (CC BY-SA 3.0) via Commons Wikipedia

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