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Sexual Reproduction Definition
Sexual Reproduction defines the process through which organisms produce progeny that has variations as a result of the recombination process involved in gamete formation. Two individuals of different sexes contribute as parents, create new offspring by combining their genetic information.
This genetic information is present within the chromosomes that are present in the organelle nucleus of sex cells known as gametes. The gametes produced by the male are known as sperm while those produced by the female are known as egg or ovum.
During the process of sexual reproduction, the syngamy of these gametes leads to the production of a zygote that later develops into an embryo. This embryo has half its genomic content from one parent and the other half from the other.
For instance, in the case of humans, 46 chromosomes are present in an individual out of which 23 are from its father and the other 23 from its mother. The result is an organism that is not a clone of either of its parents but resembles both of them.
Sexual Reproduction and Fertilization
During this process, traits are passed down to the progeny. These traits include phenotypic traits that are required for adapting to their environment and thus directly influence their survival and genotypic traits like resistance etc. The genes responsible for these traits undergo selective pressures according to the environment they live in.
This evolutionary process of natural selection helps in the survival of individuals with favorable traits and these then reproduce. Sexual reproduction aids in increasing diversity or variation that works as the raw material for different evolutionary processes like natural selection. It differs remarkedly from asexual reproduction where only one parent is involved.
There is no syngamy in asexual reproduction and as a result, the offspring are the exact clones of their parent. This kind of reproduction can be seen in certain organisms like fungi, bacteria, corals, starfish, hydras, and plants like strawberries.
Types of Sexual Reproduction
Allogamy refers to the syngamy where the involved gametes are from 2 different individuals. Ovum or egg is the female gamete while sperm is the male gamete. These are specialized sex cells that are structured in a manner that enhances fertilization probability.
These cells are haploid and consist of 23 chromosomes in humans. When they fuse they form a diploid zygote that then undergoes mitosis to form an embryo. Mitosis is a type of cell division seen in cells after they have successfully replicated their DNA.
Animals have courtship rituals that help them to breed with an organism that will reproduce to produce fertile progeny that has genes that increase their fitness and survivability. The desirable traits include aggression, speed, good health, agility, etc.
As a result of sexual reproduction, phenotypic and genotypic variations can be seen. Based on these variations, the organisms choose their mate that exhibits the desirable phenotypic traits. These variations also increase competition as a result only the best animal that shows desirable traits are chosen and this is known as sexual selection.
This also leads to sexual dimorphism, wherein different sexes are distinct morphologically. The males are generally brightly colored and often have exaggerated body parts that help them to attract a mate. Whereas the females are camouflaged with dull colors as they are primary caretakers of their offspring.
But often these sexually selected traits may also affect the survivability of a species. For instance, bright coloration not only attracts potential mates but also predators. But these traits do persist in populations where mating is essential.
a. Internal Fertilization
This occurs within one of the parent’s bodies, where the gametes meet and fuse often via sexual intercourse. It generally happens in the female body, but rarely this can also be observed in the case of seahorses. In seahorse, the female will implant her gametes into the male where the fusion will occur. The further steps of this process are species-specific.
In some organisms like reptiles, insects, and birds, they may lay fertilized eggs, wherein the rest of development occurs. These eggs also contain yolk that helps to nourish the embryo. After a certain period, the developed young ones will hatch. This is referred to as oviparity. In the case of mammalian species, the embryo grows inside the body of their mother, and the baby is born live and this is known as viviparity.
In placental mammals the embryos are supported by the placenta, which helps to provide the embryo with nutrition, is also involved in thermoregulation, and also removes the waste products. While in marsupial offspring like a koala is removed from inside the mother’s body after completing a brief gestation period.
They then complete their development in an external pouch present in front of their mother’s body. Ovoviviparity represents a third type of development whereby the embryos grow and develop in eggs within the parent’s body and ultimately hatch inside and thus giving the appearance of live birth.
b. External Fertilization
Here the syngamy or fusion of gametes occurs outside the body of the parents. Most invertebrates, amphibians, and fish employ this method, where a large number of gametes are released synchronously in proximity from both the parents.
This quick release of the gametes into an aquatic medium is referred to as spawning. The females can also lay an egg on a substrate where the males will also release their gametes. These gametes have specialized adaptations for movement like the presence of strong flagella.
Here the female and male gametes that fuse are produced by a single individual. These species that can produce gametes of both sexes are known as hermaphrodites. It is similar to asexual reproduction, in the aspect that only 1 parent is involved, but unlike asexual reproduction the progeny produced have variations resulting from the recombination of chromosomes from the male and female gametes. Animals that reproduce by allogamy include earthworms and plants. Reproduction between hermaphrodites is possible and can in this case increase genetic diversity.
Advantages of Sexual Reproduction
Mitosis can result in accumulation of some harmful deleterious mutations over time. Sexual reproduction only allows a fraction of these harmful genes to be passed in the progeny unlike asexual reproduction. This leads to an increase in fitness that also increases their survivability.
This is further increased by natural selection, that eventually eliminates these genes from the population. More genetic variations helps to increase the adaptability of an organism to its environment through selection. Some mutations that are of no present use may prove beneficial in the future circumstances. This also acts as the driving force for speciation.
Sexual Reproduction Citations
- Thyroid hormones and female reproduction. Biol Reprod . 2018 Nov 1;99(5):907-921.
- Bisphenols in the pathology of reproduction. Ceska Gynekol . Winter 2019;84(2):161-165.
- Role of oxidative stress in female reproduction. Reprod Biol Endocrinol . 2005 Jul 14;3:28.