Sexual reproduction is a mode of reproduction involving the fusion of haploid female gamete (egg cell) and haploid male gamete (sperm cell). The fusion of these gametes occurs at fertilization resulting in the formation of a diploid zygote. The zygote develops into an individual organism that is genetically distinct from the parent organisms. This is in contrast to asexual reproduction where an organism reproduces without involving gametes and the resulting offspring is a clone of the parent. Nonetheless, sexual reproduction has the advantage over asexual reproduction in increasing genetic variation and expanding the gene pool. Furthermore, it ensures that the chromosome number of a particular species will remain the same across generations. Two major types of sexual reproduction are syngamy and conjugation.
Sexual reproduction definition
Reproduction refers to a process whereby parent(s) reproduce another individual (offspring), often of the same species. It is one of the features characterizing a living thing. And by sexual, it means the reproduction involves the coming together of genetic material from two parents so as to produce an offspring. Two haploid gametes unite resulting in a genetically distinct diploid offspring.
The term sexual comes from the Late Latin sexualis, from sexus, meaning "of copulation or generation". The other term reproduction came from the Latin re, meaning "again" + productio, meaning "production". Synonym: syngenesis.
Sexual and asexual reproduction
There are two modes of reproduction: sexual and asexual. Sexual reproduction incorporates fundamental processes such as gametogenesis and fertilization. In brief, gametogenesis is a biological process that incorporates cell divisions, particularly mitosis and meiosis, to produce gametes (sex cells). It begins with the primordial cells dividing mitotically and then each of them will undergo meiosis. Meiosis is a form of cell division wherein a cell goes through two sequential cell divisions. As such, it gives rise to four haploid cells. Each haploid cell, then, undergoes maturation to become fully-differentiated gamete (sex cell). In females, the mature gamete is an egg cell (also called ovum). In males, the mature gamete is a sperm cell (also called spermatozoon). The male gamete usually has to find and swim its way to the ovum. In higher forms of animals, the male and the female usually of the same species engage in sexual intercourse to bring the gametes closer, for union. The sperm cell upon finding the egg cell will penetrate it so that their genetic material could combine as one full set (diploid). This union marks fertilization and the fertilized egg undergoes series of mitotic divisions to give rise to an embryo comprised of diploid cells.
Asexual reproduction is different from sexual reproduction in a way that an offspring has been produced through asexual means. The process is different as it does not incorporate meiosis and fertilization. Furthermore, the parent organism produces an offspring even in the absence of a mate. The offspring though is a clone of the parent. For comparison, see Table 1.
|Sexual reproduction||Asexual reproduction|
|Basic biological processes: gametogenesis (mitosis and meiosis), copulation, and fertilization||Basic biological process: mitosis|
|Involves sex cells||Does not involve sex cells|
|Offspring is genetically distinct from the parents||Offspring is a clone of the parent|
|Higher genetic variations but requires more energy in finding a mate, mating, and carrying and caring for progeny||Lower genetic variations but requires less energy as it does not need to find a mate to reproduce|
|Types are syngamy and conjugation||Types are budding, spore formation, fragmentation, and vegetative reproduction|
|Examples: mostly, higher organisms||Examples: mostly, lower organisms|
Types of sexual reproduction
Two major types of sexual reproduction are syngamy and conjugation.(1) Syngamy refers to the fusion of haploid sex cells resulting in the formation of a diploid zygote. Thus, in essence, syngamy refers to fertilization. It is the most common type of reproduction in multicellular sexual populations, including humans. Conjugation is different from syngamy in such a way that two organisms come together in a temporary fusion (e.g. by a cytoplasmic bridge) to exchange micronuclear material. This can be observed among single-celled organisms, such as bacteria, protozoans, and single-celled fungi. While syngamy is a permanent fusion of the two cells, conjugation is a temporary fusion of two cells.
Sexual reproduction in single-celled organisms
In single-celled organisms such as bacteria, sexual reproduction is done by conjugation. It is when two bacterial cells join together transiently to transfer genetic material via the plasmid of the donor cell to the recipient cell. The plasmid may either be solitary or part of a chromosome. Bacterial conjugation is essential to bacteria. It is through it that they can acquire a gene, which may be beneficial for their survival. For instance, the acquired gene might be a novel characteristic that enables the recipient cell to thrive in a rather harmful condition. Or, it may be a gene enabling the recipient to utilize a new metabolite. It is also through this process that resistance to antibiotics can be transferred from one bacterial cell to another. In protozoans, conjugation is also the process whereby two protozoans, e.g. ciliates, come together in a temporary fusion to exchange micronuclear material, then separate, each being a fertilized cell. This is also what basically occurs in other single-celled organisms. In certain algae and fungi, a male gamete unites with a female gamete resulting in the union of their nuclei and the subsequent formation of a zygote.
As for viruses, some references suggested that they were capable of sexual reproduction as there had been evidence of genetic recombination between different individual viruses. This interchange of genes was likened to the crossing over between homologous chromosomes during meiosis in higher life forms.(2) Nonetheless, the idea that viruses reproduce sexually is still contested to this day as some would not even consider viruses as a life form.
Syngamy in multicellular organisms
In multicellular organisms, the type of sexual reproduction is syngamy. It is generally a two-step process. The first step is plasmogamy. It pertains to the union of the cytoplasms of the two gametes. The second step, karyogamy, pertains to the union of nuclei of the two gametes. This union results in a single cell with two sets of chromosomes. At this point, the resulting diploid cell is called a zygote. The zygote then divides mitotically to form an embryo. This means that every parent cell component of the embryo gives rise to two daughter cells, each possessing two sets of chromosomes. There are many ways to classify syngamy. See Table 2.
|Based on the site of fertilization:|
A type of syngamy wherein the egg cell is fertilized by a sperm cell inside the body of one of the parents (usually female)
A type of syngamy that occurs outside the parent bodies.
|Based on the source of gametes:|
Also called self-fertilization, i.e. the two gametes came from one parent. This form of syngamy is uniparental and the parent is described as hermaphrodite for possessing two functional sex organs. Taenia is an example of a hermaphrodite organism.
Also called cross-fertilization, i.e. the two gametes came from two parents. This form of syngamy is biparental. This is the more common form of syngamy than endogamy.
|Based on the structure of the gametes:|
Gametes of males and females are morphologically and physiologically alike (e.g. in Monocystis).
Gametes of males and females are morphologically and physiologically different. It is more common than isogamy. The male gamete is relatively smaller than the female gamete. Thus, they are referred to as microgamete and macrogamete, respectively.
|Hologamy – when gametes are similar to the somatic cells in terms of size and shape, e.g. in yeasts.|
The plants have a life cycle comprised of two generations, the gametophyte and the sporophyte. The gametophyte generation is the sexual phase of the plant life cycle. It starts at a haploid spore that undergoes mitosis to give rise to a haploid gametophyte that bears the sex organs. The sex organs, in turn, produce gametes that will participate later in fertilization. The union of male and female gametes gives rise to a diploid zygote that later develops into a sporophyte. The sporophyte generation is the phase in the plant life cycle that starts off with the zygote up to the time that spores are produced. The sporophyte produces spores within the sporangium through meiosis. In tracheophytes (vascular plants), the sporophyte is the dominant form of the plant and is in the multicellular form. In contrast, the dominant form of the bryophytes, such as mosses and liverworts, is the gametophyte. The plant sex organ is the flower. The plant that bears both male and female reproductive organs is called monoecious. Conversely, the plant that bears only one type of reproductive organ is called dioecious. In flowering plants, the female reproductive organ is the pistil whereas the male reproductive organ is the anther. The pistil contains the ovary, which in turn, contain ovules. Inside the ovules are the egg cells. The anther bears the pollen grains. Inside the pollen grains are the sperm cells.
The sperm cells in the pollen have to reach the ovule and this is facilitated by pollination. There are two types of pollination: self-pollination and cross-pollination. Self-pollination occurs especially in monoecious flowers since the male and female organs are present in a single flower. In cross-pollination, the pollen is transferred from the male flower to the female flower. The transfer may be facilitated by wind or by insects. Based on the mode of pollination, the types of sexual reproduction in plants are autogamy (for self-fertilization) and allogamy (for cross-fertilization). Allogamy is the more common type of reproduction among higher plants.
For pollination to occur, the pollen sticks to the stigma of the pistil and grows a tube through the style of the pistil to reach the carpel containing the ovule. Fertilization occurs when the sperm cell fertilizes the egg cell whereas another sperm cell fertilizes the endosperm nuclei. Thus, the zygote will be diploid whereas the endosperm will be triploid from the union of a sperm cell and two female cells. The zygote develops into an embryo whereas the endosperm develops into nutritive tissue surrounding the embryo within the seed.
Most animals reproduce sexually. Because finding a mate is essential in reproducing by sexual means most animals display sexual dimorphism, sexual selection, and courtship rituals. Sexual dimorphism refers to the occurrence of two sexually distinct forms such that the male differs morphologically from the female of the same species. For example, male birds have colorful plumage compared with the plumage of the female birds. Female birds choose a mate based on desirable qualities. Mate selection and courtship rituals are ostensible in other animals as well, including humans.
Sexual reproduction in humans naturally is by sexual means only. The process entails courtship and mate selection, copulation, pregnancy, childbirth, and prenatal care. The partner chooses a potential mate essentially based on the qualities that ensure siring an offspring. The couple engages in sexual intercourse for internal fertilization to take place. The semen containing sperm cells is released by the male into the female's reproductive organ. Only a single sperm would be able to fertilize a viable ovum produced immediately by meiosis. The haploid sex cells form the diploid zygote that will next undergo mitosis to become an embryo. The embryo then develops organs and become a fetus inside the female womb. The fetus receives nutrients inside the mother's womb via an umbilical cord. After the gestation period (usually, about 266 days), the female gives birth by pushing the fetus out of the birthing canal. The newly born child, then, receives nutrition by lactation. Postnatal care continues until the child becomes independent.
One of the major sexual reproduction advantages is to ensure that the chromosome number of a species remains across generation. Humans, for instance, have 46 chromosomes. Half of it comes from the father and the other half from the mother. Prior to the union of the sperm cell and the egg cell, the gametes undergo meiosis to produce haploid gametes. Because of haploidy, the union of the gametes keeps up the number of chromosomes the same for all somatic cells. The sperm cell contains 23 chromosomes and the ovum has also 23 chromosomes. When they combine at fertilization, the zygote that develops into a new individual will have the same total number of chromosomes, 46. Another advantage of sexual reproduction is greater genetic variation. During meiosis, genetic recombination and interchange of genes occurring between homologous chromosomes occurs. This ensures that the newly formed zygote, while containing the original number of chromosomes, will possess a genome that is genetically distinct from, and not a clone of, either parent. This improves gene pool as it increases the chance of acquiring qualities that better equip species for survival and natural selection. Because of the pressure of acquiring superior qualities over the others in order to be selected or to be able to secure a mate for sexual reproduction, species are eventually driven to evolve.
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- Sexual Dimorphism. (2019). Retrieved from Nau.edu website: https://www2.nau.edu/~gaud/bio300b/sexdi.htm
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- Animal Reproductive Strategies. (2018, February 5). Retrieved from Biology 1520 website: http://bio1520.biology.gatech.edu/growth-and-reproduction/animal-reproduction-i-mating-systems/
- Sexual Selection. (2019). Retrieved from Stanford.edu website: https://web.stanford.edu/group/stanfordbirds/text/essays/Sexual_Selection.html
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