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Sexual reproduction

In the first stage of sexual reproduction, meiosis, the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During fertilisation, haploid gametes come together to form a diploid zygote, and the original number of chromosomes is restored.

Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid).[1] This is typical in animals, though the number of chromosome sets and how that number changes in sexual reproduction varies, especially among plants, fungi, and other eukaryotes.[2][3]

In placental mammals, sperm cells exit the penis through the male urethra and enter the vagina during copulation,[4][5] while egg cells enter the uterus through the oviduct. Other vertebrates of both sexes possess a cloaca for the release of sperm or egg cells.

Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants.[6][7] Sexual reproduction also occurs in some unicellular eukaryotes.[2][8] Sexual reproduction does not occur in prokaryotes, unicellular organisms without cell nuclei, such as bacteria and archaea. However, some processes in bacteria, including bacterial conjugation, transformation and transduction, may be considered analogous to sexual reproduction in that they incorporate new genetic information.[9] Some proteins and other features that are key for sexual reproduction may have arisen in bacteria, but sexual reproduction is believed to have developed in an ancient eukaryotic ancestor.[10]

In eukaryotes, diploid precursor cells divide to produce haploid cells in a process called meiosis. In meiosis, DNA is replicated to produce a total of four copies of each chromosome. This is followed by two cell divisions to generate haploid gametes. After the DNA is replicated in meiosis, the homologous chromosomes pair up so that their DNA sequences are aligned with each other. During this period before cell divisions, genetic information is exchanged between homologous chromosomes in genetic recombination. Homologous chromosomes contain highly similar but not identical information, and by exchanging similar but not identical regions, genetic recombination increases genetic diversity among future generations.[11]

During sexual reproduction, two haploid gametes combine into one diploid cell known as a zygote in a process called fertilization. The nuclei from the gametes fuse, and each gamete contributes half of the genetic material of the zygote. Multiple cell divisions by mitosis (without change in the number of chromosomes) then develop into a multicellular diploid phase or generation. In plants, the diploid phase, known as the sporophyte, produces spores by meiosis. These spores then germinate and divide by mitosis to form a haploid multicellular phase, the gametophyte, which produces gametes directly by mitosis. This type of life cycle, involving alternation between two multicellular phases, the sexual haploid gametophyte and asexual diploid sporophyte, is known as alternation of generations.

The evolution of sexual reproduction is considered paradoxical,[12] because asexual reproduction should be able to outperform it as every young organism created can bear its own young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation.[13] This 50% cost is a fitness disadvantage of sexual reproduction.[14] The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. However, one definite advantage of sexual reproduction is that it increases genetic diversity and impedes the accumulation of harmful genetic mutations.[15][11]

Sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates for sexual reproduction.[16][failed verification][17] It has been described as "a powerful evolutionary force that does not exist in asexual populations".[18]

  1. ^ John Maynard Smith & Eörz Szathmáry, The Major Transitions in Evolution, W. H. Freeman and Company, 1995, p 149
  2. ^ a b Chalker, Douglas (2013). "Epigenetics of Ciliates". Cold Spring Harbor Perspectives in Biology. 5 (12): a017764. doi:10.1101/cshperspect.a017764. PMC 3839606. PMID 24296171. Archived from the original on 2022-09-13. Retrieved 2022-09-13 – via Cold Spring Harbor.
  3. ^ Can Song, ShaoJun Liu (2012). "Polyploid Organisms". Science China Life Sciences. 55 (4): 301–311. doi:10.1007/s11427-012-4310-2. PMID 22566086. S2CID 17682966.
  4. ^ Naguib, Marc (2020-04-19). Advances in the Study of Behavior. Academic Press. ISBN 978-0-12-820726-0.
  5. ^ Birkhead, Tim R.; Møller, Anders Pape (1998-08-12). Sperm Competition and Sexual Selection. Elsevier. ISBN 978-0-08-054159-4.
  6. ^ Nieuwenhuis, Bart (October 19, 2016). "The frequency of sex in fungi". Philosophical Transactions B. 371 (1706). doi:10.1098/rstb.2015.0540. PMC 5031624. PMID 27619703.
  7. ^ Woods, Kerry (June 19, 2012). "Flowering Plants". Encyclopedia of Life. Archived from the original on September 13, 2022. Retrieved September 12, 2022.
  8. ^ Knop, Michael (2011). "Yeast cell morphology and sexual reproduction – A short overview and some considerations". Comptes Rendus Biologies. 334 (8–9): 599–606. doi:10.1016/j.crvi.2011.05.007. PMID 21819940. Archived from the original on 2022-09-13. Retrieved 2022-09-13 – via Elsevier Science Direct.
  9. ^ Narra, Hema (September 5, 2015). "Of What Use Is Sex to Bacteria?". Current Biology. 16 (17): R705–R710. doi:10.1016/j.cub.2006.08.024. PMID 16950097. S2CID 18268644.
  10. ^ Goodenough, Ursula (March 1, 2014). "Origins of Eukaryotic Sexual Reproduction". Cold Spring Harbor Perspectives in Biology. 6 (3): a016154. doi:10.1101/cshperspect.a016154. PMC 3949356. PMID 24591519.
  11. ^ a b "DNA Is Constantly Changing through the Process of Recombination". Nature. 2014. Archived from the original on September 15, 2022. Retrieved September 14, 2022.
  12. ^ Otto, Sarah (2014). "Sexual Reproduction and the Evolution of Sex". Scitable. Archived from the original on 28 January 2019. Retrieved 28 Feb 2019.
  13. ^ John Maynard Smith The Evolution of Sex 1978.
  14. ^ Ridley, M. (2004) Evolution, 3rd edition. Blackwell Publishing, p. 314.
  15. ^ Hussin, Julie G; Hodgkinson, Alan; Idaghdour, Youssef; et al. (4 March 2015). "Recombination affects accumulation of damaging and disease-associated mutations in human populations". Nature Genetics. 47 (4): 400–404. doi:10.1038/ng.3216. PMID 25685891. S2CID 24804649. Archived from the original on 20 January 2021. Retrieved 7 March 2021.
  16. ^ Cecie Starr (2013). Biology: The Unity and Diversity of Life (Ralph Taggart, Christine Evers, Lisa Starr ed.). Cengage Learning. p. 281.
  17. ^ Vogt, Yngve (January 29, 2014). "Large testicles are linked to infidelity". Phys.org. Archived from the original on November 12, 2020. Retrieved January 31, 2014.
  18. ^ Agrawal, A. F. (2001). "Sexual selection and the maintenance of sexual reproduction". Nature. 411 (6838): 692–695. Bibcode:2001Natur.411..692A. doi:10.1038/35079590. PMID 11395771. S2CID 4312385.

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