Sex

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Sex is the trait that determines whether a sexually reproducing organism produces male or female gametes.[1][2][3] During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent. By convention, organisms that produce smaller, more mobile gametes (spermatozoa, sperm) are called male, while organisms that produce produce larger, non-mobile gametes (ova, often called egg cells) are called female.[4] An organism, such as most flowering plants, that produces both types of gamete is a hermaphrodite.[3][5]

The male and female of a species may be physically alike (sexual monomorphism) or have physical differences (sexual dimorphism) that reflect various reproductive pressures on each sex. Sexual selection or mate choice can accelerate the evolution of differences between the sexes.

The terms 'male' and 'female' typically do not apply in sexually undifferentiated species in which the individuals are isomorphic (look the same) and the gametes are isogamous (indistinguishable in size and shape), such as the green alga Ulva lactuca. Some kinds of functional differences between individuals, such as in fungi,[6] may be referred to as mating types.[7]

There are several sex-determination systems. Most mammalian species have the XY sex-determination system, where the male usually carries an X and a Y chromosome (XY), and the female usually carries two X chromosomes (XX). Other chromosomal sex-determination systems in animals include the ZW system in birds, and the XO system in insects. Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans.[8]

Sexual reproduction

Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to eukaryotes. Genetic traits are encoded in the deoxyribonucleic acid (DNA) of chromosomes. The eukaryote cell has a set of paired homologous chromosomes, one from each parent, and this double-chromosome stage is called "diploid". During sexual reproduction, a diploid organism produces specialized haploid sex cells called gametes via meiosis,[9] each of which has a single set of chromosomes. Meiosis involves a stage of genetic recombination via chromosomal crossover, in which regions of DNA are exchanged between matched pairs of chromosomes, to form new chromosomes, each with a new combination of the genes of the parents. Then the chromosomes are separated into single sets in the gametes. Each gamete in the offspring thus has half of the genetic material of the mother and half of the father.[10] The combination of chromosomal crossover and fertilization, bringing the two single sets of chromosomes together to make a new diploid zygote, results in a new organism that contains a different set of the genetic traits of each parent.

In animals, the haploid stage only occurs in the gametes, the haploid cells that are specialized to fuse to form a zygote that develops into a new diploid organism. In a plant species, the diploid organism produces a type of haploid spore by meiosis that is capable of undergoing repeated cell division to produce a multicellular haploid organism. In either case, the gametes may be externally similar (isogamy) as in the green alga Ulva or may be different in size and other aspects (anisogamy).[11] The size difference is greatest in oogamy, a type of anisogamy in which a small, motile gamete combines with a much larger, non-motile gamete.[12]

In anisogamic organisms, by convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces large gametes is female, and one that produces small gametes is male.[13] An individual that produces both types of gamete is a hermaphrodite. In some species, a hermaphrodite can self-fertilize and produce an offspring on its own.

Animals

Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.[14] The gametes of animals have male and female forms—spermatozoa and egg cells, respectively. These gametes combine to form embryos which develop into new organisms.

The male gamete, a spermatozoon (produced in vertebrates within the testes), is a small cell containing a single long flagellum which propels it.[15] Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called fertilization.

Female gametes are egg cells. In vertebrates, they are produced within the ovaries. They are large, immobile cells that contain the nutrients and cellular components necessary for a developing embryo.[16] Egg cells are often associated with other cells which support the development of the embryo, forming an egg. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.

Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in the water can mate using external fertilization, where the eggs and sperm are released into and combine within the surrounding water.[17] Most animals that live outside of water, however, use internal fertilization, transferring sperm directly into the female to prevent the gametes from drying up.

In most birds, both excretion and reproduction are done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".[18] In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. In humans and other mammals, this male organ is known as the penis, which enters the female reproductive tract (called the vagina) to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen (a fluid containing sperm) travels. In female mammals, the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within (a process called gestation).

Because of their motility, animal sexual behavior can involve coercive sex. Traumatic insemination, for example, is used by some insect species to inseminate females through a wound in the abdominal cavity—a process detrimental to the female's health.

Plants

Like animals, land plants have specialized male and female gametes.[19][20] In seed plants, male gametes are produced by reduced male gametophytes that are contained within pollen which have hard coats that protect the male gamete forming cells during transport from the anthers to the stigma. The female gametes of seed plants are contained within ovules. Once fertilized, these form seeds which, like eggs, contain the nutrients necessary for the initial development of the embryonic plant.


The flowers of flowering plants contain their sexual organs. Most flowering plants are hermaphroditic, with both male and female parts in the same flower or on the same plant in single sex flowers, about 5% of plant species have individual plants that are one sex or the other.[21] The female parts, in the center of a hermaphroditic or female flower, are the pistils, each unit consisting of a carpel, a style and a stigma. Two or more of these reproductive units may be merged to form a single compound pistil, the fused carpels forming an ovary. Within the carpels are ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: these consist of long filaments arranged between the pistil and the petals that produce pollen in anthers at their tips. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a pollen tube that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed.

Some hermaphroditic plants are self-fertile, but plants have evolved multiple different self-incompatibility mechanisms to avoid self-fertilization, involving sequential hermaphroditism, molecular recognition systems and morphological mechanisms such as heterostyly.[22]

In pines and other conifers, the sex organs are produced within cones that have male and female forms. Male cones are smaller than female ones and produce pollen, which is transported by wind to land in female cones. The larger and longer-lived female cones are typically more durable, and contain ovules within them that develop into seeds after fertilization.

Because seed plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Some flowering plants have heavier, sticky pollen that is specialized for transportation by insects or larger animals such as hummingbirds and bats, which may be attracted to flowers containing rewards of nectar and pollen. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.

Fungi

Most species of fungus can reproduce sexually and have life cycles with both haploid and diploid phases. These species of fungus are typically isogamous, i.e. lacking male and female specialization. One haploid fungus grows into contact with another, and then they fuse their cells. In some cases, the fusion is asymmetric, and the cell which donates only a nucleus (and no accompanying cellular material) could arguably be considered male.[23] Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.[24]

Some fungi, including baker's yeast, have mating types that determine compatibility. Yeasts with the same mating types will not fuse with each other to form diploid cells, only with yeast carrying another mating type.[25]

Many species of higher fungi produce mushrooms as part of their sexual reproduction. Within the mushroom, diploid cells are formed, later dividing into haploid spores.

Sex-determination systems

XY sex determination

Humans and most other mammals have an XY sex-determination system: the Y chromosome carries factors responsible for triggering male development, making XY sex determination mostly based on the presence or absence of the Y chromosome. It is the male gamete that determines the sex of the offspring.

ZW sex determination

In birds, which have a ZW sex-determination system, the W chromosome carries factors responsible for female development, and default development is male. In this case, ZZ individuals are male and ZW are female. It is the female gamete that determines the sex of the offspring. This system is used by birds, some fish, and some crustaceans.

The majority of butterflies and moths also have a ZW sex-determination system. Females can have Z, ZZW, and even ZZWW.

XO sex determination

In the XO sex-determination system, males have one X chromosome (XO) while females have two (XX). All other chromosomes in these diploid organisms are paired, but organisms may inherit one or two X chromosomes. This system is found in most arachnids, insects such as silverfish (Apterygota), dragonflies (Paleoptera) and grasshoppers (Exopterygota), and some nematodes, crustaceans, and gastropods.

ZO sex determination

In the ZO sex-determination system, males have two Z chromosomes whereas females have one. This system is found in several species of moths.

Sex differences

Anisogamy is the fundamental difference between male and female.[26][27] Richard Dawkins has stated that it is possible to interpret all the differences between the sexes as stemming from this.[28]

Sex differences in humans include a generally larger size and more body hair in men, while women have larger breasts, wider hips, and a higher body fat percentage. In other species, there may be differences in coloration or other features, and may be so pronounced that the different sexes may be mistaken for two entirely different taxa.[29]

Sexual dimorphism

The common hill myna is sexually monomorphic, meaning that the external appearance of males and females is very similar.[30]]] In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called sexual dimorphism.[31] Sexual dimorphism in animals is often associated with sexual selection—the mating competition between individuals of one sex vis-à-vis the opposite sex.[29] In many cases, the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. Other examples demonstrate that it is the preference of females that drives sexual dimorphism, such as in the case of the stalk-eyed fly.[32]

Females are the larger sex in a majority of animals.[31] For instance, female southern black widow spiders are typically twice as long as the males.[33] This size disparity may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm: larger females are able to produce more eggs.[34][31]

In birds, males often have a more colorful appearance and may have features (like the long tail of male peacocks) that would seem to put them at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle. This hypothesis argues that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.

Sex differences in behavior

The sexes across dioecious (non-hermaphrodites) species usually differ in behavior. In most animal species females invest more in parental care,[35] although in some species, such as some coucals, the males invest more parental care.[36] Females also tend to be more choosy for who they mate with,[37] such as most bird species.[38] Males tend to be more competitive for mating than females.[39]

  1. ↑ Stevenson A, Waite M (2011). Concise Oxford English Dictionary: Book & CD-ROM Set. OUP Oxford. p. 1302. ISBN 978-0-19-960110-3. Retrieved 23 March 2018. Sex: Either of the two main categories (male and female) into which humans and most other living things are divided on the basis of their reproductive functions. The fact of belonging to one of these categories. The group of all members of either sex.
  2. ↑ Mills, Alex (1 January 2018). Biology of Sex. University of Toronto Press. pp. 43–45. ISBN 978-1-4875-9337-7. Retrieved 3 October 2023.
  3. ↑ 3.0 3.1 Purves WK, Sadava DE, Orians GH, Heller HC (2000). Life: The Science of Biology. Macmillan. p. 736. ISBN 978-0-7167-3873-2. Retrieved 23 March 2018. A single body can function as both male and female. Sexual reproduction requires both male and female haploid gametes. In most species, these gametes are produced by individuals that are either male or female. Species that have male and female members are called dioecious (from the Greek for 'two houses'). In some species, a single individual may possess both female and male reproductive systems. Such species are called monoecious ("one house") or hermaphroditic.
  4. ↑ Royle NJ, Smiseth PT, Kölliker M (2012). Kokko H, Jennions M (eds.). The Evolution of Parental Care. Oxford University Press. p. 103. ISBN 978-0-19-969257-6. The answer is that there is an agreement by convention: individuals producing the smaller of the two gamete types – sperm or pollen – are males, and those producing larger gametes – eggs or ovules – are females.
  5. ↑ Avise JC (2011). Hermaphroditism: A Primer on the Biology, Ecology, and Evolution of Dual Sexuality. Columbia University Press. pp. 1–7. ISBN 978-0-231-52715-6. Retrieved 18 September 2020.
  6. ↑ Moore D, Robson JD, Trinci AP (2020). 21st Century guidebook to fungi (2 ed.). Cambridge University Press. pp. 211–228. ISBN 978-1-108-74568-0.
  7. ↑ Kumar R, Meena M, Swapnil P (2019). "Anisogamy". In Vonk J, Shackelford T (eds.). Encyclopedia of Animal Cognition and Behavior. Cham: Springer International Publishing. pp. 1–5. doi:10.1007/978-3-319-47829-6_340-1. ISBN 978-3-319-47829-6. Anisogamy can be defined as a mode of sexual reproduction in which fusing gametes, formed by participating parents, are dissimilar in size.
  8. ↑ Hake L, O'Connor C. "Genetic Mechanisms of Sex Determination | Learn Science at Scitable". www.nature.com. Archived from the original on 19 August 2017. Retrieved 13 April 2021.
  9. ↑ Alberts et al. (2002), "V. 20. Meiosis", U.S. NIH, V. 20. Meiosis
  10. ↑ Alberts et al. (2002), U.S. National Institutes of Health, "V. 20. The Benefits
  11. ↑ Gilbert (2000), "1.2. Multicellularity: Evolution of Differentiation". 1.2.Mul NIH.
  12. ↑ Allaby M (2012). A Dictionary of Plant Sciences. OUP Oxford. p. 350. ISBN 978-0-19-960057-1.
  13. ↑ Gee, Henry (22 November 1999). "Size and the single sex cell". Nature. Archived from the original on 11 October 2017. Retrieved 4 June 2018.
  14. ↑ Alberts et al. (2002), "3. Mendelian genetics in eukaryotic life cycles", U.S. NIH, 3. Mendelian/eukaryotic Archived 2 April 2017 at the Wayback Machine.
  15. ↑ Alberts et al. (2002), "V.20. Sperm", U.S. NIH, V.20. Sperm Archived 29 June 2009 at the Wayback Machine.
  16. ↑ Alberts et al. (2002), "V.20. Eggs", U.S. NIH, V.20. Eggs Archived 29 June 2009 at the Wayback Machine.
  17. ↑ Alberts et al. (2002), "V.20. Fertilization", U.S. NIH, V.20. Fertilization Archived 19 December 2008 at the Wayback Machine.
  18. ↑ Ritchison, G. "Avian Reproduction". Eastern Kentucky University. Archived from the original on 12 April 2008. Retrieved 3 April 2008.
  19. ↑ Gilbert SF (2000). "Gamete Production in Angiosperms". Developmental Biology (6th ed.). Sunderland (MA): Sinauer Associates. ISBN 978-0-87893-243-6. Archived from the original on 21 April 2021. Retrieved 17 April 2021.
  20. ↑ Dusenbery DB (2009). Living at Micro Scale: The Unexpected Physics of Being Small. Harvard University Press. pp. 308–326. ISBN 978-0-674-03116-6.
  21. ↑ "Plants, sex & Darwin | University of Oxford". www.ox.ac.uk. Retrieved 10 January 2024.
  22. ↑ Judd, Walter S.; Campbell, Christopher S.; Kellogg, Elizabeth A.; Stevens, Peter F.; Donoghue, Michael J. (2002). Plant systematics, a phylogenetic approach (2 ed.). Sunderland MA: Sinauer Associates Inc. ISBN 0-87893-403-0.
  23. ↑ Nick Lane (2005). Power, Sex, Suicide: Mitochondria and the Meaning of Life. Oxford University Press. pp. 236–237. ISBN 978-0-19-280481-5.
  24. ↑ Watkinson SC, Boddy L, Money N (2015). The Fungi. Elsevier Science. p. 115. ISBN 978-0-12-382035-8. Retrieved 18 February 2018.
  25. ↑ Matthew P. Scott; Paul Matsudaira; Harvey Lodish; James Darnell; Lawrence Zipursky; Chris A. Kaiser; Arnold Berk; Monty Krieger (2000). Molecular Cell Biology (Fourth ed.). WH Freeman and Co. ISBN 978-0-7167-4366-8.14.1. Cell-Type Specification and Mating-Type Conversion in Yeast Archived 1 July 2009 at the Wayback Machine
  26. ↑ de la Filia A, Bain S, Ross L (June 2015). "Haplodiploidy and the reproductive ecology of Arthropods" (PDF). Current Opinion in Insect Science. 9: 36–43. doi:10.1016/j.cois.2015.04.018. hdl:20.500.11820/b540f12f-846d-4a5a-9120-7b2c45615be6. PMID 32846706. S2CID 83988416. Archived (PDF) from the original on 25 June 2021. Retrieved 25 June 2021.
  27. ↑ Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press. pp. 141–143 – via Internet Archive.
  28. ↑ Hamilton, W. D. (1967). "Extraordinary Sex Ratios: A Sex-ratio Theory for Sex Linkage and Inbreeding Has New Implications in Cytogenetics and Entomology". Science. 156 (3774): 477–488. Bibcode:1967Sci...156..477H. doi:10.1126/science.156.3774.477. JSTOR 1721222. PMID 6021675.
  29. ↑ 29.0 29.1 Kobayashi, Kazuya; Hasegawa, Eisuke; Yamamoto, Yuuka; Kazutaka, Kawatsu; Vargo, Edward L.; Yoshimura, Jin; Matsuura, Kenji (2013). "Sex ratio biases in termites provide evidence for kin selection". Nat Commun. 4: 2048. Bibcode:2013NatCo...4.2048K. doi:10.1038/ncomms3048. hdl:2123/11211. PMID 23807025.
  30. ↑ {{#if:Robin |{{#if: |[[{{{authorlink}}}|{{#if: Robin |Robin{{#if: V. V. |, V. V. }} |{{{author}}} }}]] |{{#if:Robin |Robin{{#if: V. V. |, V. V. }} |{{{author}}} }} }} }}{{#if:Robin |{{#if: | ; {{{coauthors}}} }} }}{{#if:2011 | (2011) |{{#if: |{{#if: | ({{{month}}} {{{year}}}) | ({{{year}}}) }} }} }}{{#if:Robin | . }}{{#if:Robin2011 | }}{{#ifeq: | no | | {{#if: |“|"}} }}{{#if:https://www.jstor.org/stable/24078632 |Determining the sex of a monomorphic threatened, endemic passerine in the sky islands of southern India using molecular and morphometric methods |Determining the sex of a monomorphic threatened, endemic passerine in the sky islands of southern India using molecular and morphometric methods }}{{#ifeq: | no | | {{#if:|”|"}} }}{{#if: | (in {{{language}}}) }}{{#if: | ({{{format}}}) }}{{#if:Current Science |. Current Science }}{{#if:101 | 101 }}{{#if:5 | (5) }}{{#if:676–679 |: 676–679 }}{{#if: |. {{#if: |{{{location}}}: }}{{{publisher}}} }}{{#if: |. doi:{{{doi}}} }}{{#if:0011-3891 |. ISSN 0011-3891 }}{{#if: |. PMID {{{pmid}}} }}{{#if: |. Bibcode: {{{bibcode}}} }}{{#if: |. OCLC {{{oclc}}} }}{{#if: |. {{{id}}} }}{{#if: |. Retrieved on [[{{{accessdate}}}]]{{#if: | , [[{{{accessyear}}}]] }} }}{{#if: | Retrieved on {{{accessmonthday}}}, {{{accessyear}}} }}{{#if: | Retrieved on {{{accessdaymonth}}} {{{accessyear}}} }}{{#if: |. [{{{laysummary}}} Lay summary]{{#if: | – {{{laysource}}}}} }}{{#if: | ([[{{{laydate}}}]]) }}.{{#if:Many species of birds are, however, monomorphic and difficult to sex visually, particularly in the field and some even in hand. Some examples are the Hill Mynah, Gracula religiosa and the Black-capped Chickadee, Parus atricapillus. | “Many species of birds are, however, monomorphic and difficult to sex visually, particularly in the field and some even in hand. Some examples are the Hill Mynah, Gracula religiosa and the Black-capped Chickadee, Parus atricapillus.” }}
  31. ↑ 31.0 31.1 31.2 Davis, Devra Lee; Gottlieb, Michelle and Stampnitzky, Julie; "Reduced Ratio of Male to Female Births in Several Industrial Countries" in Journal of the American Medical Association; April 1, 1998, volume 279(13); pp. 1018-1023
  32. ↑ CIA Fact Book". The Central Intelligence Agency of the United States. Archived from the original on 13 June 2007.
  33. ↑ Whitfield J (June 2004). "Everything you always wanted to know about sexes". PLOS Biology. 2 (6): e183. doi:10.1371/journal.pbio.0020183. PMC 423151. PMID 15208728. One thing biologists do agree on is that males and females count as different sexes. And they also agree that the main difference between the two is gamete size: males make lots of small gametes—sperm in animals, pollen in plants—and females produce a few big eggs.
  34. ↑ Pierce BA (2012). Genetics: A Conceptual Approach. W.H. Freeman. p. 74. ISBN 978-1-4292-3252-4.
  35. ↑ Drees BM, Jackman J (1999). "Southern black widow spider". Field Guide to Texas Insects. Houston, Texas: Gulf Publishing Company. Archived from the original on 31 August 2003. Retrieved 8 August 2012 – via Extension Entomology, Insects.tamu.edu, Texas A&M University.
  36. ↑ Stuart-Smith J, Swain R, Stuart-Smith R, Wapstra E (2007). "Is fecundity the ultimate cause of female-biased size dimorphism in a dragon lizard?". Journal of Zoology. 273 (3): 266–272. doi:10.1111/j.1469-7998.2007.00324.x.
  37. ↑ Shaw AJ (2000). "Population ecology, population genetics, and microevolution". In Shaw AJ, Goffinet B (eds.). Bryophyte Biology. Cambridge: Cambridge University Press. pp. 379–380. ISBN 978-0-521-66097-6.
  38. ↑ Schuster RM (1984). "Comparative Anatomy and Morphology of the Hepaticae". New Manual of Bryology. Vol. 2. Nichinan, Miyazaki, Japan: The Hattori botanical Laboratory. p. 891.
  39. ↑ Lehtonen J, Kokko H, Parker GA (October 2016). "What do isogamous organisms teach us about sex and the two sexes?". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 371 (1706). doi:10.1098/rstb.2015.0532. PMC 5031617. PMID 27619696.
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