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 combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid).[John Maynard Smith & Eörz Szathmáry, The Major Transitions in Evolution, W. H. Freeman and Company, 1995, p 149] 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 .
In , sperm cells exit the penis through the male urethra and enter the vagina during copulation,
Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants.
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.
During sexual reproduction, two haploid gametes combine into one diploid cell known as a zygote in a process called fertilisation. 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,[John Maynard Smith The Evolution of Sex 1978.] This 50% cost is a fitness disadvantage of sexual reproduction.[Ridley, M. (2004) Evolution, 3rd edition. Blackwell Publishing, p. 314.] 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 .
Sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing Mating for sexual reproduction.
Evolution
The first
evidence of sexual reproduction in eukaryotes is from the
Stenian period, about 1.05 billion years old.
Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing the likelihood of the Mullers ratchet of deleterious mutations, increasing rate of adaptation to changing environments,
However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing .
Sexual reproduction allows these species to exhibit characteristics that depend on the specific environment that they inhabit, and the particular survival strategies that they employ.
Sexual selection
In order to reproduce sexually, both males and females need to find a
mating. Generally in animals
mate choice is made by females while males compete to be chosen. This can lead
to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a Fisherian runaway. Thus sexual reproduction, as a form of natural selection, has an effect on
evolution. Sexual dimorphism is where the basic
vary between males and females of the same
species. Dimorphism is found in both
and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation,
behavior and other bodily traits. However, sexual selection is only implied over an extended period of time leading to sexual dimorphism.
[Dimijian, G. G. (2005). Evolution of sexuality: biology and behavior. Proceedings (Baylor University. Medical Center), 18, 244–258.]
Animals have different ways of going about sexual selection. One common example is with male peacocks fanning out their wings in order to show all their colors and attract a female mate. Lions with bigger and fuller manes are more likely to attract a female mate. Male deer with larger antlers are more likely to gain a female mate. These are just few of many examples in nature that show how sexual selection would be used in nature when females are choosing a mate.
Animals
Arthropods
Insects
Insect species make up more than two-thirds of all
Extant taxon animal species. Most insect species reproduce sexually, though some species are facultatively
parthenogenetic. Many insect species have sexual dimorphism, while in others the sexes look nearly identical. Typically they have two sexes with males producing spermatozoa and females ova. The ova develop into eggs that have a covering called the
chorion, which forms before internal fertilization. Insects have very diverse mating and reproductive strategies most often resulting in the male depositing a
spermatophore within the female, which she stores until she is ready for egg fertilization. After fertilization, and the formation of a zygote, and varying degrees of development, in many species the eggs are deposited outside the female; while in others, they develop further within the female and the young are born live.
Mammals
There are three extant kinds of mammals:
,
and
, all with internal fertilization. In placental mammals, offspring are born as juveniles: complete animals with the
present although not reproductively functional. After several months or years, depending on the species, the sex organs develop further to maturity and the animal becomes
Sexual maturity. Most female mammals are only
fertility during certain periods during their
estrous cycle, at which point they are ready to mate.
For most mammals, males and females
Promiscuity.
[Research conducted by Patricia Adair Gowaty. Reported by ]
Fish
The vast majority of fish species lay eggs that are then fertilized by the male.
Some species lay their eggs on a substrate like a rock or on plants, while others scatter their eggs and the eggs are fertilized as they drift or sink in the water column.
Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fish that have live-bearing offspring include the guppy and mollies or Poecilia. Fishes that give birth to live young can be ovoviviparous, where the eggs are fertilized within the female and the eggs simply hatch within the female body, or in , the male carries the developing young within a pouch, and gives birth to live young.
Reptiles
Reptiles generally reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs through the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis, while squamates, including snakes and lizards, possess a pair of hemipenes, only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm. Most reptiles lay amniotic eggs covered with leathery or calcareous shells.
Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother.
Plants
Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis. Male gametes are called sperm, and female gametes are called eggs or ova. In animals, fertilization of the ovum by a sperm results in the formation of a diploid zygote that develops by repeated mitotic divisions into a diploid adult. Plants have two multicellular life-cycle phases, resulting in an alternation of generations. Plant zygotes germinate and divide repeatedly by mitosis to produce a diploid multicellular organism known as the sporophyte. The mature sporophyte produces haploid spores by meiosis that germinate and divide by mitosis to form a multicellular gametophyte phase that produces gametes at maturity. The gametophytes of different groups of plants vary in size. Mosses and other pteridophytic plants may have gametophytes consisting of several million cells, while
have as few as three cells in each pollen grain.
Flowering plants
are the dominant plant form on land
and they reproduce either sexually or asexually. Often their most distinctive feature is their reproductive organs, commonly called flowers. The
Stamen produces
pollen which contain the male
that produce sperm nuclei. For pollination to occur, pollen grains must attach to the stigma of the female reproductive structure (
carpel), where the female gametophytes are located within ovules enclose within the ovary. After the pollen tube grows through the carpel's style, the
sex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a
fruit, which surrounds the seed(s). Plants may either
Self-pollination or
pollination.
In 2013, flowers dating from the Cretaceous (100 million years before present) were found encased in amber, the oldest evidence of sexual reproduction in a flowering plant. Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. The pollen was sticky, suggesting it was carried by insects.
Ferns
Ferns produce large diploid
with
, roots and leaves. Fertile leaves produce
sporangia that contain haploid
. The spores are released and germinate to produce small, thin gametophytes that are typically heart shaped and green in color. The gametophyte
Prothallus, produce motile sperm in the
antheridia and egg cells in
archegonia on the same or different plants.
After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the archegonia where they fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of different thallus. After fertilization, a
zygote is formed which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of generations.
Bryophytes
The
, which include
Marchantiophyta,
and
, reproduce both sexually and vegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm with
flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominant gametophyte form, which is a multicellular haploid body with leaf-like structures that
photosynthesis. Haploid gametes are produced in antheridia (male) and archegonia (female) by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. The
zygote divides by mitotic division and grows into a multicellular, diploid sporophyte. The sporophyte produces spore capsules (
sporangia), which are connected by stalks (
) to the archegonia. The spore capsules produce spores by meiosis and when ripe the capsules burst open to release the spores. Bryophytes show considerable variation in their reproductive structures and the above is a basic outline. Also in some species each plant is one sex (
dioicous) while other species produce both sexes on the same plant (
monoicous).
Fungi
Fungi are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production of
that are used to survive inclement times and to spread. There are typically three phases in the sexual reproduction of fungi:
plasmogamy,
karyogamy and
meiosis. The cytoplasm of two parent cells fuse during plasmogamy and the nuclei fuse during karyogamy. New haploid gametes are formed during meiosis and develop into spores. The adaptive basis for the maintenance of sexual reproduction in the
Ascomycota and
Basidiomycota (
dikaryon)
fungus was reviewed by Wallen and Perlin.
They concluded that the most plausible reason for maintaining this capability is the benefit of
DNA repair, caused by a variety of stresses, through recombination that occurs during
meiosis.
Bacteria and archaea
Three distinct processes in
are regarded as similar to eukaryotic sex: bacterial transformation, which involves the incorporation of foreign DNA into the bacterial chromosome; bacterial conjugation, which is a transfer of
plasmid DNA between bacteria, but the plasmids are rarely incorporated into the bacterial chromosome; and gene transfer and genetic exchange in archaea.
Bacterial transformation involves the recombination of genetic material and its function is mainly associated with DNA repair. Bacterial transformation is a complex process encoded by numerous bacterial genes, and is a bacterial adaptation for DNA transfer.[Bernstein, H.; Bernstein, C.; Michod, R. E. (2012) " DNA Repair as the Primary Adaptive Function of Sex in Bacteria and Eukaryotes ". Chapter 1, pp. 1–50, in DNA Repair: New Research, Editors S. Kimura and Shimizu S. Nova Sci. Publ., Hauppauge, New York. Open access for reading only. ] or from a similar process in archaea (see below).
On the other hand, bacterial conjugation is a type of direct transfer of DNA between two bacteria mediated by an external appendage called the conjugation pilus.
Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange
See also
Further reading
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Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2004
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Journal of Biology of Reproduction, accessed in August 2005.
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"Sperm Use Heat Sensors To Find The Egg; Weizmann Institute Research Contributes To Understanding Of Human Fertilization", Science Daily, 3 February 2003
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1.) The Editors of Encyclopaedia Britannica. (2025c, March 26). Sexual reproduction | Definition, Process, & Plants. Encyclopedia Britannica. https://www.britannica.com/science/sexual-reproduction 2.) Crow, James F. "Advantages of sexual reproduction." Developmental genetics 15.3 (1994): 205-213. 3.) Tedeschi, M., Limeri, L., & Chouvalova, A. (2023, January 11). Chapter 4: Sexual selection. Pressbooks. https://raider.pressbooks.pub/biology2/chapter/4-sexual-selection/ 4.) Avise, J. C., & Ayala, F. J. (2009). Mate choice and sexual selection: What have we learned since Darwin? In The Light of Evolution - NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK219729/ 5.) Mukherjee, S. (2023, December 19). Sexual Selection- Definition, Types, Examples, and graph. Science Facts. https://www.sciencefacts.net/sexual-selection.html
External links
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