Sexual reproduction

From Wikipedia for FEVERv2
Jump to navigation Jump to search

Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (such as a sperm or egg cell) with a single set of chromosomes (haploid) combines with another to produce an organism composed of cells with two sets of chromosomes (diploid). Sexual reproduction_sentence_0

Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. Sexual reproduction_sentence_1

Sexual reproduction does not occur in prokaryotes (organisms without cell nuclei), but they have processes with similar effects such as bacterial conjugation, transformation and transduction, which may have been precursors to sexual reproduction in early eukaryotes. Sexual reproduction_sentence_2

In the production of sex cells in eukaryotes, diploid mother cells divide to produce haploid cells known as gametes in a process called meiosis that involves genetic recombination. Sexual reproduction_sentence_3

The homologous chromosomes pair up so that their DNA sequences are aligned with each other, and this is followed by exchange of genetic information between them. Sexual reproduction_sentence_4

Two rounds of cell division then produce four haploid gametes, each with half the number of chromosomes from each parent cell, but with the genetic information in the parental chromosomes recombined. Sexual reproduction_sentence_5

Two haploid gametes combine into one diploid cell known as a zygote in a process called fertilisation. Sexual reproduction_sentence_6

The zygote incorporates genetic material from both gametes. Sexual reproduction_sentence_7

Multiple cell divisions, without change of the number of chromosomes, then form a multicellular diploid phase or generation. Sexual reproduction_sentence_8

In human reproduction, each cell contains 46 chromosomes in 23 pairs. Sexual reproduction_sentence_9

Meiosis in the parents' gonads produces gametes that each contain only 23 chromosomes that are genetic recombinants of the DNA sequences contained in the parental chromosomes. Sexual reproduction_sentence_10

When the nuclei of the gametes come together to form a fertilized egg or zygote, each cell of the resulting child will have 23 chromosomes from each parent, or 46 in total. Sexual reproduction_sentence_11

In plants only, the diploid phase, known as the sporophyte, produces spores by meiosis that germinate and then divide by mitosis to form a haploid multicellular phase, the gametophyte, that produces gametes directly by mitosis. Sexual reproduction_sentence_12

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. Sexual reproduction_sentence_13

The evolution of sexual reproduction is considered paradoxical, because asexual reproduction should be able to outperform it as every young organism created can bear its own young. Sexual reproduction_sentence_14

This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation. Sexual reproduction_sentence_15

This 50% cost is a fitness disadvantage of sexual reproduction. Sexual reproduction_sentence_16

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. Sexual reproduction_sentence_17

One definite advantage of sexual reproduction is that it impedes the accumulation of genetic mutations. Sexual reproduction_sentence_18

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. Sexual reproduction_sentence_19

It has been described as "a powerful evolutionary force that does not exist in asexual populations." Sexual reproduction_sentence_20

Evolution Sexual reproduction_section_0

Main article: Evolution of sexual reproduction Sexual reproduction_sentence_21

The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1 to 1.2 billion years ago. Sexual reproduction_sentence_22

Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. Sexual reproduction_sentence_23

These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, dealing with competition, DNA repair and masking deleterious mutations. Sexual reproduction_sentence_24

All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Sexual reproduction_sentence_25

Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes. Sexual reproduction_sentence_26

Maintenance of sexual reproduction has been explained by theories that work at several levels of selection, though some of these models remain controversial. Sexual reproduction_sentence_27

However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms. Sexual reproduction_sentence_28

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 reproduction_sentence_29

Sexual selection Sexual reproduction_section_1

Main article: Sexual selection Sexual reproduction_sentence_30

In order to sexually reproduce, both males and females need to find a mate. Sexual reproduction_sentence_31

Generally in animals mate choice is made by females while males compete to be chosen. Sexual reproduction_sentence_32

This can lead organisms 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. Sexual reproduction_sentence_33

Thus sexual reproduction, as a form of natural selection, has an effect on evolution. Sexual reproduction_sentence_34

Sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species. Sexual reproduction_sentence_35

Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits. Sexual reproduction_sentence_36

However, sexual selection is only implied over an extended period of time leading to sexual dimorphism. Sexual reproduction_sentence_37

Sex ratio Sexual reproduction_section_2

Apart from some eusocial wasps, organisms which reproduce sexually have a 1:1 sex ratio of male and female births. Sexual reproduction_sentence_38

The English statistician and biologist Ronald Fisher outlined why this is so in what has come to be known as Fisher's principle. Sexual reproduction_sentence_39

This essentially says the following: Sexual reproduction_sentence_40

Sexual reproduction_description_list_0

  • Sexual reproduction_item_0_0
    1. Suppose male births are less common than female.Sexual reproduction_item_0_1
    2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.Sexual reproduction_item_0_2
    3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.Sexual reproduction_item_0_3
    4. Therefore the genes for male-producing tendencies spread, and male births become more common.Sexual reproduction_item_0_4
    5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.Sexual reproduction_item_0_5
    6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.Sexual reproduction_item_0_6

Animals Sexual reproduction_section_3

Further information: Reproductive system § Animals, Fertilization in animals, and Animal sexual behavior Sexual reproduction_sentence_41

Insects Sexual reproduction_section_4

Further information: Insect § Reproduction and development Sexual reproduction_sentence_42

Insect species make up more than two-thirds of all extant animal species. Sexual reproduction_sentence_43

Most insect species reproduce sexually, though some species are facultatively parthenogenetic. Sexual reproduction_sentence_44

Many insects species have sexual dimorphism, while in others the sexes look nearly identical. Sexual reproduction_sentence_45

Typically they have two sexes with males producing spermatozoa and females ova. Sexual reproduction_sentence_46

The ova develop into eggs that have a covering called the chorion, which forms before internal fertilization. Sexual reproduction_sentence_47

Insects have very diverse mating and reproductive strategies most often resulting in the male depositing spermatophore within the female, which she stores until she is ready for egg fertilization. Sexual reproduction_sentence_48

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 are born live. Sexual reproduction_sentence_49

Birds Sexual reproduction_section_5

Further information: Avian reproduction Sexual reproduction_sentence_50

Mammals Sexual reproduction_section_6

Main article: Mammalian reproduction Sexual reproduction_sentence_51

See also: Human reproduction Sexual reproduction_sentence_52

There are three extant kinds of mammals: monotremes, placentals and marsupials, all with internal fertilization. Sexual reproduction_sentence_53

In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although not reproductively functional. Sexual reproduction_sentence_54

After several months or years, depending on the species, the sex organs develop further to maturity and the animal becomes sexually mature. Sexual reproduction_sentence_55

Most female mammals are only fertile during certain periods during their estrous cycle, at which point they are ready to mate. Sexual reproduction_sentence_56

Individual male and female mammals meet and carry out copulation. Sexual reproduction_sentence_57

For most mammals, males and females exchange sexual partners throughout their adult lives. Sexual reproduction_sentence_58

Fish Sexual reproduction_section_7

Further information: Fish § Reproductive system Sexual reproduction_sentence_59

The vast majority of fish species lay eggs that are then fertilized by the male. Sexual reproduction_sentence_60

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. Sexual reproduction_sentence_61

Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Sexual reproduction_sentence_62

Fish that have live-bearing offspring include the guppy and mollies or Poecilia. Sexual reproduction_sentence_63

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 seahorses, the male carries the developing young within a pouch, and gives birth to live young. Sexual reproduction_sentence_64

Fishes can also be viviparous, where the female supplies nourishment to the internally growing offspring. Sexual reproduction_sentence_65

Some fish are hermaphrodites, where a single fish is both male and female and can produce eggs and sperm. Sexual reproduction_sentence_66

In hermaphroditic fish, some are male and female at the same time while in other fish they are serially hermaphroditic; starting as one sex and changing to the other. Sexual reproduction_sentence_67

In at least one hermaphroditic species, self-fertilization occurs when the eggs and sperm are released together. Sexual reproduction_sentence_68

Internal self-fertilization may occur in some other species. Sexual reproduction_sentence_69

One fish species does not reproduce by sexual reproduction but uses sex to produce offspring; Poecilia formosa is a unisex species that uses a form of parthenogenesis called gynogenesis, where unfertilized eggs develop into embryos that produce female offspring. Sexual reproduction_sentence_70

Poecilia formosa mate with males of other fish species that use internal fertilization, the sperm does not fertilize the eggs but stimulates the growth of the eggs which develops into embryos. Sexual reproduction_sentence_71

Reptiles Sexual reproduction_section_8

Further information: Reptiles § Reproduction Sexual reproduction_sentence_72

Amphibians Sexual reproduction_section_9

Further information: Amphibian § Reproduction Sexual reproduction_sentence_73

Mollusks Sexual reproduction_section_10

Further information: Mollusk § Reproduction Sexual reproduction_sentence_74

Plants Sexual reproduction_section_11

Main article: Plant reproduction Sexual reproduction_sentence_75

Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis. Sexual reproduction_sentence_76

Male gametes are called sperm, and female gametes are called eggs or ova. Sexual reproduction_sentence_77

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. Sexual reproduction_sentence_78

Plants have two multicellular life-cycle phases, resulting in an alternation of generations. Sexual reproduction_sentence_79

Plant zygotes germinate and divide repeatedly by mitosis to produce a diploid multicellular organism known as the sporophyte. Sexual reproduction_sentence_80

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. Sexual reproduction_sentence_81

The gametophytes of different groups of plants vary in size. Sexual reproduction_sentence_82

Mosses and other pteridophytic plants may have gametophytes consisting of several million cells, while angiosperms have as few as three cells in each pollen grain. Sexual reproduction_sentence_83

Flowering plants Sexual reproduction_section_12

Flowering plants are the dominant plant form on land and they reproduce either sexually or asexually. Sexual reproduction_sentence_84

Often their most distinguishing feature is their reproductive organs, commonly called flowers. Sexual reproduction_sentence_85

The anther produces pollen grains which contain the male gametophytes that produce sperm nuclei. Sexual reproduction_sentence_86

For pollination to occur, pollen grains must attach to the stigma of the female reproductive structure (carpel), where the female gametophytes (ovules) are located inside the ovary. Sexual reproduction_sentence_87

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. Sexual reproduction_sentence_88

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. Sexual reproduction_sentence_89

The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Sexual reproduction_sentence_90

Plants may either self-pollinate or cross-pollinate. Sexual reproduction_sentence_91

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. Sexual reproduction_sentence_92

Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. Sexual reproduction_sentence_93

The pollen was sticky, suggesting it was carried by insects. Sexual reproduction_sentence_94

Nonflowering plants like ferns, moss and liverworts use other means of sexual reproduction. Sexual reproduction_sentence_95

Ferns Sexual reproduction_section_13

Ferns mostly produce large diploid sporophytes with rhizomes, roots and leaves. Sexual reproduction_sentence_96

Fertile leaves produce sporangia that contain haploid spores. Sexual reproduction_sentence_97

The spores are released and germinate to produce short, thin gametophytes that are typically heart shaped, small and green in color. Sexual reproduction_sentence_98

The gametophyte thalli, produce both motile sperm in the antheridia and egg cells in archegonia on the same or different plants. Sexual reproduction_sentence_99

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. Sexual reproduction_sentence_100

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. Sexual reproduction_sentence_101

After fertilization, a zygote is formed which grows into a new sporophytic plant. Sexual reproduction_sentence_102

The condition of having separate sporophyte and gametophyte plants is called alternation of generations. Sexual reproduction_sentence_103

Other plants with similar life cycles include Psilotum, Lycopodium, Selaginella and Equisetum. Sexual reproduction_sentence_104

Bryophytes Sexual reproduction_section_14

The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. Sexual reproduction_sentence_105

They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. Sexual reproduction_sentence_106

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 photosynthesize. Sexual reproduction_sentence_107

Haploid gametes are produced in antheridia (male) and archegonia (female) by mitosis. Sexual reproduction_sentence_108

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. Sexual reproduction_sentence_109

The zygote divides by mitotic division and grows into a multicellular, diploid sporophyte. Sexual reproduction_sentence_110

The sporophyte produces spore capsules (sporangia), which are connected by stalks (setae) to the archegonia. Sexual reproduction_sentence_111

The spore capsules produce spores by meiosis and when ripe the capsules burst open to release the spores. Sexual reproduction_sentence_112

Bryophytes show considerable variation in their reproductive structures and the above is a basic outline. Sexual reproduction_sentence_113

Also in some species each plant is one sex (dioicous) while other species produce both sexes on the same plant (monoicous). Sexual reproduction_sentence_114

Fungi Sexual reproduction_section_15

Main article: Mating in fungi Sexual reproduction_sentence_115

Further information: Fungus § Reproduction Sexual reproduction_sentence_116

Fungi are classified by the methods of sexual reproduction they employ. Sexual reproduction_sentence_117

The outcome of sexual reproduction most often is the production of resting spores that are used to survive inclement times and to spread. Sexual reproduction_sentence_118

There are typically three phases in the sexual reproduction of fungi: plasmogamy, karyogamy and meiosis. Sexual reproduction_sentence_119

The cytoplasm of two parent cells fuse during plasmogamy and the nuclei fuse during karyogamy. Sexual reproduction_sentence_120

New haploid gametes are formed during meiosis and develop into spores. Sexual reproduction_sentence_121

The adaptive basis for the maintenance of sexual reproduction in the Ascomycota and Basidiomycota (dikaryon) fungi was reviewed by Wallen and Perlin. Sexual reproduction_sentence_122

They concluded that the most plausible reason for maintaining this capability is the benefit of repairing DNA damage, caused by a variety of stresses, through recombination that occurs during meiosis. Sexual reproduction_sentence_123

Bacteria and archaea Sexual reproduction_section_16

Three distinct processes in prokaryotes 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. Sexual reproduction_sentence_124

Bacterial transformation involves the recombination of genetic material and its function is mainly associated with DNA repair. Sexual reproduction_sentence_125

Bacterial transformation is a complex process encoded by numerous bacterial genes, and is a bacterial adaptation for DNA transfer. Sexual reproduction_sentence_126

This process occurs naturally in at least 40 bacterial species. Sexual reproduction_sentence_127

For a bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as competence (see Natural competence). Sexual reproduction_sentence_128

Sexual reproduction in early single-celled eukaryotes may have evolved from bacterial transformation, or from a similar process in archaea (see below). Sexual reproduction_sentence_129

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. Sexual reproduction_sentence_130

Bacterial conjugation is controlled by plasmid genes that are adapted for spreading copies of the plasmid between bacteria. Sexual reproduction_sentence_131

The infrequent integration of a plasmid into a host bacterial chromosome, and the subsequent transfer of a part of the host chromosome to another cell do not appear to be bacterial adaptations. Sexual reproduction_sentence_132

Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange. Sexual reproduction_sentence_133

Ajon et al. Sexual reproduction_sentence_134

hypothesized that this cellular aggregation enhances species-specific DNA repair by homologous recombination. Sexual reproduction_sentence_135

DNA transfer in Sulfolobus may be an early form of sexual interaction similar to the more well-studied bacterial transformation systems that also involve species-specific DNA transfer leading to homologous recombinational repair of DNA damage. Sexual reproduction_sentence_136

See also Sexual reproduction_section_17

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: reproduction.