Reproduction


Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: asexual and sexual.
In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms. The cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates a genetically similar or identical copy of itself. The evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sexual reproduction is that only 50% of organisms reproduce and organisms only pass on 50% of their genes.
Sexual reproduction typically requires the sexual interaction of two specialized reproductive cells, called gametes, which contain half the number of chromosomes of normal cells and are created by meiosis, with typically a sperm cell fertilizing an egg cell from the same species to create a fertilized zygote. This produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms.

Methods

Asexual

Asexual reproduction is a process by which organisms create genetically similar or identical copies of themselves without the contribution of genetic material from another organism. Bacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras and yeasts are able to reproduce by budding. These organisms often do not possess different sexes, and they are capable of "splitting" themselves into two or more copies of themselves. Most plants have the ability to reproduce asexually and the ant species Mycocepurus smithii is thought to reproduce entirely by asexual means.
Some species that are capable of reproducing asexually, like hydra, yeast and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproductionreproduction without seeds or sporesbut can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation.
Other ways of asexual reproduction include parthenogenesis, fragmentation and spore formation that involves only mitosis. Parthenogenesis is the growth and development of embryo or seed without fertilization. Parthenogenesis occurs naturally in some species, including lower plants, invertebrates, and vertebrates.

Sexual

Sexual reproduction is a biological process that creates a new organism by combining the genetic material of two organisms in a process that starts with meiosis, a specialized type of cell division. Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes. Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as male and female. In isogamous species, the gametes are similar or identical in form, but may have separable properties and then may be given other different names. Because both gametes look alike, they generally cannot be classified as male or female. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as many fungi and the ciliate Paramecium aurelia, have more than two "sexes", called mating types.
Most animals and plants reproduce sexually. Sexually reproducing organisms have different sets of genes for every trait. Offspring inherit one allele for each trait from each parent. Thus, offspring have a combination of the parents' genes. It is believed that "the masking of deleterious alleles favors the evolution of a dominant diploid phase in organisms that alternate between haploid and diploid phases" where recombination occurs freely.
Bryophytes reproduce sexually, but the larger and commonly-seen organisms are haploid and produce gametes. The gametes fuse to form a zygote which develops into a sporangium, which in turn produces haploid spores. The diploid stage is relatively small and short-lived compared to the haploid stage, i.e. haploid dominance. The advantage of diploidy, heterosis, only exists in the diploid life generation. Bryophytes retain sexual reproduction despite the fact that the haploid stage does not benefit from heterosis. This may be an indication that the sexual reproduction has advantages other than heterosis, such as genetic recombination between members of the species, allowing the expression of a wider range of traits and thus making the population more able to survive environmental variation.

Allogamy

Allogamy is the fertilization of flowers through cross-pollination, this occurs when a flower's ovum is fertilized by spermatozoa from the pollen of a different plant's flower. Pollen may be transferred through pollen vectors or abiotic carriers such as wind. Fertilization begins when the pollen is brought to a female gamete through the pollen tube. Allogamy is also known as cross fertilization, in contrast to autogamy or geitonogamy which are methods of self-fertilization.

Autogamy

Self-fertilization, also known as autogamy, occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual, e.g., many vascular plants, some foraminiferans, some ciliates. The term "autogamy" is sometimes substituted for autogamous pollination and describes self-pollination within the same flower, distinguished from geitonogamous pollination, transfer of pollen to a different flower on the same flowering plant, or within a single monoecious gymnosperm plant.

Mitosis and meiosis

and meiosis are types of cell division. Mitosis occurs in somatic cells, while meiosis occurs in gametes.
Mitosis
The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the offspring cells is the same as that of the parent cell.
Meiosis
The resultant number of cells is four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell. A diploid cell duplicates itself, then undergoes two divisions, in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.

Gametogenesis

Animals, including mammals, produce gametes by means of meiosis in gonads. Sperm are produced by spermatogenesis and eggs are produced by oogenesis. During gametogenesis in mammals numerous genes encoding proteins that participate in DNA repair mechanisms exhibit enhanced or specialized expression. Male germ cells produced in the testes of animals are capable of special DNA repair processes that function during meiosis to repair DNA damages and to maintain the integrity of the genomes that are to be passed on to progeny. Such DNA repair processes include homologous recombinational repair as well as non-homologous end joining. Oocytes located in the primordial follicle of the ovary are in a non-growing prophase arrested state, but are able to undergo highly efficient homologous recombinational repair of DNA damages including double-strand breaks. These repair processes allow the integrity of the genome to be maintained and offspring health to be protected.

Same-sex

Scientific research is currently investigating the possibility of same-sex procreation, which would produce offspring with equal genetic contributions from either two females or two males. The obvious approaches, subject to a growing amount of activity, are female sperm and male eggs. In 2004, by altering the function of a few genes involved with imprinting, other Japanese scientists combined two mouse eggs to produce daughter mice In 2010, American scientists used genetically manipulated stem cells to produce viable mouse offspring carrying genetic contributions from two fathers. In 2018 Chinese scientists created 29 female mice from two mice mothers but were unable to produce viable offspring from two father mice. Researches noted that there is little chance these techniques would be applied to humans in the near future. In 2023, Japanese scientists created mouse pups from two mice fathers which grew into adulthood.

Strategies

There are a wide range of reproductive strategies employed by different species. Some animals, such as the human and northern gannet, do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood. For example, a rabbit can produce 10–30 offspring per year, and a fruit fly can produce up to 900 offspring per year. These two main strategies are known as K-selection and r-selection. Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population. Some organisms such as honey bees and fruit flies retain sperm in a process called sperm storage thereby increasing the duration of their fertility.

Other types

  • Polycyclic animals reproduce intermittently throughout their lives.
  • Semelparous organisms reproduce only once in their lifetime, such as annual plants, and certain species of salmon, spider, bamboo and century plant. Often, they die shortly after reproduction. This is often associated with r-strategists.
  • Iteroparous organisms produce offspring in successive cycles, such as perennial plants. Iteroparous animals survive over multiple seasons. This is more associated with K-strategists.