Reproduction

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In biology, reproduction is the process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. Although the term reproduction encompasses a great variety of means by which organisms produce new offspring, reproductive processes can be classified into two main types: Sexual reproduction and asexual reproduction.

Production of new individuals along a leaf margin of the air plant, Kalanchoë pinnata. The small plant in front is about 1 cm tall. The concept of "individual" is obviously stretched by this asexual reproductive process.

Sexual reproduction is a biological process by which organisms create descendants that have a combination of genetic material contributed from two different gametes (reproductive or sex cells), usually from two different organisms. The union (fusion) of these gametes produces an organism that is not genetically identical to the parent(s). Sexual reproduction generally involves contribution of genetic material to the offspring from two different organisms, but includes self-fertilization within one organism but involving fusion of two gametes. Humans are an example of a species that reproduces by sexual reproduction involving two individuals. Peanuts are a type of plant that can self-pollinate (self-fertilize).

In asexual reproduction, an individual can reproduce without involvement with another individual, there is no fusion of gametes, and the new organism produced inherits all of its chromosomes from one parent and thus is a genetically-similar or identical copy of the parent. The division of a bacterial cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to single-celled organisms. Most plants have the ability to reproduce asexually.

In general, more-complex organisms reproduce sexually while simpler, usually unicellular, organisms reproduce asexually. Among animals, and indeed eukaryotes in general, sexual reproduction is a nearly universal mode of reproduction. However, even lower plants, fungi, some of the protists, and bacteria likewise exhibit reproductive variances, which may be symbolized by + and - signs (rather than being called male and female), and referred to as "mating strains" or "reproductive types" or similar appellations. This polarity reflects the principle of dual characteristics, whereby existent beings exhibit the quality of positivity and negativity. (See Principles of Biology.)

While reproductive processes can be classified into two broad groupings, species exhibit a wide spectrum of mechanisms. For example, some plants alternate between sexual and asexual reproduction (see Alternation of generations). In parthenogenesis, such as found in some invertebrates (rotifers, water fleas, aphids, some bees, etc.) and some vertebrates (some amphibians, reptiles, and more rarely in birds), an embryo is produced without fertilization by a male. Generally, authors (such as Mayr 2001) list parthenogenesis as a form of asexual reproduction because it does not involve fusion of gametes of opposite sexes, nor any exchange of genetic material from two different sources; however, some authorities (McGraw-Hill 2004) classify parthenogenesis as sexual reproduction on the basis that it involves gametes or does not produce an offspring genetically identical to the parent.

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Asexual reproduction

Binary fission
Main article: Asexual reproduction

Asexual reproduction is the biological process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. In asexual reproduction, there is no fusion of gametes, and the new organism produced inherits all of its chromosomes from one parent.

Among groups of organisms that demonstrate asexual reproduction are bacteria, hydras, molds, annelid worms, yeast, mushrooms, algae, and plants. Viruses also reproduce asexually (although they are not universally considered to be living organisms).

Common examples of asexual reproduction are bacteria that divide asexually via binary fission; viruses that take control of host cells to produce more viruses; and hydras (invertebrates of the order Hydroidea of the phylum Cnidaria) and yeasts that are able to reproduce by budding. These organisms are capable of "splitting" themselves into two or more individuals. Other ways of asexual reproduction include fragmentation and spore formation that involves only mitosis.

Binary fission. In binary fission, there is a reproduction of a living cell by division into two equal or near-equal parts. It is common in bacteria. Binary fission begins with DNA replication, with the DNA double strand separated and each strand serving as a template for synthesis of a daughter strand, until the entire prokayotic DNA is duplicated. The cell then elongates and the two chromosomes locate in opposite parts of the elongated cell. The cell membrane then invaginates (grows inwards) and splits the cell into 2 daughter cells, separated by a newly grown cell plate. Baring complications, offspring would be genetically identical to the parent cell, but due to mutation and genetic recombination, daughter cells may have slightly differed genetic makeups. Bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments. Organisms that reproduce through binary fission generally grow exponentially. E. coli cells are able to divide every 20 minutes under optimum conditions.

In addition to binary fission being the reproductive method of bacteria (for example, Rickettsia species, which cause diseases such as Rocky Mountain spotted fever), various single-celled eukaryotes also reproduce by the splitting of the original cell into two cells, although this involves mitosis and cytokinesis similar to the cells of a multicellular eukaryote organisms. While historically these eukaryote single-cell divisions have been referred to as binary fission, this term today is often reserved for prokaryote reproduction, which does not involve mitosis since they lack a membrane-bounded nucleus. Among eukaryotes that reproduce by the splitting of the original cell into two are most protists (for example, Amoeba proteus); Entamoeba histolytica (a protozoan that is a human intestinal parasite); Pyrodictium abyssi (an anaerobic hyperthermophilic archaea of deep-sea hydrothermal vents); and Schizosaccharomyces pombe (a fungal organism that is a species of yeast).

In addition, the mitochondria and chloroplasts of eukaryote cells also divide by binary fission.

Budding. Budding is the formation of a new organism by the protrusion of part of another organism. This is very common in plants and fungi, but may be found in animal organisms, such as the hydra, as well. Usually, the protrusion stays attached to the primary organism for a while, before becoming free.

Fragmentation. Fragmentation occurs when an organism is split into fragments. The splitting may or may not be intentional. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. If the organism is split any further, the process is repeated. Fragmentation is seen in many organisms such as molds, some annelid worms, and starfish. Binary fission may be considered a type of fragmentation involving single-celled organisms such as bacteria, protozoa, and many algae. Molds, yeast, and mushrooms, all of which are part of the Fungi kingdom, produce tiny filaments called hyphae. These hyphae obtain food and nutrients from the body of other organisms to grow and fertilize. Then a piece of hyphae breaks off and grows into a new individual and the cycle continues.

Spore formation. A spore is a reproductive structure that is adapted for dispersion and survival for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many plants, algae, fungi, and some protozoans. In spore formation, there is the production of reproductive cells called spores that contain DNA and develop into a new organism after dispersal. Generally, such as seen in multicellular plants, spores are haploid (one-half of the genetic complement as have somatic, or body, cells) and unicellular and are produced by meiosis in the sporophyte. However, there is no fusion of these spores, nor exchange of genetic material between organisms. Once conditions are favorable, the spore can develop into a new (haploid) organism using mitotic division. In part of a fern life cycle, spore formation also can involve sporogenesis without meiosis, such that the chromosome number of the spore cell is the same as that of the parent producing the spores. (See spore.)

Asexual and sexual. Some "asexual" species, like hydra and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation.

Sexual reproduction

Main article: Sexual reproduction
Hoverflies mating in midair flight

Sexual reproduction is a biological process by which organisms create descendants that have a combination of genetic material contributed by two different gametes, usually from two different organisms. The union of these gametes produces an organism that is not genetically identical to the parent(s).

A gamete is a mature reproductive or sex cell. Typically, a gamete is haploid, while the somatic or body cell of the organism is diploid. (Some organisms exhibit polyploidy.) A diploid cell has a paired set of chromosomes. Haploid means that the cell has a single set of unpaired chromosomes, or one half the number of chromosomes of a somatic cell. In diploid organisms, sexual reproduction involves alternating haploid (n) and diploid (2n) phases, with fusion of haploid cells to produce a diploid organism. (See life cycle.)

Three important processes are involved in sexual reproduction: Meiosis, mitosis, and fertilization or fusion.

Meiosis and mitosis are an integral part of cell division. Mitosis occurs in somatic (body) cells. The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell. Meiosis occurs in reproductive or sex cells and results in gametes. It results in cells with half the number of chromosomes present in the daughter cells as are in the parent cell. Essentially, a diploid cell duplicates itself, then undergoes two divisions (tetroid to diploid to haploid), in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.

Fertilization involves the fusion of haploid gametes to give a diploid organism, which can then grow by mitosis.

Thus, in sexual reproduction, each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes that fuse to form a diploid organism. Sexual reproduction also includes self-fertilization, whereby one plant may have "male" and "female" parts, and produce different haploid gametes that fuse. Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants.

For most organisms, a gamete that is produced may have one of two different forms. In these anisogamous species, the two sexes are referred to as male, producing sperm or microspores as gametes, and female, producing ova or megaspores as gametes. In isogamous species, the gametes are similar or identical in form, but may have separable properties and may be given other names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as ciliates, have more than two kinds of gametes.

Most plants and animals (including humans) reproduce sexually. Sexually reproducing organisms have two sets of genes (called alleles) for every trait. Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. Having two copies of every gene, only one of which is expressed, allows deleterious alleles to be masked.

Allogamy and Autogamy

Allogamy is a term used in the field of biological reproduction describing the fertilization of an ovum from one individual with the spermatozoa of another individual. In humans, the fertilization event is an instance of allogamy.

By contrast, autogamy is the term used for self-fertilization. Self-fertilization or autogamy occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual. This is common in plants and certain protozoans.

In plants, allogamy is used specifically to mean the use of pollen from one plant to fertilize the flower of another plant and usually is synonymous with the term cross-fertilization or cross-pollination. However, the latter term can be used more specifically to mean pollen exchange between different plant strains or even different plant species (where the term cross-hybridization can be used) rather than simply between different individuals.

Parasites having complex life cycles can pass through alternate stages of allogamous and autogamous reproduction, and the description of a hitherto unknown allogamous stage can be a significant finding with implications for human disease (Zhong et al. 1982).

Asexual vs. sexual reproduction

Sexual reproduction is a near-universal mode of reproduction among eukaryotes. Furthermore, while asexual reproduction is widespread among fungi, certain protists and vascular plants, various invertebrates, and even some reptiles and amphibians, sexual reproduction is also seen in these same groups, and some state even in bacteria, which will exchange genetic material between donors (+ mating type) and recipients (- mating type). (However, many authorities, such as Mayr (2001), consider the unidirectional lateral transfer of genetic material in bacteria to either not be reproduction, or at least not sexual reproduction, and Mayr states that sexual reproduction is unknown in prokaryotes.) Lahr et al. (2011) contend that even in amoeboid lineages the extent of asexual reproduction in overestimated and that the evidence "demonstrates that the majority of amoeboid lineages are, contrary to popular belief, anciently sexual, and that most asexual groups have probably arisen recently and independently." Among animals, nearly all species practice sexual reproduction. Mayr (2001) notes that "above the level of the genus there are only three higher taxa of animals that consist exclusively of uniparentally reproducing clones." (An example of these higher taxa would be rotifers of the Class Bdelloidea.)

Why sexual reproduction appeared and is so prevalent is a major puzzle in modern biology. Sexual reproduction has many drawbacks, since it requires far more energy than asexual reproduction. For example, in an asexual species, each member of the population is capable of bearing young, implying that an asexual population can grow more rapidly. An additional cost of sexual reproduction is that males and females must search for each other in order to mate. Evolutionary biologist and geneticist John Maynard Smith (1978) maintains that the perceived advantage for an individual organism to pass only its own entire genome to its offspring is so great that there must an advantage by at least a factor of two to explain why nearly all animal species maintain a male sex. Mayr (2001) notes that since the 1880s evolutionists have argued over the advantage of sexual reproduction and "so far, no clear-cut winner has emerged from this controversy."

General explanations for the origin and maintenance of sexual reproduction focus on the advantages conferred due to an improvement in the quality of progeny (fitness), despite reducing the overall number of offspring (two-fold cost of sex). This enhanced fitness is explained in terms of the genetic variation that is increased through sexual reproduction. Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutations for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller amount of offspring, but the large amount of variation in their genes makes them less susceptible to disease or changing environmental stresses.

For example, many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, some species of starfish (by fragmentation), and many plants are examples. It is held that when environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival, such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH, or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources. When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, it is held that these organisms switch to sexual forms of reproduction. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.

George C. Williams introduced the the lottery principle in 1975 to explain this basic concept, using lottery tickets as an analogy. He argued that asexual reproduction, which produces little or no genetic variety in offspring, was like buying a large number of tickets that all have the same number, limiting the chance of "winning"—that is, surviving. Sexual reproduction, he argued, was like purchasing fewer tickets but with a greater variety of numbers and therefore a greater chance of success. The point of this analogy is that since asexual reproduction does not produce genetic variations, there is little ability to quickly adapt to a changing environment. The lottery principle is less accepted these days because of evidence that asexual reproduction is more prevalent in unstable environments, the opposite of what it predicts.

Conversely, Heng (2007) proposes that the resolution to the "paradox of sex" is that sexual reproduction actually reduces the drastic genetic diversity at the genome or chromosome level, resulting in the preservation of species identity, rather than the provision of evolutionary diversity for future environmental challenges. He maintains that while genetic recombination contributes to genetic diversity, it does so secondarily and within the framework of the chromosomally defined genome. That is, the asexual process generates more diverse genomes because of the less controlled reproduction systems, while sexual reproduction generates more stable genomes.

Reproductive strategies

There is a wide range of reproductive strategies employed by different species. Some animals, such as Homo sapiens 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 (mature after 8 months) can produce 10–30 offspring per year, and a fruit fly (mature after 10–14 days) can produce up to 900 offspring per year.

These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favored 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 a large number of offspring. On the other hand, animals with many offspring may devote less resources to each individual offspring; for these types of animals it, is common for a large number of offspring to die soon after birth, but normally enough individuals survive to maintain the population.

Other types of reproductive strategies include polycyclic animals' (reproduce intermittently throughout their lives), Semelparous organisms (reproduce only once in their lifetime, such as annual plants, which often die shortly after reproduction), and Iteroparous organisms (produce offspring in successive cycles, such as perennial plants, and thus survive over multiple seasons).


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