reproduction (redirected from reproducing)

reproduction, capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism. In all cases reproduction consists of a basic pattern: the conversion by a parent organism of raw materials from the environment into offspring—or into cells that develop into offspring (see meiosis; mitosis)—of a constitution similar or potentially similar to that of the parent. The reproductive process always includes the transmission of hereditary material (see nucleic acid) from the parents so the offspring too can reproduce themselves. Although the methods and complexity of the reproductive process vary tremendously, two fundamental types may be distinguished; asexual reproduction, in which a single organism separates into two or more equal or unequal parts; and sexual reproduction, in which a pair of specialized reproductive (sex) cells fuse.

Asexual Reproduction

Asexual reproduction is advantageous in allowing beneficial combinations of characteristics to continue unchanged and in eliminating the often vulnerable stages of early embryonic growth. It is found in most plants, bacteria, and protists and the lower invertebrates. In one-celled organisms it most commonly takes the form of fission, or mitosis, the division of one individual into two new and identical individuals. The cells thus formed may remain clustered together to form filaments (as in many fungi) or colonies (as in staphylococci and Volvox). Fragmentation is the process in filamentous forms in which a piece of the parent breaks off and develops into a new individual. Sporulation, or spore formation, is another means of asexual reproduction among protozoa and many plants. A spore is a reproductive cell that produces a new organism without fertilization. In some lower animals (e.g., hydra) and in yeasts, budding is a common form of reproduction; a small protuberance on the surface of the parent cell increases in size until a wall forms to separate the new individual, or bud, from the parent. Internal buds formed by sponges are called gemmules.

Regeneration is a specialized form of asexual reproduction; by regeneration some organisms (e.g., the starfish and the salamander) can replace an injured or lost part, and many plants are capable of total regeneration—i.e., the formation of a whole individual from a single fragment such as a stem, root, leaf, or even a small slip from such an organ (see cutting; grafting). F. C. Steward showed (1958) that single phloem cells from a carrot plant, when grown on an agar medium, would form a complete carrot plant. Among animals, the lower the form, the more capable it is of total regeneration; no vertebrates have this power, although clones of mammals have been produced in the laboratory (1997) from single somatic cells. Closely allied to regeneration is vegetative reproduction, the formation of new individuals by various parts of the organism not specialized for reproduction. In some plants structures that form on the leaves give rise to young plantlets. Rhizomes, bulbs, tubers, and stolons are other forms of vegetative reproduction.

Sexual Reproduction

Sexual reproduction occurs in many one-celled organisms and in all multicellular plants and animals. In higher invertebrates and in all vertebrates it is the exclusive form of reproduction, except in the few cases in which parthenogenesis is also possible. Sexual reproduction is essentially cellular in nature, i.e., it involves the fertilization of one sex cell (gamete) by another, producing a new cell (called a zygote), which develops into a new organism. The union of two isogametes (structurally identical but differing physiologically) is called isogamy, or conjugation, and occurs only in some lower forms (e.g., Spirogyra and some protozoa). Heterogamy is the fusion of two clearly differing kinds of gametes, distinguished as the ovum and the sperm.

Multicellular plants alternate sexually reproducing, or gametophyte, and asexually reproducing, or sporophyte, generations. The gametophyte produces gametes, and the union of gametes results in the growth of a sporophyte; the sporophyte produces spores that give rise to a gametophyte. The prominent generation in lower plants (e.g., mosses, liverworts) and the complex fungi is the gametophyte; in the vascular plants (ferns, conifers, grasses, and flowering plants) it is the sporophyte. The less prominent generation may be an independent plant, as is the small inconspicuous gametophyte of ferns, or a reduced organism consisting of only a few cells and dependent for survival on the prominent form, like the pollen grain, which is the male gametophyte of seed plants.

Many organisms exhibit special reproductive mechanisms to ensure fertilization; among higher plants the process of pollination may involve extremely complex interaction between the flower and the pollen-bearing agent (e.g., the yucca plant and the yucca moth). Among land-dwelling animals internal fertilization (copulation) is necessary in order to provide the fluid environment essential to fertilization.

Sexual reproduction is of great significance in that, because of the fusion of two separate parental nuclei, the offspring inherit endlessly varied combinations of characteristics that provide a vast testing ground for new variations that may not only improve the species but ensure its survival. This probably explains the predominance of sexual reproduction among higher forms. Even in those microorganisms that reproduce asexually (e.g., bacteria) exchanges of hereditary material take place; in the hermaphroditic plants and animals (e.g., the earthworm) self-fertilization is almost always prevented by anatomical specializations or by differing maturation times for male and female gametes.

See also genetics, recombination, and sex.

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reproduction

Process by which organisms replicate themselves, assuring continuation of their species. The two basic forms are asexual and sexual. Asexual reproduction (e.g., fission, spore formation, regeneration, and vegetative reproduction) produces an offspring genetically identical to its single parent. Sexual reproduction produces a new individual through the union of special sex cells (gametes), usually from different parents. Gametes result from meiosis. Gamete union results in a zygote, the first cell of a new organism. Sexual reproduction ensures that each offspring is genetically unique (except in cases of multiple offspring derived from divisions of one zygote). Most animals, including all vertebrates, reproduce sexually.

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reproduction

1. Biology any of various processes, either sexual or asexual, by which an animal or plant produces one or more individuals similar to itself

2. 

a. an imitation or facsimile of a work of art, esp of a picture made by photoengraving

b. (as modifier): a reproduction portrait

3. the quality of sound from an audio system

4. a revival of an earlier production, as of a play

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reproduction [¦rē·prə¦dək·shən]

(biology)

The mechanisms by which organisms give rise to other organisms of the same kind.

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Reproduction (animal)

The formation of new individuals, which may occur by asexual or sexual methods. In the asexual methods, which occur mainly among the lower animals, the offspring are derived from a single individual. Sexual methods are general throughout the animal kingdom, with offspring ordinarily derived from the paired union of special cells, the gametes, from two individuals. Basic to all processes of reproduction is the origin of the new individual from one or more living cells of the parent or parents.

Asexual reproduction

Asexual processes of reproduction include binary fission, multiple fission, fragmentation, budding, and polyembryony. Among the protozoa and lower metazoa, these are common methods of reproduction. However, the last-mentioned process can occur in mammals, including humans.

Binary fission involves an equal, or nearly equal, longitudinal or transverse splitting of the body of the parent into two parts, each of which grows to parental size and form. This method of reproduction occurs regularly among protozoans.

Multiple fission, schizogony, or sporulation produces several new individuals from a single parent. It is common among the Sporozoa, such as the malarial parasite, which form cystlike structures containing many cells, each of which gives rise to a new individual.

Fragmentation is a form of fission occurring in some metazoans, especially the Platyhelminthes, or flatworms; the Nemertinea, or ribbon worms; and the Annelida, or segmented worms. The parent worm breaks up into a number of parts, each of which regenerates missing structures to form a whole organism.

Budding is a form of asexual reproduction in which the new individual arises from a relatively small mass of cells that initially forms a growth or bud on the parental body. The bud may assume parental form either before separation from the body of the parent as in external budding, or afterward, as in internal budding. External budding is common among sponges, coelenterates, bryozoans, flatworms, and tunicates. Internal budding occurs among fresh-water sponges and bryozoans. In the sponges the internal buds, termed gemmules, consist of groups of primitive cells surrounded by a dense capsule formed by the body wall. If the parent animal dies as a result of desiccation or low temperature, the cells of the gemmules can later be released and form new sponges. In the bryozoans the similarly functioning buds are known as statoblasts.

Polyembryony is a form of asexual reproduction, occurring at an early developmental stage of a sexually produced embryo, in which two or more offspring are derived from a single egg. Examples are found scattered throughout the animal kingdom, including humans; in humans it is represented by identical twins, triplets, or quadruplets.

Sexual reproduction

Sexual reproduction in animals assumes various forms which may be classified under conjugation, autogamy, fertilization (syngamy), and parthenogenesis. Basically, the various processes all involve the occurrence of certain special nuclear changes, termed meiotic divisions, preliminary to the production of the new individual. See Gametogenesis, Meiosis

Conjugation occurs principally among the ciliate protozoans, such as Paramecium, and involves a temporary union of two individuals during which each is “fertilized” by a micronucleus from the other.

In autogamy the nuclear changes described for conjugation take place, but since there is no mating, there is no transfer of micronuclei. Instead, the prospective migratory micronucleus reunites with the stationary one. The process may be considered related to parthenogenesis.

Fertilization, or syngamy, comprises a series of events in which two cells, the gametes, fuse and their nuclei, which had previously undergone meiotic divisions, fuse. In metazoans, the gametes are of two morphologically distinct types: spermatozoa, or microgametes, and eggs, also called ova or macrogametes. These types are produced by male and female animals, respectively, but in some cases both may be produced by a single, hermaphroditic individual. The nucleus of the spermatozoon has half the number of chromosomes characteristic of the ordinary (somatic) cells of the animal. The nucleus of the ripe egg in some animals, for instance, coelenterates and echinoderms, also has attained this haploid condition, but in most species of animals it is at an early stage of the meiotic divisions when ready for fertilization. In the latter situation, the meiotic divisions of the egg, characterized by formation of small, nonfunctional cells termed polar bodies, are completed after the sperm enters, whereupon the haploid egg nucleus fuses with the haploid sperm nucleus. Fertilization thus produces a zygote with the diploid chromosome number typical of the somatic cells of the species (23 pairs in humans), and this is maintained during the ensuing cell divisions.

Parthenogenesis is the development of the egg without fertilization by a spermatozoon. It is listed as a form of sexual reproduction because it involves development from a gamete. Rotifers, crustaceans, and insects are the principal groups in which it occurs naturally. It has also been induced (artificial parthenogenesis) in species from all the major phyla by various kinds of chemical or physical treatment of the unfertilized egg. Even in mammals, several adult rabbits have reportedly been thus produced. See Estrus, Oogenesis, Ovum, Sperm cell, Spermatogenesis

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Reproduction (plant)

The formation by a plant of offspring that are either exact copies or reasonable likenesses. When the process is accomplished by a single individual without fusion of cells, it is referred to as asexual; when fusion of cells is involved, whether from an individual or from different donors, the process is sexual.

Asexual reproduction

Using the technique of tissue culture, higher green plants can be regenerated from a single cell and can usually flower and set seed normally when removed and placed in soil. This experiment shows that each cell of the plant body carries all the information required for formation of the entire organism. The culture of isolated cells or bits of tissue thus constitutes a means of vegetative propagation of the plant and can provide unlimited copies identical to the organism from which the cells were derived.

All other vegetative reproductive devices of higher plants are elaborations of this basic ability and tendency of plant cells to produce tissue masses that can organize into growing points (meristems) to yield the typical patterns of differentiated plant organs. For example, a stem severed at ground level may produce adventitious roots. Similarly, the lateral buds formed along stems can, if excised, give rise to entire plants. The “eyes” of the potato tuber, a specialized fleshy stem, are simply buds used in vegetative propagation of the crop. In many plants, cuttings made from fleshy roots can similarly form organized buds and reconstitute the plant by vegetative propagation. Thus, each of the vegetative organs of the plant (leaf, stem, and root) can give rise to new plants by asexual reproduction. See Plant propagation

Sexual reproduction

While in asexual reproduction, the genetic makeup of the progeny rarely differs greatly from that of the parent, the fusion of cells in sexual reproduction can give rise to new genetic combinations, resulting in new types of plants. The life cycle of higher green plants consists of two distinct generations, based on the chromosomal complement of their cells. The sporophyte generation is independent and dominant in the flowering plants and ferns, but small, nongreen, and dependent in the mosses, and contains the 2n number of chromosomes. The diploidy results in each case from the fusion of sperm and egg to form the zygote, which then develops into an embryo and finally into the mature sporophyte. The sporophyte generation ends with the formation of 1n spores by reduction division, or meiosis, in a spore mother cell. The spore then develops into the gametophyte generation, which in turn produces the sex cells, or gametes. The gametophyte generation ends when gametes fuse to form the zygote, restoring the 2n situation typical of sporophytes. See Meiosis

In flowering plants, the gametophyte or 1n generation is reduced to just a few cells (generally three for the male and eight for the female). The male gametophyte is formed after meiosis occurs in the microspore mother cells of the anther, yielding a tetrad of 1n microspores. Each of these microspores then divides mitotically at least twice. The first division produces the tube nucleus and the generative nucleus. The generative nucleus then divides again to produce two sperms. These nuclei are generally not separated by cell walls, but at this stage the outer wall of the spore becomes thickened and distinctively patterned—a stage typical of the mature male gametophyte, the pollen grain. See Flower, Mitosis, Pollen, Pollination

Each pollen grain has a weak pore in its wall, through which the pollen tube emerges at the time of germination. Pollen germinates preferentially in the viscous secretion on the surface of the stigma, and its progress down the style to the ovary is guided through specific cell-to-cell recognition processes. Throughout its growth, which occurs through the deposition of new cell wall material at the advancing tip, the pollen tube is controlled by the tube nucleus, usually found at or near the tip. When the pollen tube, responding to chemical signals, enters the micropyle of the ovule, its growth ceases and the tip bursts, discharging the two sperms into the embryo sac, the female gametophyte of the ovary.

The female gametophyte generation, like the male, arises through meiotic division of a 2n megaspore mother cell. This division forms four 1n megaspores, of which three usually disintegrate, the fourth developing into an eight-nucleate embryo sac by means of three successive mitotic divisions. The eight nuclei arrange themselves into two groups of four, one at each pole of the embryo sac. Then one nucleus from each pole moves to the center of the embryo sac. One of the three nuclei at the micropylar end of the embryo sac is the female gamete, the egg, which fuses with one of the sperm nuclei to form the zygote, the first cell of the sporophyte generation, which produces the embryo. The second sperm fuses with the two polar nuclei at the center of the embryo sac to form a 3n cell that gives rise to the endosperm of the seed, the tissue in which food is stored. The entire ovule ripens into the seed, with the integuments forming the protective seed coat. The entire ovary ripens into a fruit, whose color, odor, and taste are attractive to animals, leading to dispersal of the seeds. The life cycle is completed when the seed germinates and grows into a mature sporophyte with flowers, in which meiotic divisions will once again produce 1n microspores and megaspores.

Nonflowering higher plants such as the ferns and mosses also show a distinct alternation of generations. The familiar fern plant of the field is the sporophyte generation. Meiosis occurs in sporangia located in special places on the leaves, generally the undersides or margins. A spore mother cell produces a tetrad of 1n spores, each of which can germinate to produce a free-living, green gametophyte called a prothallus. On the prothallus are produced male and female sex organs called antheridia and archegonia, which give rise to sperms and eggs, respectively. Sperms, motile because of their whiplike flagella, swim to the archegonium, where they fertilize the egg to produce the zygote that gives rise to the sporophyte generation again.

In mosses, by contrast, the dominant green generation is the gametophyte. Antheridia or archegonia are borne at the tips of these gametophytes, where they produce sperms and eggs, respectively. When suitably wetted, sperms leave the antheridium, swim to a nearby archegonium, and fertilize the egg to produce a 2n zygote that gives rise to a nongreen, simple, dependent sporophyte. The moss sporophyte consists mainly of a sporangium at the end of a long stalk, at the base of which is a mass of tissue called the foot, which absorbs nutrients from the green, photosynthetic gametophyte. Meiosis occurs in the sporangium when a spore mother cell gives rise to four reduced spores. Each spore can germinate, giving rise to a filamentous structure from which leafy gametophytic branches arise, completing the life cycle.

Various members of the algae that reproduce sexually also display alternation of generations, producing sperms and eggs in antheridia and oogonia. Sporophyte and gametophyte generations may each be free-living and independent, or one may be partially or totally dependent on the other. See Fruit, Plant physiology, Population dispersal, Seed