REPRODUCTION
CONCEPT
The term reproduction encompasses the entire variety of means by which plants and animals produce offspring. Reproductive processes fall into two broad groupings: sexual and asexual, the latter being the means by which bacteria and algae reproduce. Many plants reproduce sexually by means of pollination, and some plants alternate between sexual and asexual forms of reproduction. Other creatures, such as bees and ants, reproduce through a form of reproduction called parthenogenesis, which is neither fully sexual nor asexual.
HOW IT WORKS
ASEXUAL REPRODUCTION
Asexual reproduction involves only one organism, as opposed to two in sexual reproduction. It occurs when a single cell divides to form two daughter cells that are genetically identical to the parent cell. This process is known as fission, and it may take the form either of binary fission, in which two new cells are produced, or multiple fission, which results in the creation of many new cells. Since there is no fusion of two different cells, the daughter cells produced by asexual reproduction are genetically identical to the parent cell. Asexual reproduction usually takes place by mitosis, a process during which the chromosomes in a cell's nucleus are duplicated before cell division. (Mitosis, chromosomes, and many other topics referred to in this essay are discussed in considerably more detail in Genetics.)
Whereas sexual reproduction is extremely complex—and human sexual reproduction is much more so, freighted as it is with degrees of meaning that go far beyond mere biology—asexual reproduction is a fairly simple, cellular process. Of course, nothing in nature is really simple, and, in fact, the dividing and replication of DNA (deoxyribonucleic acid, the genetic blueprint material found in each cell) is a complicated subject; however, that subject, too, is discussed in the essay Genetics. DNA is located at the cell nucleus, which is the cell's control center, and the nucleus is the first part of the cell to divide in asexual reproduction. After the nucleus splits, the cytoplasm, or the cellular material external to the nucleus, then divides. The result is the formation of two new daughter cells whose nuclei have the same number and kind of chromosomes as the parent.
The adaptive advantage of asexual reproduction is that organisms can reproduce quickly and by doing so colonize favorable environments rapidly. (See Evolution for more about the importance of adaptation and environment in shaping species.) For example, some bacteria can double their numbers every 20 minutes. In addition to bacteria, which are discussed in more detail in Infection, other life-forms that reproduce asexu-ally include protozoa (varieties of which are examined in Parasites and Parasitology), blue-green algae, yeast, dandelions, and flatworms.
SEXUAL REPRODUCTION
Sexual reproduction involves the union of two organisms rather than the splitting of one. Like asexual reproduction, it is a process that takes place at the cellular level. In sexual reproduction it is not binary fission that occurs, but the fusion of two cells. Nor are the two cells identical; rather, the cells—known as gametes—can be identified as either male or female according to the makeup of their chromosomes. The male gamete is called a sperm cell, and the female gamete is termed an egg cell. In sexual reproduction, the sperm cell fuses, or bonds, with the egg cell to produce a cell that is genetically different from either of the parent cells. This process of fusion is known as fertilization, and the fertilized egg is called a zygote. Gametes are produced in the male testes and female ovaries by a splitting process called meiosis. (Meiosis and other terms mentioned briefly in these paragraphs are discussed in much more detail in Genetics.)
Meiosis produces haploid cells, or ones that have half the number of chromosomes as are in a normal cell for that species. When the haploid sperm and egg cells fuse at fertilization, however, the chromosomes from both combine, so that the normal number of chromosomes appears in the zygote. The shuffling of the parents' genetic material that happens during meiosis allows for new gene combinations in offspring that account for variations between offspring (which is why you don't look just like your siblings) and which, over time, can improve a species' chances of survival.
REAL-LIFE APPLICATIONS
EXAMPLES OF ASEXUAL REPRODUCTION
As we noted earlier, bacteria, blue-green algae, most protozoa, yeast, and flatworms all reproduce asexually, as do mosses and starfish. (The last actually reproduce both sexually and asexually by means of alternation of generations, discussed later.) The products of asexual reproduction are known as clones—an example of the fact, discussed in Genetic Engineering, that cloning and the concept of clones are not as new as one might imagine. (See that essay for much more about artificial cloning.) A starfish can regenerate and eventually produce a whole new organism from a single severed appendage, while flatworms divide in two and regenerate to form two new flatworms. This formation of a separate organism is obviously much more complex than the simple splitting of single bacteria cells, but it is still a form of asexual reproduction.
VEGETATIVE PROPAGATION.
Strawberries reproduce by forming growths called runners, which grow horizontally and generate new stalks. At some point, the runner decomposes, leaving a new plant that is a clone of the original. This is an example of vegetative propagation, a term for a number of processes by which crop plants are produced asexually. Vegetative propagation is used for such crops as potatoes, bananas, raspberries, pineapples, and some flowering plants. Its advantage to farmers is that the crops will be more uniform than those grown from seed. Furthermore, some plants are difficult to cultivate from seed, and the vegetative propagation of those plants makes it possible to grow crops that otherwise would not be available for commercial marketing.
In reproducing potatoes through vegetative propagation, farmers plant the so-called eyes to produce duplicates of the parent. With banana plants, the farmer separates the suckers that grow from the root of the plant and plants them. The farmer raising raspberry bushes bends the branches and covers them with soil, whereupon a process not unlike that of the runner growth of mosses takes place: the branches eventually grow into a separate plant, with their own root system, and ultimately can be detached from the parent plant.
BETWEEN ASEXUAL AND SEXUAL
The example of vegetative propagation suggests that there is not a sharp dividing line between sexual and asexual reproduction—that is, that many organisms can reproduce either way. This is true even of humans, who, in theory, could be cloned, though the technology to do so—let alone resolution of the ethical issues of the procedure—lies in the far distant future. (See Genetic Engineering for more on this subject.) Even humans, however, can use external fertilization, which is sexual reproduction without sexual intercourse (see Sexual Reproduction).
Plants go through a process known as alternation of generations, in which they alternate as sexual and asexual reproducers, or gametophytes and sporophytes, respectively. In the asexual stage, the sporophyte produces diploid reproductive cells called spores, which develop into gametophytes. These gametophytes produce haploid gametes, which then unite sexually to form a diploid zygote that grows into a sporophyte. In
At one phase in the alternation of generation for mosses, for instance, male and female moss plants grow from spores. Male moss plants produce sperm cells, which, when the moss receives rainfall, are able to propagate because they have a medium (water) in which to move. They fertilize the female plants, producing zygotes. The zygote grows on top of the female moss plant, which helps to store moisture and thus provides a hospitable environment in which the zygote can develop. The zygote eventually produces haploid spores, which it releases into the air. These tiny spores, carried by the wind, float away from their point of origin until they come to rest, and soon the cycle begins once again.
PARTHENOGENESIS.
There are also organisms, including bees, ants, wasps, and other insects, that reproduce in a way that is neither fully sexual nor asexual. This is parthenogenesis, a type of reproduction in which a gamete develops without fertilization. In other words, a sex cell is reproduced without actual intercourse between male and female. The gamete is almost always female—a fact indicated in the name itself, which comes from parthenos, Greek for "maiden."
The Parthenon in Athens, like the city itself, is named after the goddess Athena (also called Minerva), who was known by the nickname Parthenos. She is said to have been born fully formed, having sprung from the head of her father, Zeus, dressed in armor and ready for battle. Thus, her own birth was a form of parthenogenesis, a word whose second half (a name well known from the Bible) means "beginning."
POLLEN AND POLLINATION
Pollen is a fine, powdery substance consisting of microscopic grains containing the male gametophyte of certain plants that reproduce sexually. These plants include angiosperms, a type of plant that produces flowers during sexual reproduction, and gymnosperms, which reproduce sexually through the use of seeds that are exposed and not hidden in an ovary, as with an angiosperm. Pollen is designed for long-distance dispersal from the parent plant, so that fertilization can occur. Pollination is the transfer of pollen from the male reproductive organs to the female reproductive organs of a plant, and it precedes fertilization. In other words, pollination is the equivalent of sexual intercourse for seed-bearing plants. Actually, cross-pollination, or the transfer of pollen from one plant to another, would perhaps be analogous to sexual intercourse in animals. Pollination occurs in seed-bearing plants, as opposed to the more primitive spore-producing plants, such as ferns and mosses. Gymnosperms, such as pines, firs, and spruces, produce male and female cones, whereas angiosperms produce flowers containing a male organ called the stamen and a female organ called the pistil. Both types of plants rely on insects and other creatures to aid in the pollen transfer.
DARWIN'S MOTH.
The German physician and botanist Rudolf Jakob Camerarius (1665-1721) was the first scientist to demonstrate that plants reproduce sexually, and he pioneered the study of pollination. One of the scientists influenced by his work was the English naturalist Charles Darwin (1809-1882), who discussed the subject in The Various Contrivances by which Orchids Are Fertilized by Insects (1862). Darwin wrote this book partly to support the ideas on evolution presented in his much more well known book Origin of Species (1859). In Various Contrivances, he suggested that orchids and their insect pollinators evolved by interacting with one another over many generations.
As an example, he discussed Angraecum sesquipedale, an orchid native to Madagascar. Darwin had not seen the plant in its native habitat, however; he had looked only at its dried leaves. The white flower of this orchid has a foot-long (30 cm) tubular spur with a small drop of nectar at its base, and from observing this, he hypothesized that the orchid had been pollinated by an insect with a foot-long tongue. This hypothesis, he wrote, "has been ridiculed by some entomologists," or scientists who study insects. After all, no such creature had been found in Madagascar. But then, around the turn of the nineteenth century—some two decades after Darwin's death—it was found. A Madagascan moth was discovered that had a foot-long tongue that uncoils to sip the nectar of A. sesquipedale as it cross-pollinated the flowers.
PLANTS AND THEIR POLLINATORS.
Angiosperms and gymnosperms are discussed in Ecosystems and Ecology, where each is compared in terms of its degree of adaptation to its environment. Angiosperms seem to be the hands-down winner: by enlisting the aid of insects and other pollinators, they manage to pollinate much more efficiently than gymnosperms, which have to produce vast quantities of pollen for each grain that reaches its target. Typically, pollination benefits the animal pollinator by supplying it with sweet nectar and, of course, benefits the plant by providing direct transfer of pollen from one plant to the pistil of another plant. For this reason, specific plant and animal species have developed a relationship of mutualism, a form of symbiosis in which each participant reaps benefits (see Symbiosis). In many cases, plant and pollinator have evolved together, and it is possible to determine which animal pollinates a certain flower species simply by studying the morphologic features (shapes), color, and odor of the flower.
For example, some flowers are pure red, or nearly pure red, and have very little odor. In most such situations, the pollinator is a bird species, since birds have excellent vision in the red region of the spectrum but a rather undeveloped sense of smell. It so happens that Europe, which has no pure red native flowers, also has no bird-pollinated native flower species. Not all bird-pollinated flowers are red, but they are all characterized by striking, and sometimes contrasting, colors that readily catch the eye. Examples of plants pollinated by birds include the cardinal flower, the red columbine, the hibiscus, the eucalyptus, and varieties of orchid, cactus, and pineapple.
Some flowering plants have a very strong odor but are very dark, or at least drab, in color. These flowers and plants—examples include the saguaro cactus, century plant, or cup-and-saucer vine—are often pollinated by bats, which have very poor vision, are typically active during the night, and have a very well developed sense of smell. The flowers of many plant species are marked with special pigments called flavonoids, which absorb ultraviolet light and appear to direct the pollinator toward the pollen and nectar. These pigments are invisible to humans and most animals, but bees' eyes have special ultraviolet photoreceptors that enable the bees to detect patterns and so pollinate these flowers.
WHERE TO LEARN MORE
"Asexual Reproduction Lab." Lester B. Pearson College of the Pacific (Web site). <http://www.pearson-college.uwc.ca/pearson/biology/asex/asex.htm>.
Canine Reproduction (Web site). <http://www.labbies.com/canine_reproduction_table_of_con.htm>.
CRES: The Center for Reproduction of Endangered Species/ San Diego Zoo (Web site). <http://www.sandiegozoo.com/conservation/cres_home.html>.
Elia, Irene. The Female Animal. New York: Henry Holt, 1988.
"Flowering Plant Reproduction." Estrella Mountain Community College (Web site). <http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookflowers.html>.
Kevles, Bettyann. Females of the Species: Sex and Survival in the Animal Kingdom. Cambridge, MA: Harvard University Press, 1986.
Kimball, Jim. "Asexual Reproduction ." Kimball's Biology Pages (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/A/AsexualReproduction.html>.
Maxwell, Kenneth E. The Sex Imperative: An Evolutionary Tale of Sexual Survival. New York: Plenum, 1994.
The Pollination Home Page (Web site). <http://pollinator.com/>.
Reproduction (Web site). <http://www.factmonster.com/ce6/sci/A0841565.html>.
Topoff, Howard R. The Natural History Reader in Animal Behavior. New York: Columbia University Press, 1987.
Walters, Mark Jerome. The Dance of Life: Courtship in the Animal Kingdom. New York: Arbor House, 1988.
KEY TERMS
ALTERNATION OF GENERATIONS:
A process whereby plant generations alternate as sexual and asexual reproducers—gametophytes and sporophytes, respectively.
ASEXUAL REPRODUCTION:
One of the two major varieties of reproduction (along with sexual reproduction), In contrast to sexual reproduction, which involves two organisms, asexual reproduction involves only one. Asexual reproduction occurs when a single cell divides through mitosis to form two daughtercells, which are genetically identical to the parent cell.
BINARY FISSION:
The process in asexual reproduction whereby a single cell divides to form two daughter cells that are genetically identical to the parent cell.
CLONE:
A cell, group of cells, or organism that contains genetic information identical to that of its parent cell or organism.
CLONING:
A specialized genetic process whereby clones are produced. Cloning is a form of asexual reproduction.
CHROMOSOME:
A DNA-containing body, located in the cells of most living things, that holds most of the organism's genes.
CROSS-POLLINATION:
The transfer of pollen from one plant to another.
CYTOPLASM:
The material inside a cell that is external to the nucleus.
DIPLOID:
A term for a cell that has the basic number of doubled chromosomes.
DNA:
Deoxyribonucleic acid, a molecule in all cells, and many viruses, that contains genetic codes for inheritance.
EGG CELL:
A female gamete.
ENZYME:
A protein material that speeds up chemical reactions in the bodies of plants and animals without itself taking part in or being consumed by those reactions.
FERTILIZATION:
The process of cellular fusion that takes place in sexual reproduction. The nucleus of a male reproductive cell, or gamete, fuses with the nucleus of a female gamete to produce a zygote.
GAMETE:
A reproductive cell—that is, a mature male or female germ cell that possesses a haploid set of chromosomes and is prepared to form a new diploid by undergoing fusion with a haploid gamete of the opposite sex. Sperm and egg cells are, respectively, male and female gametes.
GAMETOPHYTE:
In alternation of generations, a gametophyte is a plant that reproduces sexually.
GERM CELL:
One of two basic types of cells in a multicellular organism. In contrast to somatic or body cells, germ cells play a part in reproduction.
HAPLOID:
A term for a cell that has half the number of chromosomes that appear in a diploid or somatic cell.
MEIOSIS:
The process of cell division that produces haploid genetic material. Compare with mitosis.
MITOSIS:
A process of cell division that produces diploid cells, as in asexualreproduction. Compare with meiosis.
NUCLEUS:
The control center of a cell, where DNA is stored.
OVARY:
Female reproductive organ that contains the eggs.
OVULE:
Female haploid gametophyte of seed plants, which develops into a seed upon fertilization by a pollen grain.
PARTHENOGENESIS:
A type of reproduction that involves the development of a gamete without fertilization. In other words, a sex cell (usually female) is reproduced without actual intercourse between male and female.
POLLEN:
Male haploid gametophyte of seed plants (including angiosperms and gymnosperms), which unites with the ovule to form a seed. Pollen is a fine, powdery substance consisting of microscopic grains.
POLLINATION:
The transfer of pollen from the male reproductive organs to the female reproductive organs of a plant. Pollination precedes fertilization. See also cross-pollination.
REGENERATION:
A biological process among some lower animals whereby a severed body part is restored by the growth of a new one.
SEXUAL REPRODUCTION:
One of the two major varieties of reproduction (along with asexual reproduction). In contrast to asexual reproduction, which involves a single organism, sexual reproduction involves two. Sexual reproduction occurs when male and female gametes undergo fusion, a process known as fertilization, and produce cells that are genetically different from those of either parent.
SOMATIC CELL:
One of two basic types of cells in a multicellular organism. In contrast to germ cells, somatic cells (also known as body cells) are not involved in reproduction; rather, they make up the tissues, organs, and other parts of the organism.
SPERM CELL:
A male gamete.
SPOROPHYTE:
In alternation of generations, a sporophyte is a plant that reproduces asexually.
ZYGOTE:
A diploid cell formed by the fusion of two gametes.
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