Major Issues in Parthenogenesis - Essay Example

Parthenogenesis: Nature's Cloning Unravelled Thesis ment: In nature, some plants and animals appear to "clone" themselves when they reproduce without the fertilizing their egg - this process is called parthenogenesis. Introduction In this age where the frontiers of science are slowly being unravelled, the area of genetics is now presently at the centerstage because many scientists believe the genes may hold the answers to all our questions pertaining to life. Issues like cloning and stem cell research have caused numerous debates because of the unknown repercussions and the social/ethical boundaries that these processes may cause. However, many of us may have not known that one type of "cloning" have been occurring in nature since time immemorial. In nature, some plants and animals appear to "clone" themselves when they reproduce without the fertilizing their egg - this process is called parthenogenesis. What is parthenogenesis Why is it a form of asexual, rather than a sexual type of reproduction What are the organisms that have the ability to undergo parthenogenesis How can the knowledge of this type of reproduction benefit genetics, biology and science as a whole These are just some of the questions we will try to answer in this paper dedicated to delve deeper into the process of parthenogenesis. Sexual vs. Asexual The most commonly known type of reproduction is the sexual reproduction. Two parents, one male and one female, are needed to produce an offspring. The Oxford Dictionary of Biology (2004) expounds that sexual reproduction is "a form of reproduction that involves the fusion of two reproductive cells (gametes) in the process of fertilization". In general, the gametes consist of a sperm from one individual and an egg from another. In a process called fertilization, the gametes unite to form a cell called the zygote, which develops into the offspring. The new individual is genetically different from its parents. Gametes are produced through a type of cell division called meiosis. In a cell produced by this kind of division, there are only half as many chromosomes as were in the cell that produced it. Sexual reproduction occurs normally, especially in animals. However, most plants bear both male and female reproductive organs and self-fertilization may occur, as it does in hermaphrodite animals. Gametes are formed by meiosis, a special kind of cell division in the parent reproductive organs that both reassorts the genetic material and halves the chromosome number. Meiosis thus ensures genetic variability in the gametes and therefore in the offspring resulting from their subsequent fusion. Sexual reproduction, unlike asexual reproduction, therefore generates variability within a species. However, it depends on there being reliable means of bringing together male and female gametes, and many elaborate mechanisms have evolved to ensure this (Oxford Dictionary of Biology, 2004). On the other hand, asexual reproduction is "a form of organic reproduction in which the parent organism does not exchange genetic material with another organism of the same species" (Ashworth and Little, 2001). In fact, there are four general types of asexual reproduction: 1.) Binary fission - Commonly occurring in protists and other unicellular organisms, binary fission closely resembles the process of mitosis, by which the cells of multicellular animals divide. The organism's chromosomes replicate (duplicate themselves) within the cell nucleus; the nucleus elongates with a group of identical chromosomes in each end; and finally the cell splits down the middle, along the short axis of the elongated nucleus, forming two "daughter cells", which are exact copies of the parent cell. 2.) Fragmentation - This occurs when an organism's body or body part breaks into two or more pieces; each part then develops into a completely new organism. The regrowth of tissue is referred to as regeneration. 3.) Budding - This process produces a small copy of the parent that begins as a growth on the parent's side and then breaks free and pursues an independent existence (such as in yeasts). In one-celled organisms this is simply a sort of sack in the cell wall into which replicated genetic material flows, while in higher organisms a complete multicelled copy of the parent usually develops before separation. Occasionally, separation never takes place. For example, in a strawberry plant, buds from runners remain permanently attached to the parent plant. 4.) Parthenogenesis - In this type of asexual reproduction, eggs are produced and develop into young exactly as in the higher forms of sexual reproduction, but without the crucial step of fertilization (Ashworth and Little, 2001). Both sexual and asexual reproduction has its pros and cons. As we all know, asexual reproduction permits rapid population growth that can be controlled entirely by the availability of food and suitable habitat, without the need to seek out and mate with a certain organism. However, since every daughter organism produced asexually is genetically identical to its parent, species variation cannot be achieved except by mutation and undesirable traits cannot be bred out. For this reason, almost all organisms-even one-celled forms such as amoebas and paramecia -utilize some form of sexual reproduction at least part of the time. Parthenogenesis in Focus According to the Oxford Dictionary of Biology (2004), parthenogenesis is the process of an organism to develop from an unfertilized egg. As a form of asexual reproduction, it occurs sporadically in many plants (e.g. dandelions and hawkweeds) and in a few animals. However, some species appear that it is the main and sometimes only method of reproduction. For example, in some species of aphid, males are absent or very rare. The eggs formed by the females contain the full (diploid) number of chromosomes and are genetically identical. Variation is consequently very limited in species that reproduce parthenogenetically. Moreover, parthenogenesis, as compared to the usual sexual reproduction, has the advantage of being both rapid and independent of contact with other members of the reproducing organism's species. It is therefore useful for rapid population expansion in new areas, and is employed for this purpose by a number of insects and plants, especially pioneer species (such as the dandelion) and social species (such as aphids). Aphids employ it in order to spread their population rapidly in advantageous environments, utilizing sexual reproduction only in the fall, apparently to create hardier eggs that can live through the winter before hatching. This is because parthenogenesis, like all forms of inbreeding, tends to reveal the weak traits of organisms. Since no genetic material is exchanged in parthenogenesis, the individuals that result are exact genetic copies of the individual that laid the eggs. Thus, the offsprings are almost always females, although when parthenogenesis has been forced in birds in the laboratory it has resulted not in females but in sterile males (Ashworth and Little, 2001). History of Parthenogenesis In Greek, parthenogenesis means "virgin birth". The term parthenogenesis was first coined by the biologist Richard Owens in 1849 (Gale Encyclopedia of Science, 2001). However, the existence of parthenogenetic phenomena has been recognized since the time of Charles Bonnet in his treatise called Trait d'insectolologie in 1745. But it was only in the nineteenth century that the interest passed from the evident external phenomena to the cell, and especially to the nucleus. It was observed as early as 1869 that the parthenogenetic eggs of aphids formed only one polar body, while eggs normally fertilized formed two. In 1886, August Weismann confirmed these observations, and regarded the formation of only one polar body as characteristic of all types of parthenogenesis. This fits well with our modern knowledge that the omission of meiosis that will keep the eggs diploid since only one cell division will be concerned in gametogenesis. If eggs are to be fertilized meiosis will take place, the cells will become haploid, and two polar bodies will be formed. The diploid number of chromosomes is restored by union with the sperm. Weismann's observations and conclusions stand as true for most cases of parthenogenesis. In them meiosis is normally omitted and the eggs are diploid (Singer, 1959, p. 544). However, Singer (1959) recounted that there are cases of parthenogenesis in which the eggs do undergo meiosis, and give off two polar bodies. In these cases the diploid chromosome number is restored either by fusion of one polar body with the egg, as observed by O. Hertwig in a starfish in 1890, or by a fusion in pairs of the cells of the developing embryo. The organism is thus exercising in these cases a sort of self-fertilization (p. 545). It was in 1900 that Jacques Loeb "accomplished the first clear case of artificial parthenogenesis when he pricked unfertilized frog eggs with a needle and found that in some cases normal embryonic development ensued". This paved the way for artificial parthenogenesis that has "since been achieved in almost all major groups of animals, although it usually results in incomplete and abnormal development". To induce the artificial parthenogenesis, various "mechanical and chemical agents" were "used to stimulate unfertilized eggs". Then, Gregory Pincus successfully promoted parthenogenesis in "mammalian (rabbit) eggs by temperature change and chemical agents". In history, however, not one successful experiment have been linked with human parthenogenesis. It is also said that parthenogenesis is "rarer among plants (where it is called parthenocarpy) than among animals" (The Columbia Encyclopedia, 2000). Parthenogenesis in Animals Most animal species that reproduce parthenogenetically also display a phase of sexual behaviour and sexual reproduction. In most cases, parthenogenetic reproduction occurs when environmental conditions are favorable and there is plenty of food that can sustain the generation of large numbers of individuals in a short period of time. When external conditions change and food supplies become less abundant, or when the environment becomes unpredictable, these species shift to a sexual mode of reproduction. Although sexual reproduction is considerably slower and generates fewer organisms, it gives rise to individuals containing variations in their genetic material. Some of these individuals might be at an advantage over their predecessors, because they might be more able to adapt to new conditions. Some lizards have traditionally displayed that they could exhibit the process of parthenogenesis. The first reports on this all-female population of whiptail lizards appeared in 1962 (Duellman and Zweifel, 1962). David Crews in 1979 reported that some types of parthenogenetic whiptail lizards of the genus Cnemidophorus are imitating males in order to reproduce. While at Harvard University, he found that captive members of the all-female C. uniparens imitated mounting and mating postures of the male C. tigris, a species that containing both males and females that reproduce sexually (Edwards, 1987). In some species of insects, such as the aphids, parthenogenetic reproduction occurs in the spring and summer, when conditions are favorable for rapid population growth. As time goes by and conditions become less favourable, the parthenogenetically born individuals mate and lay fertilized eggs. These eggs hatch the following spring, when conditions are again favourable for another cycle of parthenogenetic reproduction. Thus, parthenogenesis helps to keep things moving along because there's no need for such time-consuming frivolities as mate location, courtship or copulation. Also, "parthenogenesis engenders another upward leap in reproductive efficiency called paedogenesis, whereby offspring start developing inside their parents even before those parents have shed their larval skins" (Schwartz, 1985). Moreover, some species of ants, bees, and wasps also has the ability to reproduce both sexually and asexually that is part of the mechanism establishing sexual differences. Usually, females develop from unfertilized eggs, containing only half of the genetic material of the mother, whereas males develop from fertilized eggs, containing the genetic contributions of both mother and father. Bees, in particular, the difference between males and females are that males are produced parthenogenetically and are haploid, whereas females are diploid. The difference between a queen and a worker is due solely to the different diet they receive at the larval stage, which results in workers having underdeveloped reproductive systems. These differences are critical to the social organization of honeybee colonies (Hounsome, April 2004). Some fish also exhibit parthenogenesis. Minnows found in the southwestern United States living in rivers that dry to the point where only puddles remain, demonstrate parthenogenetic reproduction so eliminating the need for a suitable mate to be present in a given puddle. Minnows take advantage of parthenogenetic reproduction by making most offsprings survive the dry months, regardless of whether or not sexual recombination occurs. Therefore organisms which produce a greater quantity of offspring are more likely to have one survive to the next generation (Hughes, 1989). However, the best known examples of parthenogenetic reproduction could be found among rotifers (plankton-like animals). The Encyclopdia Britannica Online (2007) explained that: The males are completely unknown in some genera; in others, they appear in the population only for brief periods and more or less seasonally. Females are the dominant form or are the only sex present in a population throughout most of the year. Because no reductional division (meiosis) occurs in the course of egg maturation, the eggs are diploid-thatis, they have the full number of chromosomes; they give rise to new diploid individuals with no chromosomal contribution from a male gamete (diploid parthenogenesis). Even if males were present, sperm could not fertilize the eggs because the latter are already diploid. Under conditions of environmental stress such as seasonal changes, some females form eggs that undergo reductional division, resulting in eggs with the haploid number of chromosomes; such eggs must be fertilized by a male gamete to produce a new female. When the new individual matures, it will probably reproduce parthenogenetically. If, however, there are no males in the population, the haploid eggs can develop into haploid males (haploid parthenogenesis), which then participate in bisexual reproduction. Bisexually produced eggs are often referred to as winter eggs since they have a thick covering that protects the embryo during adverse environmental conditions. Summer eggs, produced parthenogenetically, are thin shelled. Bisexual reproduction occurs, therefore, only often enough to ensure survival of the species. Parthenogenesis in Plants In certain ferns, algae, and fungi, the parthenogenesis occurs during the development of the sporophyte directly from a cell of the gametophyte, so fusion of gametes is bypassed. It frequently occurs in gametophytes that have been produced aposporously and are thus diploid. This process is also known as apogamy (Bailey, 2003). Pathenogenesis is also exhibited by higher blackberries, hawkweeds and most dandelions. However, these plants still produce pollen. A nonadaptive explanation for the retention of the male function of such parthenogenic plants has been given by Maynard Smith (1978), who suggested that the asexual lineages in these species may be too young and may not yet have had the time to accumulate the necessary mutations for male sterility. Alternatively, male function could be retained selectively, if pollen or sperm are used to fertilize sexual relatives and if such matings lead to the production of new clonal lineages or reduce the fitness of sexual competitors. Male function can also be adaptive in pseudogamous species, where parthenogens require sperm or pollen to trigger embryo development (Weinzierl et al., 1998). Another form of parthegenesis, which is called parthenocarpy, occurs in fruit-bearing plants. Parthenocarpy pertains to the "development of fruit without fertilization". According to Encyclopdia Britannica Online (2007), the parthenocarpic fruit resembles a "normally produced fruit but is seedless": Varieties of the pineapple, banana, cucumber, grape, orange, grapefruit, persimmon, and breadfruit exemplify naturally occurring parthenocarpy. Seedless parthenocarpic fruit can be induced in nonparthenocarpic varieties and in naturally parthenocarpic varieties out of season by a type of artificial pollination with dead or altered pollen or by pollen from a different type of plant. The application of synthetic growth substances in paste form, by injection, or by spraying, also causes parthenocarpic development. Artificial Parthenogenesis Artificial parthenogenesis refers to the "artificial induction of the development of an unfertilized egg by chemical or physical stimulation" (King et al., 2007). The artificial induction of parthenogenesis was first observed by Boursier (1847) in female silkworms maintained under sun exposure, and by Tichomirov (1886) in unfertilized eggs treated by sulphuric acid. Many experimental treatments have since been shown empirically to attain this goal: action of chemicals, oxygenation, electric pulses, mechanical wrapping, centrifugation or cooling. The work of Astaurov (1940) defined a suite of relevant methods for inducing parthenogenesis, mainly by precise spatio-temporal temperature activation of fertilized or unfertilized eggs (as cited in Grenier et al., August 2004) However, it was Jacques Loeb who made waves using artificial parthenogenesis and commanded worldwide publicity of a "scientific basis for the doctrine of immaculate conception". His production of fatherless sea urchins and other organisms by using artificial parthenogenesis were targeted to the extension of life. These experiments were not efforts to reduce biology to physicochemical laws, although they have often been understood as such; rather they were attempts to manipulate life processes in the absence of an understanding of mechanisms (Servos, July 1987). Just recently, a Japanese scientist made waves when he artificially induced parthenogenesis in mice. Tomohiro Kono of Tokyo University of Agriculture, took an immature female egg from a genetically engineered mouse. Two of the mouse's genes had been altered so that the egg had a pattern of gene activity similar to that of a sperm. The team then fused the nucleus of this pseudosperm with that of a normal, mature egg from another female. After 457 attempts, the team produced 10 live pups. One survived to adulthood. The mouse was called Kaguya, after the Japanese tale of a girl miraculously discovered in a bamboo stump. "It was a fantastic surprise, I couldn't believe my eyes!" Kono exclaimed (Pearson, January 2005). Conclusion Parthenogenesis is one of nature's wonders that have been cracked open by human beings. Although it is an asexual reproduction that occurs in nature to some species, scientists have duplicated the process and make it applicable other species. For example, in the past, attempts to create live mice by parthenogenesis, by stimulating mature eggs with chemicals, have failed. Although embryos start to develop, these do not survive implantation. The barrier to parthenogenesis in mammals, it seems, is something known as genetic imprinting. In a normal embryo, there are two copies of each chromosome--one from the mother and one from the father. Each has a slightly different chemical imprint which is vital for normal development. This difference only seems to exist in mammals. However, Tomohiro Kono of Japan had already been successful in making this near-impossible thing happen just last year. Thus, further studies in the process of parthenogenesis should be forwarded because parthenogenesis might hold the key in offering an important new therapeutic strategy for a host of medical conditions that the world is experiencing at present. Works Cited Ashworth, William, and Little, Charles E. "Parthenogenesis." Encyclopedia of Environmental Studies, New Edition. New York: Facts On File, Inc., 2001. Ashworth, William, and Little, Charles E.. "Asexual Reproduction." Encyclopedia of Environmental Studies, New Edition. New York: Facts On File, Inc., 2001. Bailey, Jill. "Apogamy." The Facts On File Dictionary of Botany. New York: Facts On File, Inc., 2003. Duellman, W. E. and Zweifel, R.G.. "A Synopsis of the Lizards of the Cnemidophorus cozumela Complex". Copeia 1969(1962):519-535. Edwards, Diane D."Leaping Lizards and Male Impersonators: Are There Hidden MessagesScience News131(May 30, 1987):348-50. Encyclopdia Britannica. "Reproductive System, Animal." 2007. 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