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Types and Significance of Parthenogenesis and Alternation of Generation
The process by which young ones develop from the egg without fertilization is known as parthenogenesis.
Parthenogenesis is a fairly common phenomenon in the animal kingdom, with parthenogenetic reproduction being found in all animal groups. Normally, the ovum develops into a new individual only after the fertilization but in certain cases, the development of the egg takes place without the fertilization. This special and peculiar mode of sexual reproduction in which egg development occurs without fertilization is considered parthenogenesis.
In lower groups of plants like Mucor, Spirogyra, and Chlamydomonas and in animals like bees, ants, wasps, etc. parthenogenesis occurs during sexual reproduction. An unfertilized egg (female gamete) is called Parthenospore or Azygospore. Sometimes the development of the ovary takes place in a seedless fruit like an apple, banana, vine, etc. without fertilization, are called parthenocarpic fruits and the process is called parthenocarpy.
Types of Parthenogenesis
1. Natural Parthenogenesis
When parthenogenesis occurs in the life cycle of living beings in normal natural conditions, it is called natural parthenogenesis. Natural parthenogenesis is of two kinds:
(i) Complete Parthenogenesis: Certain insects have no sexual phase and no males. They depend exclusively on parthenogenesis for self-reproduction. This type of parthenogenesis is known as complete or obligatory parthenogenesis. It is found in some species of earthworms, bdelloid rotifers, grasshoppers, roaches, phasmids, moths, gall flies, fishes, salamanders, and lizards.
(ii) Incomplete Parthenogenesis: The life cycle of certain insects include two generations, the sexual generation, and the parthenogenetic generation, both of which alternate with each other. In such cases, the diploid eggs produce females and the unfertilized eggs produce males. This type of parthenogenesis is known as partial or incomplete parthenogenesis.
- Arrhenotoky: Males are parthenogenetically developed. e.g., honey bee (Apis indica), parasitic wasp, mite, etc.
- Thelytoky: Only females are parthenogenetically developed, e.g., Caucasian rock lizard.
- Amphitoky: Parthenogenesis may result in any sex. e.g., Aphis.
Cyclic parthenogenesis shows several variations in the alternation of sexual (S) and parthenogenetic (P) generation:
- In gall flies (e.g., Neuroterus) there is an alternation of one sex and one parthenogenetic generation per year (P, S…..P, S, …….P, S).
- In aphids (plant lice), daphnids, and rotifers, the sexual generation may come after many parthenogenetic generations during the summer of the year (P, P, P, P, P, P, S,… P, P, P, P, P, P, P, S,……).
- In gall midge (Miaster) the larvae reproduce indefinitely by paedogenetic parthenogenesis. In this case, germ cells within the larvae develop parthenogenetically giving rise to parasitic larvae which feed on fungus. Under favorable conditions winged males and females are produced. These stages reproduce sexually and help in dispersion.
- In some groups, there is no regularity between parthenogenetic and sexual generations.
Natural Parthenogenesis in Vertebrates: A few cases of natural parthenogenesis have also been reported in vertebrates. The fish Carassius auratus gibelio is reported to consist of females only. Likewise, males are found totally lacking in the lizard Lacerta Mexico armeniaca. In it, females are reported to be originated by parthenogenesis. In turkeys, 80% of incubated eggs show early cleavage stages. Such parthenogenetic forms have hatched and grown to reproducing adults which are found to be diploid males with ZZ sex chromosomes. In mammals too, up to 60% of hamster eggs become spontaneously activated and develop upto the two-cell stage.
Complete and Incomplete Types of Natural Parthenogenesis:
The complete and incomplete types of natural parthenogenesis may be of the following two types:
- Haploid Parthenogenesis: When an unfertilized egg (n) directly develops into a new individual, it is called haploid parthenogenesis, e.g., Spirogyra, Mucor, Bee, Ant, etc.
- Diploid Parthenogenesis: When an unfertile diploid egg (without meiotic division) develops into a new individual directly, it is called diploid parthenogenesis, e.g., Aphids, Daphnia, etc.
2. Artificial Parthenogenesis
By the application of artificial factors like organic acids, rays, hormones, electric current, temperature, etc. the ova of various plants and animals begin to develop into a new individual without fertilization. It is called artificial parthenogenesis, e.g., Petunia, Strawberry, Annelida, Mollusca, etc.
The eggs that always develop into young individuals by fertilization sometimes may develop parthenogenetically under certain artificial conditions. This type of parthenogenesis is known as artificial parthenogenesis. Such artificial parthenogenesis may be induced by various physical and chemical means.
Physical means: The following physical means may cause parthenogenesis.
- The range of temperature may induce parthenogenesis in the eggs. For instance, when the egg is transferred from 30°C to 0-10°C parthenogenesis is induced.
- Electric shocks can cause parthenogenesis.
- When the eggs are pricked by the fine glass needles the development of young ones takes place parthenogenetically.
Chemical means: The following chemicals have been found to cause parthenogenesis in normal eggs:
- Hypertonic and Hypotonic sea waters
- Chlorides of K+, Ca++, Na++, Mg++ etc.
- Acids such as butyric acid, lactic acid, oleic acid, and other fatty acids
- Fat solvents, e.g., toluene, ether, alcohol, benzene, and acetone
- Urea and sucrose.
Artificial parthenogenesis has been induced by above mentioned physical and chemical means by various workers in the eggs of most echinoderms, mollusks, annelids, amphibians, birds, and mammals.
Significance of Parthenogenesis
- Parthenogenesis serves as the means for the determination of sex in honey bees, wasps, etc., and it supports the chromosome theory of inheritance.
- There is no complexity like sexual reproduction for the development of an organism through parthenogenesis.
- No need for fertilization.
- It also arises artificially through the application of external factors.
- It is the most simple, stable, and easy process of reproduction, e.g., aphids (insects).
- It eliminates the variation from the population but encourages development of the advantageous mutant characters.
- It causes polyploidy in organisms.
- Due to parthenogenesis, there is no need for the organisms to waste their energy in the process of mating but it allows them to utilize that amount of energy in feeding and reproduction.
- Honey bee and other social insects also control their sex ratio by parthenogenesis.
Disadvantages of Parthenogenesis
- A new combination of genes can not be produced because of parthenogenesis.
- Avoids selection in the population.
- Lack of adaptability followed by extinction.
Differences between Parthenogenesis and Sexual Reproduction:
|1. No need for fertilization.||1. Fertilization is necessary.|
|2. Gametes are not required.||2. Gametic union takes place.|
|3. Offspring is formed directly from the unfertilized egg.||3. Offspring is formed from the diploid zygote.|
|4. Only mother’s characters are developed in the off-springs.||4. Both parental characters are developed in the off-springs.|
|5. It does not help in the alternation of generation.||5. It helps in the alternation of generation.|
Alternation of Generation
The phenomenon in which the two phases of life-cycle, i.e., gametophytic generation and sporophytic generation occur in regular succession alternating with each other, is known as alternation of generation.
Alternation of generation (also known as alternation of phases or metagenesis) is a term primarily used to describe the life cycle of plants (taken here to mean the Archaeplastida). A multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process that reduces the number of chromosomes to half, from 2n to n. Because meiosis is a key step in the alternation of generations, it is likely that meiosis has a fundamental adaptive function. The nature of this function is still unresolved, but the two main ideas are that meiosis is adaptive because it facilitates repair of DNA damages, and/or that it generates genetic variation.
Generally sexually reproducing organism exhibits two distinct phases of development to complete their lifecycle. These phases are:
- the sporophytic generation or diploid generation (2n) or simply the sporophyte where the nucleus bears two chromosome complements (2n).
- the gametophytic generation or haploid generation (n) or simply the gametophyte where the nucleus bears one chromosome complement (n).
These two generations follow each other in a regular sequence in the life cycle. The gametophyte or the haploid generation begins with meiotic division, while the sporophyte or diploid generation starts with sexual fusion and with the formation of the zygote.
The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a zygote, which develops into a diploid sporophyte. This cycle, from gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many algae undergo sexual reproduction.
The relationship between the sporophyte and gametophyte varies among different groups of plants. In those algae which have an alternation of generation, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. In liverworts, mosses, and hornworts, the sporophyte is less well-developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesize, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for the supply of water, mineral nutrients, and nitrogen.
By contrast, in all modern vascular plants, the gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity. In ferns, the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is briefly dependent for its nutrition. In flowering plants, the reduction of the gametophyte is much more extreme; it consists of just a few cells that grow entirely inside the sporophyte.
All animals develop differently. A mature animal is diploid and so is, in one sense, equivalent to a sporophyte. However, an animal directly produces haploid gametes by meiosis. No haploid spores capable of dividing are produced, so neither is a haploid gametophyte. There is no alternation between diploid and haploid forms. Other organisms, such as fungi, can have life cycles in which different kinds of generations alternate. The term ‘alternation of generation’ has also been applied to these cases.
Alternation of Generation in Higher Organism
Higher plants and animals are mainly diploid (2n) in nature. Before sexual reproduction, meiotic division takes place in the gamete mother cells for producing the haploid (n) male and female gametes. These two different gametes, after their union, produce a zygote or diploid (2n) generation again.
Alternation of Generation in Lower Organism
Most of the lower groups of plants (Algae, Fungi, etc.) are characterized by the presence of two individuals which are morphologically identical, but one of them is haploid (gametophyte) producing gametes and the other is diploid (sporophyte) producing spores. Gametes formed from this haploid generation, unite and give rise to the diploid individual, i.e., sporophyte. The meiotic division takes place in the diploid generation during the formation of spores and each of the spores on germination produces a haploid individual, i.e., gametophyte again.
Life cycles of plants and algae with alternating haploid and diploid phases are referred to as diplohaplontic (the equivalent terms haplodiplontic, diplobiontic, or abiotic are also in use). Life cycles, such as those of animals, in which there is only a diploid multicellular phase are referred to as diplontic. Life cycles in which there is only a haploid multicellular phase are referred to as haplontic. The discussion of ‘alternation of generations’ above treats the alternation of a multicellular diploid form with a multicellular haploid form as the defining characteristic, regardless of whether these forms are free-living or not.
In some species, such as the alga Ulva lactuca, the diploid and haploid forms are indeed both free-living, independent organisms, essentially identical in appearance and therefore said to be isomorphic. The free-swimming, haploid gametes form a diploid zygote which germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by meiosis that germinate into haploid gametophytes.
- Haplontic: There is a single somatic phase which is haploid. Diploid condition is present only in zygote or zygospore where meiosis occurs to produce haploid condition again, e.g., Spirogyra, Chlamydomonas, Ulothrix, Cham.
- Diplontic: There is a single somatic phase which is diploid. The haploid condition occurs in gametes that fuse to form a diploid state, e.g., Cladophora, Fucus.
A Complex Life Cycle of Alternation of Generation
The following schematic diagram shows the alternation of generation in a species that are heteromorphic, sporophytic, oogametic, heterosporic, and dioecious in nature. This diagram shows a particular process which is discussed as follows:
1. An immobile egg, contained in the archegonium, fuses with a mobile sperm, released from an antheridium. The resulting zygote is either ‘male’ or ‘female’.
2. A ‘male’ zygote develops by mitosis into a microsporophyte, which at maturity produces one or more microsporangia. Microspores develop within the microsporangium by meiosis.
In a willow (like all seed plants most species of genus Salix are dioecious) the zygote first develops into an embryo microsporophyte within the ovule (a megasporangium enclosed in one or more protective layers of tissue known as integument). At maturity, these structures become the seed. Later the seed is shed, germinates, and grows into a mature tree. A ‘male’ willow tree (a microsporophyte) produces flowers with only stamens, the anthers of which are the microsporangia.
3. Microspores germinate producing microgametophytes; at maturity, one or more antheridia are produced. Sperm develop within the antheridia. In a willow, microspores are not liberated from the anther (the microsporangium), but develop into pollen grains (mi¬crogametophytes) within it. The whole pollen grain is moved (e.g., by an insect or by the wind) to an ovule (megagametophyte), where sperm is produced which moves down a pollen tube to reach the egg.
4. A ‘female’ zygote develops by mitosis into a megasporophyte, which at maturity produces one or more megasporangia. Megaspores develop within the megasporangium; typically one of the four spores produced by meiosis gains bulk at the expense of the remaining three, which disappear. ‘Female’ willow trees (mega-sporophytes) produce flowers with only carpels (modified leaves that bear the megasporangia).
5. Megaspores germinate producing megagametophytes; at maturity, one or more archegonia are produced. Eggs develop within the archegonia. The carpels of a willow produce ovules, megasporangia enclosed in integuments. Within each ovule, a megaspore develops by mitosis into a megagametophyte. An archegonium develops within the megagametophyte and produces an egg. The whole of the gametophytic ‘generation’ remains within the protection of the sporophyte except for pollen grains (which have been reduced to just three cells contained within the microspore wall).
Significance of Alternation of Generations
- The alternation of generations is significant. Each generation has their own way of thinking and there is no wrong way. It is just a matter of these generations working together. New ways of thinking create new ideas and new inventions.
- Alternation of generation is very important in plants because it results in the formation of a variety of new organisms.
- The alternation of generations is a reproductive cycle in plants. It occurs when a plant switches from an asexual to a sexual way of reproduction.
- The regular alternation of forms or modes of reproduction in the life cycle of an organism, especially the alternation between sexual and asexual reproductive phases in plants and some invertebrates.
- In many non-vascular plants, such as mosses and liverworts, the sporophyte is a relatively small plant that grows in or on top of the gametophyte, which is larger. In gymnosperms and angiosperms, however, the sporophyte is the main plant form and the gametophyte is dependent on the sporophyte.
- Alternation of generation, also called Metagenesis, or Heterogenesis, in biology, is the alternation of a sexual phase and an asexual phase in the life cycle of an organism. The two phases, or generations, are often morphologically and always chromosomally, distinct.
- In algae, fungi, mosses, ferns, and seed plants, alternation of generations is common; it is not always easy to observe however, since one or the other of the generations is often very small, even microscopic. The sexual phase called the gametophyte, produces gametes, or sex cells; the asexual phase, or sporophyte, produces spores asexually. In terms of chromosomes, the gametophyte has a single (i.e., monoploid, or haploid) set, and the sporophyte has a double (diploid) set.
- Among animals, many invertebrates have an alternation of sexual and asexual generation (e.g., protozoans, jellyfish, flatworms), but the alternation of haploid and diploid generations is unknown.