What is parthenogenesis? Parthenogenesis is derived from two Greek words, parthenos, "virgin" and genesis, " creation". Simply, it is defined as reproduction without fertilization. It is said to occur, when a new individual is formed from the female gamete only; there is no involvement of the male gamete. It is often called as a form of asexual reproduction but actually, it is an incomplete form of sexual reproduction. This is because production, activation and development of female egg takes place which is a specialized reproductive cell. Parthenogenesis should not be confused with hermaphroditic species which can also reproduce by themselves. Hermaphroditic species produce both the male and female gametes which enables them to reproduce on their own. On the other hand, parthenogenesis involves the production of a female egg only.
Organisms in which parthenogenesis takes place Following are the organisms in which parthenogenesis occurs: -
Mechanism of parthenogenesis There are two ways by which parthenogenesis can take place: -
1. Apomixis: - In apomixis, mitosis is responsible for the formation of eggs; the female sex cell replicates via mitosis resulting in the formation of two diploid cells. These diploid cells have the full compliment of chromosomes which are required for the development of an embryo. The offspring produced by this method are clones of the parent cell. It takes place in aphids, flowering plants etc.
2. Automixis: - In automixis, the female sex cell replicates via meiosis. The resulting daughter cells are unequally divided during the process, which results in the formation of one large cell known as oocyte and smaller cells known as polar bodies. The oocyte thus formed is haploid in nature. It can convert to diploid on fertilisation by male sperm but in parthenogenesis, there is no involvement of male; the haploid oocyte converts to diploid by fusing with one of the polar bodies or by duplicating its genetic material. Due to meiosis, genetic recombination occurs and hence the offspring are not an exact clone of the parent cell.
Involvement of sexual activity during parthenogenesis. It is interesting to note that some organisms that reproduce by parthenogenesis, require sexual activity for parthenogenesis to occur. This is known as pseudogamy or gynogenesis. In this process, the function of male sperm is to stimulate the egg cell for further development. Although male gamete is required in the process, no genetic material is exchanged between the male sperm and the female gamete; male sperm does not fertilise the female gamete. The egg cell develops into an embryo by the process of parthenogenesis only. Some organisms showing this behaviour are ticks, mites, cicadas, bees, ants, wasps, stick insects etc.
Types of parthenogenesis Parthenogenesis is generally divided into following types: -
1. Facultative parthenogenesis: - This is observed in organisms which are capable of reproducing by both ways, sexually as well as asexually. Depending on the environmental conditions, they switch between the sexual mode and parthenogenesis mode. The eggs created by the species can develop either by fertilisation or by parthenogenic activation. Ex: - Komodo dragon switch between the two modes depending on the availability of a mate.
2. Cyclic parthenogenesis: - It is also known as heterogony. In this type, the alternate generations are formed by parthenogenesis. If the first generation is formed by parthenogenesis, then the next one will be formed via fertilisation which will be followed by parthenogenetic generation. In these species, certain eggs are capable of fertilisation while others are not. It is observed in water fleas.
3. Artificial parthenogenesis: - Parthenogenesis can be artificially induced in almost all animal phyla. Jeaques Leon was the first one to demonstrate artificial parthenogenesis. He did this by pricking unfertilized frog eggs by a needle. Artificial parthenogenesis can also occur by other factors such as seawater solutions, temperature change, diluted acids etc. It is the only way by which parthenogenesis has been shown to occur in mammals.
Advantages 1. When conditions are not favourable for sexual reproduction, parthenogenesis is the process which ensures that the organism is able to reproduce.
2. If an organism, for its survival, has to live in a particular environment, where the mates are scarce, parthenogenesis is very advantageous to that organism.
3. Numerous offspring can be produced in a very little amount of time without costing much loss of energy to the organism.
Disadvantage Offspring produced by parthenogenesis lack genetic variation. In the case of changing environmental conditions, the offspring that are genetically variable will adapt much better than those that lack genetic variation. It may result in the extinction of the species.
Can Parthenogenesis occur in humans? Yes, it is possible. It does seem a bit odd, but it is possible; at least, theoretically it is possible. For the female egg to fertilise on its own, a number of rare biological events should take place in quick succession which is very unlikely. So, although it is theoretically possible the chances of it to occur in practical life are virtually zero.
Very interesting subject. I am amazed as for me it is a new area to read and understand the process. Nature has so many things in it and our quest to understand them is really something which is being delivered by the scientific and research community.
A rare subject but very engrossing to the detailed manner in which the author has presented it.