What is parthenogenesis?
What is parthenogenesis?
Parthenogenesis is most simply defined as reproduction without fertilization. More specifically, it occurs when a female gamete develops a new individual without being fertilized by a male gamete. It is often called a form of "asexual reproduction," but it is more accuratley defined as an incomplete form of sexual reproduction. This is because it involves the production, activation and development of a female egg which is a specialized reproductive cell. Parthenogenesis is not to be confused with hermaphroditic species which can also reproduce by themselves. Hermaphroditic species reproduce by themselves because an organism can produce both the male and female gamete. Parthenogenesis soley involves the production and stimulation of a female egg.
The name itself comes from the Greek words parthenos or "virgin" and genesis or "creation".
How normal Fertilisation and parthenogenesis differ
Normal Fertilisation
Note that this image is a demonstration of parthenogenesis in sharks. The mechanism of arthenogenesis varies between organisms
Types of Parthenogenesis
Facultative Parthenogenesis
This is seen when a species has the ability to reproduce sexually through fertilization and asexually through parthenogenesis. The switch between sexual and asexual reproduction is sometimes based on the organism's environment. Thses species can create eggs that are capable of development from either fertilization or parthenogenic activation. Several species of insects demonstrate this as their unfertilized eggs will develop into one sex, while their fertilized eggs will develop into the other.
Facultative parthenogenesis is seen in komodo dragons. They are able to switch between regular and parthenogenetic reproduction based on the availibilty of a mate.
Cyclic Parthenogenesis / Heterogony
In some species, parthenogenetic generations alternate with generations in which fertilization is able to take place. In these species certain eggs are capable of fertilization, while certain eggs are not. Each cyclic parthenogenetic organism has its own process that determines how and when each type of egg is laid and what type of organism each egg will develop into. For example, water fleas lay eggs that are morphologically distinct. Certain eggs are capable of being fertilized. These eggs are slow developing, yolk-rich, and lie dormant through the winter. After fertilization by males, these eggs will develop and hatch as females. Other eggs are laid parthenogenically. These eggs are smaller, grow faster and are laid in the summer. These will develop into embryos without fertilization by a male. Some will develop into females and some will develop into males.
Chromosomal Differences
There are also chromosomal differences in different types of parthenogenesis. Some species undergo haploid parthenogenesis after chromosomal reduction has already occurred. Here progeny arise from haploid eggs and turn into haploid individuals. This is seen in male honey bees and some flowering plants. It is rarer than diploid parthenogenesis (Rose, 1989).
Other species undergo diploid parthenogenesis. This can occur by a variety of mechanisms. In automictic parthenogenesis, chromosome reduction occurs in the normal manner and the diploid chromosome number is restored by the fusion of two haploid nuclei. In apomicitic parthenogenesis, there is no chromosome reduction or nuclear fusion. In this case, meosis does not occur completely during egg formation so there is no chromosomal reduction of the egg. Both mechanisms will create diploid individuals (Rose, 1989).
Parthenogenesis is most simply defined as reproduction without fertilization. More specifically, it occurs when a female gamete develops a new individual without being fertilized by a male gamete. It is often called a form of "asexual reproduction," but it is more accuratley defined as an incomplete form of sexual reproduction. This is because it involves the production, activation and development of a female egg which is a specialized reproductive cell. Parthenogenesis is not to be confused with hermaphroditic species which can also reproduce by themselves. Hermaphroditic species reproduce by themselves because an organism can produce both the male and female gamete. Parthenogenesis soley involves the production and stimulation of a female egg.
The name itself comes from the Greek words parthenos or "virgin" and genesis or "creation".
How normal Fertilisation and parthenogenesis differ
Normal Fertilisation
Note that this image is a demonstration of parthenogenesis in sharks. The mechanism of arthenogenesis varies between organisms
Types of Parthenogenesis
Facultative Parthenogenesis
This is seen when a species has the ability to reproduce sexually through fertilization and asexually through parthenogenesis. The switch between sexual and asexual reproduction is sometimes based on the organism's environment. Thses species can create eggs that are capable of development from either fertilization or parthenogenic activation. Several species of insects demonstrate this as their unfertilized eggs will develop into one sex, while their fertilized eggs will develop into the other.
Facultative parthenogenesis is seen in komodo dragons. They are able to switch between regular and parthenogenetic reproduction based on the availibilty of a mate.
Cyclic Parthenogenesis / Heterogony
In some species, parthenogenetic generations alternate with generations in which fertilization is able to take place. In these species certain eggs are capable of fertilization, while certain eggs are not. Each cyclic parthenogenetic organism has its own process that determines how and when each type of egg is laid and what type of organism each egg will develop into. For example, water fleas lay eggs that are morphologically distinct. Certain eggs are capable of being fertilized. These eggs are slow developing, yolk-rich, and lie dormant through the winter. After fertilization by males, these eggs will develop and hatch as females. Other eggs are laid parthenogenically. These eggs are smaller, grow faster and are laid in the summer. These will develop into embryos without fertilization by a male. Some will develop into females and some will develop into males.
Chromosomal Differences
There are also chromosomal differences in different types of parthenogenesis. Some species undergo haploid parthenogenesis after chromosomal reduction has already occurred. Here progeny arise from haploid eggs and turn into haploid individuals. This is seen in male honey bees and some flowering plants. It is rarer than diploid parthenogenesis (Rose, 1989).
Other species undergo diploid parthenogenesis. This can occur by a variety of mechanisms. In automictic parthenogenesis, chromosome reduction occurs in the normal manner and the diploid chromosome number is restored by the fusion of two haploid nuclei. In apomicitic parthenogenesis, there is no chromosome reduction or nuclear fusion. In this case, meosis does not occur completely during egg formation so there is no chromosomal reduction of the egg. Both mechanisms will create diploid individuals (Rose, 1989).
