What is parthenogenesis?
Note that this image is a demonstration of parthenogenesis in sharks. The mechanism of parthenogenesis varies between organisms (Picture from: www.nova.edu/.../images/bbc_virginshark_1.gif)
Types of 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 sepecies 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. (Bookrags Staff)
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 cyclicly parthenogenetic organism has its own process that determines how and when each type of eggs 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 parthenogenicly. 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. (Bookrags Staff)
Picture from: www.waterflea.org/lifecycle
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 automicitic 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 completley during egg formation so there is no chromosomal reduction of the egg. Both mechanisms will create diploid individuals. (Rose, 1989)
Parthenogenesis has been been artificially induced in practically all animal phyla. In 1900, Jeauqes Leon did the first clear case of artificial parthenogenesis by pricking unfertilized frog eggs with a needle. He found that in some cases normal development ensued. Eggs can also be stimulated to mature using things such as temperature changes, seawater solutions, and diluted acids. Artificial parthenogenesis is is the only way parthenogenesis has been seen to occur in mammals. In 1936, Gregory Pincus induced parthenogenesis in mammalian (rabbit) eggs by temperature change and chemical agents. No successful experiments with human parthenogenesis have been reported (Columbia Encylcopedia).
A parthenogenetic embryo (Picture taken from: www.highlighthealth.com)
In what organisms does parthenogenesis occur?