A parasite lives in a close relationship with another organism, its host, and causes it harm. The parasite is dependent on its host for its life functions. For example, viruses are common parasites. The parasite has to be in its host to live, grow, and multiply. Parasites rarely kill their hosts. A common, well-known type of a parasite is a hookworm. It is possible for humans or their pets to get them. Hookworms attach themselves in the lining of the small intestine, and cause diseases, and malnutrition as well, as they eat the nutrients and keep them from going to the host.
Here, we'll be talking about a different type of parasite, called a parasitoid. The parasitoid Encarsia pergandiella is a tiny wasp that lays its eggs in developing whiteflies. The wasp larva eventually kills the whitefly, and then emerges as free-living adults. This is why it is said that the wasps are parasitic. If you have ever seen Aliens, the developing aliens are parasitoids. Parasitods are different from normal parasites because they kill their hosts when they develop.
Encarsia pergandiella develops in a strange way. Unlike most parasitoid species, they are autoparasitoids. This means females develop on whitefly immatures and males develop on immature parasitoids. This means males are parasitoids of other parasitoids, including females of their own species!
The fertilization process is a female decision. The female stores the male sperm for later fertilization of eggs.
All bees and wasps, including parasitoid wasps are haplodiploid. This means not all eggs have to be fertilized to develop. Unfertilized eggs become males, which are haploid. Fertilized eggs become female, and are diploid. In autoparasitoids, placement is also important in sex development. Female eggs can only develop on whitefly larvae. This makes the whitefly a primary host, meaning that the wasp generally places its eggs on this host. The humans from the Aliens movies are the primary host for the aliens.
Male wasps can only develop on wasp larvae. This makes the wasp larva the secondary host. The secondary host can be female wasps of the same species or other parasitoid species developing on the whitefly. If the aliens also had secondary hosts, then they would lay the male eggs on the aliens developing in the humans.
Female autoparasitoids can choose to lay female or male eggs on the basis of environmental conditions. Pretty sophisticated behavior for a wasp the size of a speck of dust, huh?!
PSR - The Paternal Sex Ratio Chromosome
In human beings, almost every cell in the body is diploid meaning it contains two sets of nearly identical copies of 23 chromosomes, a total of 46 being both female and male. The exception to this are gametes which are eggs or sperm. They are called haploid for they only contain one copy of each of the 23 chromosomes, the egg being female and the sperm being male.
When the female egg is...