Female Nasonia parasitic wasps have almost complete control over the determination of the sex of their offspring. Biologists at the Evolutionary Genetics department at the University of Groningen have discovered how this species of parasitic wasp determines its offspring’s gender. Nasonia parasitic wasps are used as a biological pest control agent in the agricultural sector, but are also, like fruit flies, ideal for genetic research. The research will be published this week in the academic journal Science.
Nasonia wasps lay their eggs in fly larvae that are found in bird’s nests and corpses. They are also, however, easy to breed in laboratories, making them excellent model organisms for genetic and evolutionary research.
Nasonia DNA has recently been completely mapped (Science, 15 January 2010). Based on this data, researchers at the University of Groningen were able to identify the genes that are important in the development of a male or female ichneumon wasp. This development is potentially important for the use of parasitic wasps in biological pest control. Only female parasitic wasps kill off pest insects, meaning that there is an economic advantage in increasing the number of female offspring.
Like fruit flies, Nasonia wasps are easy to cross-breed, which is important for genetic research. However, there is an important characteristic that sets Nasonia apart from fruit flies: because Nasonia males come from unfertilized eggs (as is the case with, for example, honey bees), males only have one set of chromosomes instead of two (like humans and fruit flies). Female Nasonia, on the other hand, come from fertilized eggs and do have two sets of chromosomes. This method of reproduction is known as ‘haplodiploid’ and is very useful when searching for particular genes and researching interactions between genes.
Like humans, these wasps are also able to change the chemical structure of their DNA through methylation. This process plays an important part in switching genes on or off during development, and can lie at the root of birth defects. This makes Nasonia very important for research into the epigenetic mechanisms of gene regulation.
Surprisingly, Nasonia’s sex determination method turns out to be based on this type of DNA modification. A crucial role is played by the transformer (tra) gene, which produces the TRANSFORMER (TRA) enzyme. The TRA enzyme ensures that the organism develops into a female. The TRA enzyme also ensures that there is always enough TRA present through self-regulation. If a Nasonia female lays an egg, it will contain the tra messenger RNA, enabling TRA to be produced. Without the father’s set of chromosomes, however, the embryo cannot manufacture enough TRA by itself. This is why an unfertilized egg will always produce a male.
If the egg has been fertilized, the embryo will start producing its own TRA, causing it to develop into a female. Apparently the mother, when passing on her genes, has switched off her tra gene by changing the DNA chemically. This is known as epigenetic modification and appears to be more common than previously thought.
So while fruit flies (and humans) need a father to be involved for sons to be produced, parasitic wasps thus need a father to produce daughters. The mothers, however, have complete control: they decide whether or not to fertilize an egg, using sperm that they have stored in their body. In doing so they regulate their offspring’s TRA production. In the world of Nasonia, mothers are in charge!
More information:- Dr Louis van de Zande (louis.van.de.zande rug.nl, ++3150-3632126), Centre for Ecological and Evolutionary Studies (CEES)- Eveline Verhulst (e.c.verhulst rug.nl , ++3150-3632119), Centre for Ecological and Evolutionary Studies (CEES)- Prof. Leo Beukeboom, (l.w.beukeboom rug.nl, ++3150-3638448), Centre for Ecological and Evolutionary Studies (CEES)
The research was partly facilitated by support from NWO. High resolution photographs for publication are available on request.
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