Parasitoid wasps are haplodiploid. This means that the male wasps develop from unfertilised eggs and only have one set of chromosomes, namely those of their mother. Female wasps develop from fertilised eggs and have two sets of chromosomes: one from their mother and one from their father.
A special egoistic B chromosome is present in some males of the parasitoid wasp species Trichogramma kaykai and Nasonia vitripennis. Although this chromosome has the same effect and a similar structure in both wasps, Van Vugt demonstrated that both chromosomes do not share any DNA sequence homology. From this she concluded that both types of the chromosome have a different origin. Therefore the existence of more such B chromosomes in other insect species seems likely. This could also point to a simple molecular mechanism of these chromosomes which has developed more than once during the course of evolution.
More males due to selfish chromosome
The B chromosome ensures that only male offspring develop. In eggs fertilised with sperm from these males, the complete genome from the father is destroyed during the first nuclear division. However, the B chromosome remains intact and is incorporated into the chromosome set of the mother. Such fertilised eggs develop into males with the B chromosome. Accordingly this chromosome is only inherited from father to son.
This selfish chromosome is called the Paternal Sex Ratio (PSR) chromosome and was first discovered in N. vitripennis. At the end of 1997 a second PSR chromosome was discovered in the unrelated parasitoid wasp T. kaykai. This made a comparative study of PSR chromosomes possible.