A "selfish" B chromosome that enhances its transmission by eliminating the paternal genome.

In the parasitic wasp, Nasonia vitripennis, males are haploid and usually develop from unfertilized eggs, whereas females are diploid and develop from fertilized eggs. Some individuals in this species carry a genetic element, termed psr (paternal sex ratio), which is transmitted through sperm and causes condensation and subsequent loss of paternal chromosomes in fertilized eggs, thus converting diploid females into haploid males. In this report the psr trait was shown to be caused by a supernumerary chromosome. This B chromosome contains at least three repetitive DNA sequences that do not cross-hybridize to each other or to the host genome. The psr chromosome apparently produces a trans-acting product responsible for condensation of the paternal chromosomes, but is itself insensitive to the effect. Because the psr chromosome enhances its transmission by eliminating the rest of the genome, it can be considered the most "selfish" genetic element yet described.