A "Selfish" B Chromosome That Enhances Its Transmission by Eliminating the Paternal Genome

Abstract

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.

Evolution of Tandemly Repeated Sequences: What Happens at the End of an Array?

“…PSR is continually maintained in a haploid cell lineage because it is always carried by (haploid) male wasps. The chromosome is transmitted in sperm, and after fertilization PSR causes loss of the paternal autosomes in the first zygotic division (Werren et al 1987;Nur et al 1988). The result is development of a male wasp containing the haploid maternal autosomes and PSR.…”

“…Three of these tandem repeats (PSR2, PSR18, and PSR22) are classic ''satellite'' DNAs and they apparently evolved specifically on PSR, because they are not present on the autosomes of N. vitripennis or related species (Nur et al 1988;Eickbush et al 1992). The three repeated sequences are approximately 171-214 bp in length (Eickbush et al 1992) and are present in large localized arrays on the PSR chromosome (Beukeboom and Werren 1993).…”

Hybrid origin of a B chromosome (PSR) in the parasitic wasp Nasonia vitripennis

“…Paternal sex ratio (PSR) is a B chromosome in the parasitic wasp Nasonia vitripennis (Nur et al 1988;Werren 1991). The transmission pattern of this chromosome is an extreme example of the independent nature of B chromosomes.…”

“…Loss of the paternal chromosomes results in the transformation of a diploid zygote into a haploid zygote that develops into a male, owing to haplodiploid sex determination. Molecular analyses of PSR have characterized a number of repetitive sequences from this chromosome (Nur et al 1988;Eickbush et al 1992;McAllister 1995). Three different families of PSR-specific tandemly repeated sequences (PSR2, PSR18, PSR22), a tandem repeat (PSR79) that is also present in the A complement, and a retrotransposable element (NATE) have all been cloned and characterized from PSR.…”

“…In previous papers (Nur et al 1988;Werren 1991;McAllister 1995), a hybrid origin of the PSR chromosome has been proposed. Based upon the geographic distribution of PSR (restricted to western North America in an area of sympatry with the sibling species Nasonia longicornis) and evidence that interspecific cytoplasmic incompatibility results in the generation of de novo chromosomal fragments (Ryan et al 1985;Beukeboom and Werren 1993b), it was proposed that PSR arose as a chromosomal fragment from N. longicornis generated by interspecific hybridization with N. vitripennis.…”