TRANSMISSION AND EXPRESSION OF THE PARASITIC PATERNAL SEX-RATIO (PSR) CHROMOSOME

B-chromosomes are often considered genomic parasites. They are extra to the normal chromosomal complement, are unnecessary for survival of an individual, and are often inherited at higher than Mendelian rates. Paternal Sex Ratio (PSR) is an extreme example of a parasitic B-chromosome in the wasp Nasonia vitripennis. It is transmitted via sperm but then destroys the other paternal chromosomes in the early fertilized egg. PSR disrupts the normal haplodiploid sex determination system of this wasp by converting diploid (female) eggs into haploid eggs that develop into PSR-bearing males. Transmission and expression of PSR was measured in single pair crosses between carrier males and standard females. Presence of the chromosome was detected by probing offspring with PSR-specific repetitive DNA. Most (equal to or more than 90 per cent) PSR males produced all-male offspring. Overall transmission rates of PSR to fertilized eggs varied beween 0.94 and 1.0. Some males (up to 10 per cent) produced daughters at varying frequencies. Of 226 daughters tested, only one carried PSR (and this may have been a laboratory error) indicating that daughters result from failure of transmission rather than loss of expression. Transmission of PSR to males in families that included female offspring varied from 0 to 94 per cent. Incomplete transmission is most likely the result of loss of PSR in some spermatogonial cell lineages and indicates some mitotic instability. Implications of the results to the aetiology and population genetics of PSR are discussed.