Paternal transmission of an extrachromosomal factor in a wasp: evolutionary implications.

Females of the haplodiploid wasp, Nasonia vitripennis, lay both fertilised eggs (= female) and unfertilised eggs (= male) following matings with ordinary males. Matings with males that carry the extrachromosomal paternal-sex-ratio (psr) factor result in the production of all-male broods. These male offspring are haploid and inherit only the maternal genome, yet some also inherit the psr factor. The proportion of males inheriting psr is highly correlated with the proportion of eggs fertilised in non-psr matings, indicating that transmission of psr is effected by egg fertilisation and concomitant destruction of the paternally-derived chromosomes. A simple model suggests that the frequency of the factor ought to decline rapidly in a population producing only 50 per cent fertilised eggs; this was observed.