Meiotic and mitotic instability of two EMS-produced centric fragments in the haplodiploid wasp Nasonia vitripennis

Terminal deletions that result in chromosomal fragments with centromeres (c entric fragments) arc relatively easy to generate and study in the haplodip loid insect Nasonia. We investigated the transmission stability of two chro mosomal fragments generated by chemical mutagenesis. Visible mutations at t he R locus (peach-233 and St-DR) and a linked body-colour mutant (purple) w ere used to track transmission of the centric fragments (which lack the pur ple locus and are wild-type at the R locus). Transmission rates in meiotic oogenesis were low (medians 0.15-0.18) and comparable to previous data on c entric fragments in this species. The homologous chromosome genetic backgro und strongly affected meiotic stability of one centric fragment (CF2) but n ot the other (CF1). Specifically, in peach/scarlet R locus heterozygous fem ales, CF2 showed a normal segregation proportion with the chromosome bearin g the scarlet allele (0.16), but near complete failure to segregate with pe ach (0.0002). Data show that this is due to loss of CF2 in eggs receiving p each, rather than to preferential segregation of CF2 with scarlet or mortal ity of CF2-bearing males. CF1 shows typical segregation ratios with both ch romosomes. We hypothesize that deletions (or rearrangements) associated wit h the peach-233 mutant inhibit proper pairing and segregation of CF2. Consi stent with the model, CF2 did segregate with chromosomes that had undergone recombination between peach and purple (a body-colour mutation 10 cm from peach), indicating that the domains inhibiting segregation are closely link ed to peach. Mitotic instability also differed between the two fragments; r educed mitotic stability may relate to absence of telomeres on these centri c fragments. Given the relative case of generating and tracking terminal de letions in Nasonia, we propose this as a good system for studying mitotic a nd meiotic stability of centric fragments. Finally, results are discussed i n relation to the evolution of B chromosomes from centric fragments.