Terminal deletions that result in chromosomal fragments with centromeres (centric fragments) are relatively easy to generate and study in the haplodiploid insect Nasonia. We investigated the transmission stability of two chromosomal fragments generated by chemical mutagenesis. Visible mutations at the R locus (peach-233 and St-DR) and a linked body-colour mutant (purple) were used to track transmission of the centric fragments (which lack the purple 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 centric fragments in this species. The homologous chromosome genetic background strongly aected meiotic stability of one centric fragment (CF2) but not the other (CF1). Speci®cally, in peach/scarlet R locus heterozygous females, CF2 showed a normal segregation proportion with the chromosome bearing the scarlet allele (0.16), but near complete failure to segregate with peach (0.0002). Data show that this is due to loss of CF2 in eggs receiving peach, rather than to preferential segregation of CF2 with scarlet or mortality of CF2-bearing males. CF1 shows typical segregation ratios with both chromosomes. We hypothesize that deletions (or rearrangements) associated with the peach-233 mutant inhibit proper pairing and segregation of CF2. Consistent with the model, CF2 did segregate with chromosomes that had undergone recombination between peach and purple (a bodycolour mutation 10 cM M from peach), indicating that the domains inhibiting segregation are closely linked to peach. Mitotic instability also diered between the two fragments; reduced mitotic stability may relate to absence of telomeres on these centric fragments. Given the relative ease of generating and tracking terminal deletions in Nasonia, we propose this as a good system for studying mitotic and meiotic stability of centric fragments. Finally, results are discussed in relation to the evolution of B chromosomes from centric fragments.Keywords: B-chromosome, centric fragment, EMS, haplodiploid, mitotic instability, mutagen.
IntroductionA variety of chemical and environmental mutagens result in chromosomal deletions and rearrangements (Sankaranarayanan, 1996; Grant & Owens, 1998). Terminal deletions are an extreme form; one or both arms of a chromosome are deleted, leaving the centromeric region and¯anking regions of variable size. Such largescale deletions are typically lethal or highly detrimental, because they are often associated with aneuploidy or dosage compensation eects, including partial trisomy of pericentric regions in humans (Stankiewicz et al., 2000). Under some circumstances, major terminal deletions can result in a chromosome containing only the centromere and a relatively small¯anking region. These are referred to as`centric fragments'. In humans, they are referred to as`marker chromosomes' and are implicated in the aetiology of abnormal phenotypes (Li et al., 2000; Stankiewicz et al., 2000). Terminal deletions also typically lack telomeres on the dele...