Chromosome fission associated with growth of ribosomal DNA in Neodiprion abietis (Hymenoptera : Diprionidae)

The haploid complement consists of seven metacentric chromosomes in most di prionid species but has evolved to n=8 by fission in Neodiprion abietis. Th is fission generated a small telocentric chromosome and a large pseudoacroc entric chromosome with a short arm carrying a satellite. In situ hybridizat ion indicated that the location of the rRNA gene cluster corresponds to the whole short arm. This suggests that (i) the breaking paint was located clo se to an rRNA gene cluster, and (ii) fission was associated with growth of rDNA. These results suggest rDNA as a preferential breaking point but with a role in the healing of naked chromosome ends.