Chromosomal location of rRNA genes in Diprion pini detected by in situ hybridization

Diprion pini belongs to a small family of sawflies with n = 7 as the modal chromosome number. This species has 14 acrocentric chromosomes and one carr ies a satellite. In situ hybridization to mitotic chromosomes of a Drosophi la rDNA probe was carried out according to two protocols. The biotinylated probe was detected with peroxidase-conjugated extravidin and diaminobenzidi ne or with fluorescein-conjugated extravidin (FISH). These two techniques s howed that only one chromosome per haploid complement responds to the probe used. The centric fission hypothesis is consequently more likely than poly ploidization in order to explain the chromosome number doubling in D. pini. The sites of probe hybridization are located on the satellite and its carr ier short arm, which are heterochromatin-rich. The propensity of rDNA and h eterochromatin for self duplication and accretion most likely created the s atellite and the short arms. This process is also known to be one of the me chanisms by which extra segments may arise. (C) Academie des Sciences / Els evier, Paris.