In the present study, the origin of recurrent rearrangements involving chromosome 6 in 3.2% of cells of Melolontha melolontha (Coleoptera, Scarabaeidae) was investigated. Various chromosome staining techniques, including C-banding, Giemsa and silver staining, as well as fluorescence in situ hybridization with a human 28S rDNA probe, were applied to M. melolontha chromosome spreads. In addition, related species of the genera Melolontha and Protaetia were studied. On chromosome 6 of M. melolontha, there is a fragile site-like structure which corresponds to an interstitial nucleolus organizer region (NOR). Despite this instability, the NOR remains unique and interstitial in this species, as well as in the other species studied. It is proposed that the intercalary position of the NOR both facilitates the detection of its fragile site-like instability and correlates with its relative stability during evolution. We explain this apparent paradox by strong counter-selection for imbalances of the chromosome fragment distal to the interstitial NORs, which would recurrently occur in the progeny of translocation carriers. Thus, the frequent telomeric position of the NORs in most animal and plant taxa would have no functional rationale but would be the consequence of selection against the meiotic transmission of chromosome imbalances.

Keywords:
Chromosome 6, Fragile site, Interstitial NOR, Melolontha, Protaetia
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