Abstract : Amongst 15 species of birds, belonging to 7 orders, recurrent breakages evocating the presence of fragile sites were detected in the chromosomes of the 5 species belonging to Passeriformes. These breaks appear when BrdU (5-bromodeoxyuridine) is added to the cell culture medium, at a dose inefficient for inducing chromosome structure alterations in other birds and mammals. They involve, similarly in male and female, 3 loci on the Z chromosome of 3 species of the genus Turdus (Turdidae). The labeling by BrdU antibody confirms the correlation between BrdU incorporation into DNA and breakage, especially around and in the sites of breakage. Thus, 3 BrdU-sensitive fragile sites are present on the Z chromosome of these birds. Three fragile sites were also detected at different locations in the Z chromosomes of the European Robin (Erithacus rubecula Muscicapidae), which suggests that a structural rearrangement occurred during the evolution of Turdidae and Muscicapidae. Chromosome banding confirms this interpretation. Finally, in the more distantly related species Parus major (Paridae), the almost acrocentric Z chromosome displays a single BrdU-sensitive fragile site on its short arm and the W appears to be pulverized by BrdU incorporation. Although it cannot be excluded that the BrdU-sensitive fragile sites may be involved in rearrangements, their conservation in many species and possibly all Passeriformes, provides evidence that they do not constitute a pejorative character during evolution.
