Cytogenetic repartition of chicken CR1 sequences evidenced by PRINS in Galliformes and some other birds

Chicken repeat 1 (CR1) belongs to the non-long repeat class of retrotransposons. Nearly 100000 repeats interspersed in the chicken genome are subdivided into at least six distinct subfamilies, each 300 bp long and all sharing substantial sequence similarity. CR1-like elements were found in genomes from invertebrates to mammals, suggesting their importance for genome structure and/or function. Moreover, numerous data support the hypothesis of their implication in regulation of gene expression. So, the chromosomal distribution of these CR1 sequences in vertebrates is of great interest to improve our knowledge about the genome structure, function and evolution. A comparison of the cytogenetic distribution of CR1 sequences was performed by PRINS using consensus chicken primers on the chromosomes of chicken and species of several bird orders: Galliformes, Anseriformes, Passeriformes and Falconiformes. The study revealed that CR1 repeats are spread over nearly all chicken chromosomes with a higher density on the macrochromosomes and in particular with hot spots on subtelomeric regions of chromosome 1, 2, 3q, 4q, 5q. Their distribution on the macrochromosomes forms a kind of banding pattern, which was not systematically matched with R- or G-banding. This banding pattern appears to be conserved on the chromosomes of the Galliformes studied, irrespective of their karyotypes, rearranged or not. CR1 primers also show similar signals on the chromosomes of birds phylogenetically more distant (Anseriformes, Passeriformes and Falconiformes). This fact confirms the importance of these sequences at the large scale of bird evolution and in the chromosomal structure. The location of CR1 sequences, and in particular of the hot spots, mainly within the richest CG areas are in conformity with the data on an epigenetic role of these highly conserved sequences.