Organization of repetitive DNA sequences at pachytene chromosomes of gilthead seabream Sparus aurata (Pisces, Perciformes)

A method of preparing two-dimensional surface spreads of fish synaptonemal complexes (SCs) associated with fluorescent in-situ hybridization is described. This technique permits a novel approach to the analysis of chromatin organization and the construction of physical maps at meiosis, since surface-spread pachytene chromosomes are several times the length of metaphase chromosomes and the decondensed chromatin loops are attached to the lateral elements of the SC. We have applied this technique to analyze the location and organization of three different repetitive DNA sequences, rDNA, an EcoRI satellite DNA of the Sparidae family and telomere DNA in the gilthead seabream Sparus aurata. Our observations indicate that, depending on the type of sequence, the chromatin has different properties with regard to anchorage to the SC.