Identification of 16 chicken microchromosomes by molecular markers using two-colour fluorescence in situ hybridization (FISH)

A feature of avian karyotypes is the presence of microchromosomes. As a typical avian genome, the chicken karyotype (2n = 78) consists of nine pairs of macrochromosomes, including the W and Z sexual chromosomes, and 30 pairs of indistinguishable microchromosomes usually ordered arbitrarily by decreasing size. Despite their reduced size, microchromosomes represent one-third of the genome and have a high gene density. So as to provide a tool to identify them, we developed a set of large insert-containing clones to be used as tags in two-colour fluorescence in situ hybridization experiments. Seventeen clones, six of which contain a microsatellite sequence and two others the fatty acid synthase gene or genes from the major histocompatibility complex, all presenting a strong hybridization signal, were selected for this purpose and enabled us to identify 16 different microchromosomes. The ability to recognize individual microchromosomes will be of great value for cytogenetic gene mapping, assignation of linkage groups from genetic maps and other studies on avian genome structure.