The chicken as a model to study microchromosomes in birds: a review

The typical avian karyotype is composed of a few macrochromosomes and around 60 indistinguishable small microchromosomes. Due to its economic importance, the chicken is the avian species for which cytogenetic and genetic maps are the most developed. Based on these genome studies, it has been shown that the chicken microchromosomes are carriers of dense genetic information. Indeed, they probably bear at least 50 % of the genes and exhibit high recombination rates. Because of the presence of microchromosomes, the genetic size of the chicken genome seems higher than first estimated and could reach more than 4000 cM for 1200 Mb. Thus, it is worth developing the microchromosome map. From an evolutionary point of view, comparative mapping data raise many questions about the origin of microchromosomes. They could be ancestral chromosomes, from which large chromosomes formed by fusions, or conversely they could be the result of the splitting of macrochromosomes. (C) Inra/Elsevier, Paris.