Chromosome homeologies and high variation in heterochromatin distribution between Citrus L. and Poncirus Raf. as evidenced by comparative cytogenetic mapping

The genus Citrus is well-known for its economic importance and complex taxonomy. Only three to six Citrus taxa are considered true biological species, among them is Citrus medica, the citron. Previous studies based on chromomycin A3 (CMA)/4',6-diamidino-2-phenylindole staining showed that the citron has a homomorphic karyotype, consisting of four distinct chromosome types according to the CMA(+) heterochromatin distribution. Based on the pattern of CMA(+) bands and the hybridization of 25 bacterial artificial chromosomes (BACs) from a genomic library of Poncirus trifoliata, a closely related species to the genus Citrus, we were able to identify each chromosome pair and build a comparative cytogenetic map for C. medica. The data showed a high degree of sequence conservation between these genera, enabling heterologous hybridization of BACs, and the establishment of chromosomal homeologies. It was thus possible to visualize changes in the position of some BACs in relation to CMA(+) bands. Since no breakdown of synteny was observed between these species, expansions and contractions in repetitive DNA sequences seem to be the major driving force of chromosomal evolution since the separation of these two genera.