University of Naples, Federico II, Department of Soil, Plant, Environmental and Animal Production Sciences, Via Università 100, 80055 Portici, Italy. *Corresponding author: [email protected]

The cultivated commercial potato Solanum tuberosum is a polysomic tetraploid (2n = 4x = 48) crop with basic chromosome number x = 12, and a haploid genome size of approximately 840 Mbp, which makes it a mediumsized plant genome. Each of the four copies of homologous chromosomes segregates from multivalents or random bivalents. Due to the small size (about 1-3.5 µm at mitotic metaphase) of individual chromosomes, it has been generally regarded as a cytologically diffi cult species to study. Indeed, potato mitotic chromosomes cannot be identifi ed based on their morphology. Mohanty et al. (2004) found high variability in cytological characteristics (e.g., total chromosome length and volume, number of secondary restricted chromosomes) between various cultivars and breeding lines. On the basis of size and position of constrictions, they classifi ed potato mitotic chromosomes into four groups. Advanced approaches are now the most suitable alternatives to the strategies traditionally used for potato chromosome analysis. Reliable identifi cation of the potato somatic metaphase chromosomes has been achieved using a set of restriction fragment length polymorphism (RFLP) marker-anchored bacterial artifi cial chromosome (BAC) clones as fl orescence in situ hybridization (FISH) probes (Dong et al. 2000). This work enabled researchers to assign the genetic linkage groups of potato to specifi c chromosomes, and the potato chromosomes were consequently numbered according to their correspondent linkage group (this numbering system is generally accepted and was also adopted by the International Potato Genome Sequence Consortium).