Carrot Molecular Cytogenetics

In this chapter, we review the contribution of cytogenetics to our understanding of the genome organization of the carrot (Daucus carota subsp. sativus) and its wild Daucus relatives. The genus Daucus includes about 40, mainly diploid, species with basic chromosome numbers ranging from n = 8 to n = 11. Early studies have suffered the difficulty to distinguish individual carrot chromosomes. Thanks to the development of carrot genomic resources, reliable chromosome identification and high-resolution karyotyping were obtained by using fluorescence in situ hybridization (FISH) and bacterial artificial chromosomes (BACs) as well as cocktails of repetitive sequences. These advances have contributed to study the organization and distribution of several repeat elements, such as miniature inverted-repeat transposable elements (MITEs) and retrotransposons, identify candidate centromeric and knob-associated repeats in carrot and other Daucus species, and begin uncovering syntenic chromosome regions between carrot and other Daucus species. Genome size analysis of about ten diploid species indicated a three-fold difference across Daucus. However, for many species, basic cytological data remain sketchy. Given the difficult taxonomy and the ongoing revision of the entire genus, we briefly argue that expanding such data as well as comparative cytogenetics studies in Daucus will contribute to clarify the phylogeny and perform a more effective exploitation and management of the Daucus germplasm.