Chromosomal sex determination and heterochromatin structure in date palm

In the date palm, a dioecious mode (separate male and female individuals) and the late initial reproductive age (5–10 years) are major practical constraints for genetic improvement. Early selection on young seedlings could enhance breeding programmes and generate experimental male and female genetic stocks, but no cytogenetic protocol exists for sex determination in an immature date palm. Here we describe a cytological method based on chromomycin staining which demonstrates the occurrence of sexual chromosomes carrying distinctive nucleolar heterochromatin and thus offers, for the first time, the possibility of identifying male and female individuals by simple analysis of root meristems. This observation has been extended by in situ rDNA hybridization, confocal microscopy and dual-label flow cytometry of nuclei.