Papaya sex chromosomes

Papaya is a semi - woody tree that produces fruit rich in vitamins and minerals. It is trioecious with male, female, and hermaphrodite plants. Though many theories have been suggested in the past, papaya sex is determined by a pair of nascent sex chromosomes; females have two X chromosomes, males have an X and a Y, and hermaphrodites have an X and a Y_h, which varies slightly from the male Y. Any combination of the Y and Y_h genotype, YY, Y Y_h, or Y_h Y_h, is lethal. The X and Y chromosomes have a small non -recombining region in the centromeric and pericentromeric region. The hermaphrodite -specific region of the Y_h chromosome (HSY) is gene poor and has an increased amount of retroelements and chromosomal rearrangements compared to its X counterpart and the genome wide average. The X and Y chromosomes were estimated to have diverged relatively recently about 2 - 3 million years ago (MYA), and even more so for the Y and Y_h (73,000 years ago). Physical maps of the HSY and the corresponding X region have been produced and sequenced, showing the HSY sequence has expanded. Genes in these regions are being mined with a special focus on identifying the two sex determination genes, one promoting maleness and one suppressing femaleness. The impact of identifying the sex determination genes would be high for both the commercial production of papaya and the field of sex chromosome evolution.