Low X/Y divergence in four pairs of papaya sex-linked genes

Sex chromosomes in flowering plants, in contrast to those in animals, evolved relatively recently and only a few are heteromorphic. The homomorphic sex chromosomes of papaya show features of incipient sex chromosome evolution. We investigated the features of paired X- and Y-specific bacterial artificial chromosomes (BACs), and estimated the time of divergence in four pairs of sex-linked genes. We report the results of a comparative analysis of long contiguous genomic DNA sequences between the X and hermaphrodite Y (Y-h) chromosomes. Numerous chromosomal rearrangements were detected in the male-specific region of the Y chromosome (MSY), including inversions, deletions, insertions, duplications and translocations, showing the dynamic evolutionary process on the MSY after recombination ceased. DNA sequence expansion was documented in the two regions of the MSY, demonstrating that the cytologically homomorphic sex chromosomes are heteromorphic at the molecular level. Analysis of sequence divergence between four X and Y-h gene pairs resulted in a estimated age of divergence of between 0.5 and 2.2 million years, supporting a recent origin of the papaya sex chromosomes. Our findings indicate that sex chromosomes did not evolve at the family level in Caricaceae, and reinforce the theory that sex chromosomes evolve at the species level in some lineages.