Physical Mapping of Papaya Sex Chromosomes

Abstract

Papaya sex chromosomes have recently evolved and suitable for studying the early events of sex chromosome evolution that are not detectable in ancient sex chromosomes like those of humans. The papaya hermaphrodite-specific Y chromosome region (HSY) is pericentromeric, heterochromatic, and suppressed for recombination. Complete sequencing and analysis of the papaya sex chromosomes will advance our knowledge of sex determination mechanism(s) in plants and the evolutionary process of young sex chromosomes. Physical mapping of HSY and its X counterpart is essential for complete sequencing of the papaya sex chromosomes. The maps of bacterial artificial chromosome (BAC) in minimal tiling path for both HSY and the corresponding X region were constructed in papaya through chromosome walking. The HSY map contained 68 anchored overlapped BAC clones and spanned approximately 8.5 Mb, while the physical map for the corresponding X region had 44 BAC clones and extended about 5.4 Mb with a small gap in the middle of the map unfilled. The borders of the MSY/X region were defined by fine mapping with a large F2 population. The HSY exhibited about 89 % expansion of DNA sequence compared to the corresponding X region, indicating expansion of the Y chromosome at an early evolutionary stage. These physical maps of HSY and X corresponding region in papaya provide the foundation for sequencing and analysis of the heterochromatic young sex chromosomes in papaya.