Papaya (Carica papaya L.) is a principal fruit crop of tropical and subtropical regions worldwide. Papaya fruit is a major export commodity in Hawaii, and is also in production in Florida, the Rio Grande Valley of Texas, and southern California. Papaya trees are grown for both fruit and papain, a commercially valuable proteolytic enzyme. Papaya is one of the few plant species which fruit throughout the year and can produce ripe fruit in as little as nine months from planting. A papaya tree may live for 25 years or longer, bearing continuously with one or more fruit in each leaf axil.
Papaya has male, female, and hermaphrodite sexes, with a primitive Y chromosome that provides a unique opportunity to study sex chromosome evolution and sex determination. Sex chromosomes in animals are ancient - about 300 million years old, and Y-chromosomes are genetically eroded. Flowering plants appeared about 130-200 million years ago and plant sex chromosomes evolved more recently. The Silene genus with its estimated 20 - 25 million-year ancestry has generally been thought to contain the most recently evolved XY system known in eukaryotes. Yet 90% of the Y chromosome in Silene is degenerated and suppressed for recombination. The recently discovered primitive Y chromosome in papaya has a small male specific region that comprises only about 10% of the Y chromosome. This male specific region (MSY) shows much more moderate degeneration and suppression of recombination. Thus the papaya system appears to be the most recently evolved XY system known to date, offering potential insights into the question of how evolution of the sex chromosome began. Unraveling plant sex determination processes will have direct applications in production of crop plants such as papaya, asparagus, spinach, black pepper, yam, and pistachio.
This project seeks to explore the nature of incipient sex chromosomes. The papaya MSY and the corresponding X-specific region will be sequenced and characterized. To estimate the antiquity of the non-recombining region, the extent of divergence of X and Y gene pairs along the region will be calculated, giving special attention to candidate gene(s) for sex determination. Comparing the X and Y DNA sequences and their surroundings will reveal types and extent of chromosomal rearrangements and the role of transposable elements in the degeneration of MSY. The relative position of MSY to the centromere of the primitive Y chromosome will be clarified by pachytene and fiber Fluorescent In Situ Hybridization (FISH) analyses. It is expected that this project will generate candidate genes for sex determination and embryo lethality for the papaya research community.
Access to project outcomes DNA sequence data and trace files will be deposited in GenBank, and project data made available at the websites of Hawaii Agriculture Research Center (www.hawaiiag.org/harc) and University of Georgia (www.plantgenome.uga.edu/). Bacterial Artificial Chromosome (BAC) clones will be distributed by the Plant Genome Mapping Laboratory at the University of Georgia and also deposited in a public BAC distribution center.
