title: Construction of physical maps for the sex-specific regions of papaya sex chromosomes.
abstract: BACKGROUND
  Papaya is a major fruit crop in tropical and subtropical regions worldwide. It is trioecious with three sex forms: male, female, and hermaphrodite. Sex determination is controlled by a pair of nascent sex chromosomes with two slightly different Y chromosomes, Y for male and Yh for hermaphrodite. The sex chromosome genotypes are XY (male), XYh (hermaphrodite), and XX (female). The papaya hermaphrodite-specific Yh chromosome region (HSY) is pericentromeric and heterochromatic. Physical mapping of HSY and its X counterpart is essential for sequencing these regions and uncovering the early events of sex chromosome evolution and to identify the sex determination genes for crop improvement.
  
  
  RESULTS
  A reiterate chromosome walking strategy was applied to construct the two physical maps with three bacterial artificial chromosome (BAC) libraries. The HSY physical map consists of 68 overlapped BACs on the minimum tiling path, and covers all four HSY-specific Knobs. One gap remained in the region of Knob 1, the only knob structure shared between HSY and X, due to the lack of HSY-specific sequences. This gap was filled on the physical map of the HSY corresponding region in the X chromosome. The X physical map consists of 44 BACs on the minimum tiling path with one gap remaining in the middle, due to the nature of highly repetitive sequences. This gap was filled on the HSY physical map. The borders of the non-recombining HSY were defined genetically by fine mapping using 1460 F2 individuals. The genetically defined HSY spanned approximately 8.5 Mb, whereas its X counterpart extended about 5.4 Mb including a 900 Kb region containing the Knob 1 shared by the HSY and X. The 8.5 Mb HSY corresponds to 4.5 Mb of its X counterpart, showing 4 Mb (89%) DNA sequence expansion.
  
  
  CONCLUSIONS
  The 89% increase of DNA sequence in HSY indicates rapid expansion of the Yh chromosome after genetic recombination was suppressed 2-3 million years ago. The genetically defined borders coincide with the common BACs on the minimum tiling paths of HSY and X. The minimum tiling paths of HSY and its X counterpart are being used for sequencing these X and Yh-specific regions.
authors.affiliation: Department of Plant Biology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA.
authors.forename: Jong-Kuk
authors.initials: JK
authors.lastname: Na
authors.forename: Jianping
authors.initials: J
authors.lastname: Wang
authors.forename: Jan E
authors.initials: JE
authors.lastname: Murray
authors.forename: Andrea R
authors.initials: AR
authors.lastname: Gschwend
authors.forename: Wenli
authors.initials: W
authors.lastname: Zhang
authors.forename: Qingyi
authors.initials: Q
authors.lastname: Yu
authors.forename: Rafael
authors.initials: R
authors.lastname: Navajas-Pérez
authors.forename: F Alex
authors.initials: FA
authors.lastname: Feltus
authors.forename: Cuixia
authors.initials: C
authors.lastname: Chen
authors.forename: Zdenek
authors.initials: Z
authors.lastname: Kubat
authors.forename: Paul H
authors.initials: PH
authors.lastname: Moore
authors.forename: Jiming
authors.initials: J
authors.lastname: Jiang
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Paterson
authors.forename: Ray
authors.initials: R
authors.lastname: Ming
mesh_terms: D001483:Base Sequence; D029441:Carica / Q000235:genetics*; D022202:Chromosomes, Artificial, Bacterial; D032461:Chromosomes, Plant / Q000235:genetics*; D003433:Crosses, Genetic; D005819:Genetic Markers; D018895:Microsatellite Repeats / Q000235:genetics; D020161:Physical Chromosome Mapping / Q000379:methods*; D011995:Recombination, Genetic / Q000235:genetics
chemical_list: D005819:Genetic Markers