Abstract
BackgroundPapaya 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.ResultsA 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.ConclusionThe 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.
Highlights
Papaya is a major fruit crop in tropical and subtropical regions worldwide
Physical mapping of hermaphrodite-specific Yh chromosome region (HSY) Physical mapping started with screening the hermaphrodite bacterial artificial chromosome (BAC) library using the sex co-segregating sequence characterized amplified region (SCAR) marker W11 by which four positive BACs were identified
The BAC end sequences (BES) of all four positive BACs were used for sorting out the configuration to select the two most extended ends as probes for the round of screening of the BAC library
Summary
Papaya is a major fruit crop in tropical and subtropical regions worldwide. It is trioecious with three sex forms: male, female, and hermaphrodite. Most flowering plant species are hermaphrodites having both male and female sexual organs in the same flower. It is proposed that the dioecious species have been evolved from ancestral hermaphrodite species along with the evolution of sex chromosomes [2]. The dioecious plant species provide a unique opportunity for understanding the evolution of plant sex and sex chromosomes because of the co-existence of hermaphrodite, gynodioecious, androdioecious, dioecious, or even trioecious species and the possession in some families of sex chromosomes at various evolutionary stages [3,4]
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