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A high-density genetic map of papaya (Carica papaya L.) was constructed using microsatellite markers derived from BAC end sequences and whole-genome shot gun sequences. Fifty-four F 2 plants derived from varieties AU9 and SunUp were used for linkage mapping. A total of 707 markers, including 706 microsatellite loci and the morphological marker fruit flesh color, were mapped into nine major and three minor linkage groups. The resulting map spanned 1069.9 cM with an average distance of 1.5 cM between adjacent markers. This sequence-based microsatellite map resolved the very large linkage group 2 (LG 2) of the previous high-density map using amplified fragment length polymorphism markers. The nine major LGs of our map represent papaya's haploid nine chromosomes with LG 1 of the sex chromosome being the largest. This map validates the suppression of recombination at the male-specific region of the Y chromosome (MSY) mapped on LG 1 and at potential centromeric regions of other LGs. Segregation distortion was detected in a large region on LG 1 surrounding the MSY region due to the abortion of the YY genotype and in a region of LG6 due to an unknown cause. This high-density sequencetagged genetic map is being used to integrate genetic and physical maps and to assign genome sequence scaffolds to papaya chromosomes. It provides a framework for comparative structural and evolutional genomic research in the order Brassicales. Papaya is a perennial plant species that flowers as early as 3 months and produces fruit in 9 months; it is trioecious with an intriguing sex determination system with three basic sex types: male, female, and hermaphrodite. A high-density genetic map of papaya, constructed using 1498 AFLP and three morphological markers revealed severe suppression of recombination at the sex determination locus with 225 cosegregating markers accounting for 66% of the markers on linkage group 1 (LG 1) and 15% of the markers genomewide . This AFLP map provided a critical piece of evidence for defining the primitive Y chromosome in papaya . Sequence comparison of Y-specific fragments from males and hermaphrodites confirmed that 1

Papaya (Carica papaya L.) is a major tree fruit crop of tropical and subtropical regions with an estimated genome size of 372 Mbp. We present the analysis of 4.7% of the papaya genome based on BAC end sequences (BESs) representing 17 million high-quality bases. Microsatellites discovered in 5,452 BESs and flanking primer sequences are available to papaya breeding programs at http://www.genomics.hawaii.edu/papaya/BES . Sixteen percent of BESs contain plant repeat elements, the vast majority (83.3%) of which are class I retrotransposons. Several novel papaya-specific repeats were identified. Approximately 19.1% of the BESs have homology to Arabidopsis cDNA. Increasing numbers of completely sequenced plant genomes and BES projects enable novel approaches to comparative plant genomics. Paired BESs of Carica, Arabidopsis, Populus, Brassica and Lycopersicon were mapped onto the completed genomes of Arabidopsis and Populus. In general the level of microsynteny was highest between closely related organisms. However, papaya revealed a higher degree of apparent synteny with the more distantly related poplar than with the more closely related Arabidopsis. This, as well as significant colinearity observed between peach and poplar genome sequences, support recent observations of frequent genome rearrangements in the Arabidopsis lineage and suggest that the poplar genome sequence may be more useful for elucidating the papaya and other rosid genomes. These insights will play a critical role in selecting species and sequencing strategies that will optimally represent crop genomes in sequence databases.

BackgroundPapaya is a major fruit crop in tropical and subtropical regions worldwide and has primitive sex chromosomes controlling sex determination in this trioecious species. The papaya genome was recently sequenced because of its agricultural importance, unique biological features, and successful application of transgenic papaya for resistance to papaya ringspot virus. As a part of the genome sequencing project, we constructed a BAC-based physical map using a high information-content fingerprinting approach to assist whole genome shotgun sequence assembly.ResultsThe physical map consists of 963 contigs, representing 9.4× genome equivalents, and was integrated with the genetic map and genome sequence using BAC end sequences and a sequence-tagged high-density genetic map. The estimated genome coverage of the physical map is about 95.8%, while 72.4% of the genome was aligned to the genetic map. A total of 1,181 high quality overgo (overlapping oligonucleotide) probes representing conserved sequences in Arabidopsis and genetically mapped loci in Brassica were anchored on the physical map, which provides a foundation for comparative genomics in the Brassicales. The integrated genetic and physical map aligned with the genome sequence revealed recombination hotspots as well as regions suppressed for recombination across the genome, particularly on the recently evolved sex chromosomes. Suppression of recombination spread to the adjacent region of the male specific region of the Y chromosome (MSY), and recombination rates were recovered gradually and then exceeded the genome average. Recombination hotspots were observed at about 10 Mb away on both sides of the MSY, showing 7-fold increase compared with the genome wide average, demonstrating the dynamics of recombination of the sex chromosomes.ConclusionA BAC-based physical map of papaya was constructed and integrated with the genetic map and genome sequence. The integrated map facilitated the draft genome assembly, and is a valuable resource for comparative genomics and map-based cloning of agronomically and economically important genes and for sex chromosome research.

Sex chromosomes in flowering plants evolved recently and many of them remain homomorphic, including those in papaya. We investigated the chromosomal location of papaya's small male specific region of the hermaphrodite Y (Yh) chromosome (MSY) and its genomic features. We conducted chromosome fluorescence in situ hybridization mapping of Yh-specific bacterial artificial chromosomes (BACs) and placed the MSY near the centromere of the papaya Y chromosome. Then we sequenced five MSY BACs to examine the genomic features of this specialized region, which resulted in the largest collection of contiguous genomic DNA sequences of a Y chromosome in flowering plants. Extreme gene paucity was observed in the papaya MSY with no functional gene identified in 715 kb MSY sequences. A high density of retroelements and local sequence duplications were detected in the MSY that is suppressed for recombination. Location of the papaya MSY near the centromere might have provided recombination suppression and fostered paucity of genes in the male specific region of the Y chromosome. Our findings provide critical information for deciphering the sex chromosomes in papaya and reference information for comparative studies of other sex chromosomes in animals and plants.