Construction and characterization of a papaya BAC library as a foundation for molecular dissection of a tree-fruit genome

A bacterial artificial chromosome (BAC) library was constructed from high-m olecular-weight DNA isolated from young leaves of papaya (Carica papaya L.) . This BAC library consists of 39168 clones from two separate ligation reac tions. The average insert size of the library is 132 kb; 96.5% of the 18700 clones from the first ligation contained inserts that averaged 86 kb in si ze, 95.7% of the 20468 clones from the second Ligation contained inserts th at averaged 174 kb in size. Two sorghum chloroplast probes hybridized separ ately to the library and revealed a total of 504 chloroplast clones or 1.4% of the library. The entire BAC library was estimated to provide 13.7 x pap aya-genome equivalents, excluding the false-positive and chloroplast clones . High-density filters were made containing 94% or 36864 clones of the libr ary with 12.7 x papaya-genome equivalents. Eleven papaya-cDNA and ten Arabi dopsis-cDNA probes detected an average of 22.8 BACs per probe in the librar y. Because of its relatively small genome (372 Mbp/1 C) and its ability to produce ripe fruit 9 to 15 months after planting, papaya shows promise as a model plant studying genes that affect fruiting characters. A rapid approa ch to locating fruit-controlling genes will be to assemble a physical map b ased on BAC contigs to which ESTs have hybridized. A physical map of the pa paya genome will significantly enhance our capacity to clone and manipulate genes of economic importance.