| Papaya is a popular fruit tree for home garden and commercial production throughout the tropics. Papaya received top ranking among 39 common fruits for overall nutrition and, depending on variety, one medium papaya can supply more than the minimum daily requirement of vitamin A for adults. For this reason, papaya is one of the crops recommended by the World Health Organization as part of a sustainable strategy combining crop bio-fortification and dietary education programs to combat Vitamin A Deficiency (VAD). VAD is the leading cause of preventable childhood blindness, affecting nearly 0.5M children every year with approximately 50% mortality within a year of diagnosis. VAD is a serious public health concern in many of the countries where papaya is grown (Appendix C, Figure C.1) and development of new varieties that combine high vitamin A content with fruit qualities that affect consumer acceptance (e.g. fruit size, shape, aroma) is being pursued. Papaya fruit size and shape are quantitative traits controlled by multiple genes while fruit flesh color is controlled by a single major gene. Quantitative trait locus (QTL) mapping of a F2 population segregating for fruit size identified 13 QTL for papaya fruit weight, diameter, length or shape. These QTL mapped across five major linkage groups of the papaya genetic map and account for 5.5 to 37.1% of the phenotypic variation of the fruit trait. The 1-LOD interval surrounding each QTL was searched for candidate genes. Five candidate genes (without QTL association) that show homology to one of three previously identified tomato loci ( ovate, sun or fw2.2) affecting tomato fruit size and shape have been identified in the papaya genome. Additionally, a chromoplast-specific lycopene beta-cyclase, CpCYC-b, has been identified as the single major gene determining papaya fruit flesh color. The benefit that papaya provides for vitamin A nutrition is derived from its carotenoid content that determines fruit flesh color. A co-dominant DNA marker, CPFC, with tight linkage to CpCYC-b has been developed for marker-assisted selection (MAS) in papaya breeding programs. The research presented here provides initial tools for MAS of papaya fruit size, shape and flesh color as well as additional genetic and sequence information for further genomic studies. |
