Developing DNA Markers for a Wild Source of Resistance to Papaya Ringspot Virus

Production of papaya (Carica papaya) is limited throughout its growing regions by the devastating disease of Papaya Ringspot Virus type P (PRSV-P). Papaya breeding programmes for PRSV-P resistance target two sources of resistance from crop wild relatives. The resistance loci from V. pubescens were identified in a segregating population resulting from an interspecific cross between V. pubescens and V. parviflora. The resistance loci in V. pubescens were located on supercontig (SC) 28 of the papaya whole genome sequence although SC28 is not yet anchored to any linkage group (LG). Recent investigations indicate that SC28 could be located at the end of LG5. Another species V. quercifolia also carries PRSV-P resistance, although more quantitative in action, and this species produces some fertile hybrids when crossed to C. papaya. The aim of this study was to target polymorphism at the end of LG5 and on SC28 in C. papaya and V. quercifolia. Five candidate resistant genes were used, corresponding to three nucleotide binding sites (NBS; NBS9.305, NBS9.308 and NBS18.235) at the end of LG5 and one NBS (NBS28.12) and one kinase gene (STK105) on SC28. Primers were designed for each gene and used to amplify DNA of papaya variety '2.001' and an accession of V. quercifolia. Resultant DNA sequences were aligned by BLASTN to the papaya whole genome sequence, indicating 98-100% and 89-99% identities for the '2.001' and V. quercifolia amplicons, respectively, and 82-93% identity between these two species.