Progress in backcrossing between Carica papaya × Vasconcellea quercifolia intergeneric hybrids and C. papaya

Papaya is a major tropical fruit crop worldwide, however, all genotypes are susceptible to Papaya ringspot virus type P (PRSV-P). Protocols have been developed to produce large numbers of intergeneric hybrids between Carica papaya L. and Vasconcellea quercifolia, which is PRSV-P resistant. Although pollen fertility of these hybrids was low, backcross populations were produced in both Australia and the Philippines when papaya flowers were pollinated with pollen from 11 hybrid plants that had been identified as having some pollen fertility and were PRSV-P resistant. Plantlets were produced after embryo rescue and culture in vitro. Second backcross generations (BC2) were easier to produce than first backcross (BC1) generations as pollen fertility was >80% in male BC1 plants. Variation in time to PRSV-P symptom expression and severity of virus symptoms in the backcross generations was observed in both countries. Commercially acceptable fruit were produced in the BC2 generation. The results demonstrate that efforts in wide hybridisation to transfer PRSV-P resistance to C. papaya, are better directed towards crosses between C. papaya and V. quercifolia than with other Vasconcellea species.