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An attempt to develop Carica papaya L. resistant to Papaya ringspot virus type P (PRSV-P) has been established via an intergeneric cross using Vasconcellea pubescens as the resistance donor. V. pubescens has consistently been reported to be resistant to PRSV-P, unlike the resistance in other species (d’Eeckenbrugge et al., 2014). The immunity to PRSV-P is also controlled by a single dominant gene (Dillon et al., 2006), which should be easy to transfer across crops. Because of the genetic distance between the two genera and genetic incompatibility, Vasconcellea parviflora was introduced as a bridging species to transfer the PRSV-P resistance gene(s) to papaya (C. papaya). Backcrossing has been achieved to BC4 in order to maximize the V. parviflora genetic background to improve fertility. The PRSV-P resistance genotype of the BC4 was determined with previously published sequences linked to the prsv-1 locus (Dillon et al., 2006). One of the heterozygous lines, BC4#113, was selected to use as a resistance donor to cross with C. papaya line 2.001. Embryos were rescued, initiated and multiplied in vitro. The cross was successfully micropropagated and produced 18 F1 lines [C. papaya × (V. parviflora backcrossed to V. parviflora × V. pubescens)]. These F1s were genotyped at the prsv-1 locus as previously mentioned. They were subsequently acclimatized and planted in a field in north Queensland. True hybridity was revealed through morphological characterization, including pink flowers, which is a dominant trait from V. parviflora. The hybrid F1s carrying the PRSV-P locus were then successfully backcrossed to C. papaya to generate BC1 generations. They were also sib-crossed with the F1 to generate F1:F2 generations. These lines will be planted in a quarantine laboratory at Griffith University, Nathan, for evaluation of virus resistance. Fertile plants that carry the virus-resistance alleles will be used as parental lines in future breeding strategies.

Beyond their own commercial potential, highland papayas (Vasconcellea spp.) bear genes for resistances against important diseases, cold hardiness, and monoecy, which are absent from the common papaya genome. While the two genera share many morphological traits, strong reproductive barriers have considerably limited the success of introgression programs, imposing the use of special techniques for hybridizations. In fact, all cytogenetic and genetic studies have consistently shown that Vasconcellea is relatively distant from Carica as compared to other Caricaceae genera. A few years after the first breakthrough obtained in the field, the present chapter reviews the potential of the different highland papaya species as sources of genes for papaya improvement, the considerable experience accumulated in the different hybridization programs, the advantages of this difficult approach, and its current perspectives. It also stresses the need for further exploration within Vasconcellea and closer genera of the Caricaceae. After the successful introgression of papaya ring spot virus resistance from V. quercifolia, the combination of marker-assisted selection, new breeding schemes (e.g., bridging species), and a deeper knowledge of Caricaceae gene pools should give access to more genes of considerable interest to the papaya industry.

Molecular markers have been applied to papaya crop improvement for two decades. They were initially used to study the genetic diversity among the Caricaceae, containing the cultivated Carica papaya and 21 related wild species. Originally, these were all placed within the genus Carica. However, in 2000, molecular markers were employed to show that several members of this genus were genetically distant from C. papaya, and they were subsequently placed in a new genus Vasconcellea. C. papaya plants exhibit three potential sex types, and the ability to identify sex of a plant at an early growth stage would greatly benefit commercial producers in terms of planting management. Therefore, much marker research has been applied to the identification of DNA markers for the differentiation and selection of male, female, and hermaphrodite plants. Papaya is an ideal model tropical fruit species for genomic studies. It has a relatively small genome size of 372 Mb, is diploid 2n = 18, produces climacteric fruit, and has a short generation time. The papaya genome was recently sequenced and several genetic and physical maps exist. These have been developed specifically to identify DNA-based markers for assisted selection of important traits such as resistance to papaya ringspot virus and flesh color. Although a substantial quantity of data on papaya genomics has been produced, this is often trait or genome specific and not always widely applicable due to problems associated with irreproducibility and non-transferability. Future papaya genomics efforts must be focused on the identification of functional genetic components that control the traits of interest. Tools based on the expressed genome sequences that are closely associated with significant trait loci will become the highly efficient and transferable markers of the future for enhanced papaya breeding objectives.