Marker-Assisted Breeding for Papaya Ringspot Virus Resistance in Carica papaya L.

Abstract

There have been numerous attempts to transfer Papaya ringspot virus-Type P (PRSV-P) resistance from wild Vasconcellea relatives (Vasconcellea pubescens, Vasconcellea stipulata, Vasconcellea cauliflora and Vasconcellea quercilfolia) to Carica papaya. Success has been limited by the high degree of genetic divergence and thus incompatibility between Vasconcellea species and C. papaya. This has resulted in infertility of intergeneric F1 hybrids and failure to perform backcrosses which are essential to transfer PRSV-P resistance to C. papaya. However, there has been success in producing intergeneric hybrid populations of C. papaya x V. pubescens; C. papaya x V. parviflora; and intrageneric populations of V. pubescens x V. parviflora. The aim of this research was to develop a resistant V. parviflora population containing the PRSV-P allele from V. pubescens and then transfer the resistance into C. papaya. In this research, F2 and F3 populations have been produced from the V. pubescens x V. parviflora [F1]. Individuals selected for their allele of the PRSV-P resistance gene i.e. homozygous resistant (RR) and heterozygous (Rr) were backcrossed to V. parviflora (rr) or outcrossed to C. papaya (rr) and their seedlings evaluated for morphological characteristics and PRSV-P resistance. Dominant traits were transferred from all species to progeny, e.g., leaf vein number (7) from V. pubescens (RR); pink flower colour from V. parviflora (rr); petiole colour (red-green) and plant size from C. papaya (rr). Other traits were intermediate in hybrids, e.g., flower shape and fruit size. When the wild species or their hybrids were outcrossed to C. papaya (rr), inheritance patterns did not always follow Mendelian ratios, suggesting abnormal pairing of chromosomes or preferential elimination of the Vasconcellea genes. For example, all flowers were female in crosses between C. papaya (rr) x V. pubescens (RR) and the cream flower colour of C. papaya (rr) was dominant over the pink colour of V. parviflora (rr). However the PRSV-P resistance gene from V. pubescens (RR) was transferred into V. parviflora (rr) from their F2 hybrids and pollen fertility was obtained in hybrids between C. papaya (rr) and V. parviflora (rr), thus V. parviflora (rr) is proposed as a bridging species between the other two. A co-dominant CAPS marker has been developed which is closely linked to PRSV-P resistance in V. pubescens (RR). This marker was used to facilitate these intergeneric and intrageneric hybridisation programmes and was used at the in vitro stage after embryo rescue of wide crosses.