Molecular Markers in Papayas

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.