Construction of a Genetic Larkage Map of Papaya and Mapping Traits of Horticultural Importance

Abstract

A genetic linkage map of papaya based on 61 randomly amplified polymorphic DNA markers and one morphological marker has been constructed by using a computer program MAPMAKER/EXP. The map is based on a F2 segregating population of a cross between a Hawaiian cultivar Sunrise and Line 356, a disease (papaya ringspot virus) tolerant selection from Florida. The map is comprised of 11 linkage groups covering a total distance of 1000 cM. The sex locus is mapped on linkage group 1, within a marker bracket of 14 cM. Assay for these flanking markers can be used to forecast the sex of the plant at the seedling stage. Analysis of QTLs based on MAPMAKER/QTL reveals the presence of several loci affecting vigor and precocity. With regard to vigor defined as plant height and stem diameter, environmentally sensitive and stable QTLs have been observed. Analysis of growth rates for height and for diameter improved the efficiency of resolving for neighboring peaks in QTL analysis for vigor. The analysis of yield and yield related traits like carpellody, sterility and fruit weight, has indicated the occurrence of multiple QTLs. QTLs affecting carpellody indicate a possible interaction between qualitative and quantitative factors in influencing the phenotype. In the majority of the traits studied, genetic factors with an effect opposite to the overall effect have been detected in parent Line 356. Genetic analysis of different components of papaya ringspot virus tolerance (vigor, severity of symptoms and ELISA titer) has confirmed the complex nature of disease tolerance. QTLs affecting plant vigor (diameter and height) specific to the disease environment have been detected in Line 356. Stem diameter appears to be an important index of disease tolerance. QTLs in Line 356 with gene-dosage dependent effects in lowering PRV load as measured by ELISA titer show the presence of suppressive virus resistance, indicating a positive type mechanism of disease resistance. In light of these findings, disease tolerance in Line 356 is reclassified as resistance or 'tolerant to symptoms and resistant to virus'.