Molecular polymorphism and linkage analysis in sweet passion fruit, an outcrossing species

One of the current challenges of tropical fruit crop improvement is to incorporate molecular marker-based approaches into conventional breeding programmes. This study was designed to build an integrated genetic map of the sweet passion fruit (Passiflora alata), a diploid (2n=18) outcrossing species which is greatly appreciated for in natura consumption, and reported to inspire cosmetic and pharmaceutical companies to create plant-derived compounds. With this in mind, a full-sib family of 180 individuals was genotyped using different molecular marker types, such as amplified fragment length polymorphisms (AFLP), microsatellite-AFLP (M-AFLP), simple sequence repeats (SSR), resistance gene analogues (RGA) and target region amplification polymorphism (TRAP). On average, the rate of polymorphism between the parental genotypes was 20.3%. We also searched for single nucleotide polymorphisms (SNPs) in some AFLP bands and in seven gene fragments, and found one SNP every 87bp. All SNPs were biallelic and occurred most frequently in putative gene fragments (81.5%) rather than in AFLP bands (60.0%) analyzed. Excellent gel profiles were obtained allowing the recognition of all types of segregation expected for a progeny of an outcrossing species. Multipoint linkage analysis was performed using OneMap software, with logarithm of the odds (LOD) score5.6 and recombination fraction <0.5. The resulting integrated map consists of 549 markers, 2.0% of which fit a segregation ratio of 1:1:1:1, 1.3% a ratio of 1:2:1, 27.3% a ratio of 3:1 and 69.4% a ratio of 1:1. The map spanned a total of 2073.0cM, with an average distance between adjacent markers of 3.8cM. This is the first linkage study on sweet passion fruit and should prove useful for quantitative trait loci mapping.