THE USE OF EST-DERIVED MICROSATELLITES AS MARKERS IN THE DEVELOPMENT OF A GENETIC MAP IN KIWIFRUIT

Green and gold kiwifruit represent two species of the genus Actinidia. They are important horticultural crops in New Zealand and a large investment in breeding and genomics programmes has been made to develop new types for fresh fruit markets, and gene products for food-related industries. To facilitate rapid and efficient production of new cultivars, construction of a genetic map was undertaken to provide markers for desired fruit traits. Microsatellites, or simple sequence repeats (SSRs), are favoured markers in the construction of genetic maps as they are co-dominant, highly polymorphic and occur frequently in all eukaryotic genomes. We sampled microsatellites from expressed sequence tags (ESTs) to evaluate their frequency of occurrence and level of polymorphism. A sample of 50 perfect dinucleotide repeats of 22 or more bases, and either 5' or 3' of the open reading frame, were the microsatellites chosen as possible markers, and when these were amplified 93% of loci produced were polymorphic and segregating in the mapping population. The information content of the markers varied, giving four categories. In this sample, fully informative markers made up 32% of the total, 15% were female informative, 24% were male informative and 29% were partly informative. Some primer pairs amplified more than one locus, with some loci mapping to separate linkage groups, indicating the presence of duplicated regions within the genome, and supporting the suggestion of paleopolyploidy in the genus. The linkage map was constructed using Joinmap3 and a log of the odds (LOD) value of 7. Distribution of markers over the linkage groups obtained appeared to be random, taking into consideration that the linkage groups exceeded the chromosome number of x = 29, and that a number of markers were not assigned to any linkage group. EST-derived microsatellite markers were found to be economic to produce through bioinformatics approaches, simple to use and highly polymorphic in Actinidia.