Simple Sequence Repeat (SSR) Variation in a Collection of Malus Species and Hybrids

Abstract

A diverse collection of 133 Malus species and hybrids from the USDA Plant Genetic Resources Unit's core subset collection was screened with five simple sequence repeat (SSR) primer pairs in order to determine genetic identities and overall levels of genetic variation. The number of amplification products (alleles) per locus (primer pair) in this collection ranged from 6 to 39, with some genotypes showing complex banding patterns of up to four products per locus, suggesting that duplication events may have occurred within the genome. Five primer sets unequivocally differentiated all but 10 pairs of genotypes in the collection, with seven of these 10 being pairs of the same species. Within three of the species holdings surveyed, M. honanensis, M. sargentii, and M. sikkimensis, no genetic variation was revealed with the SSR markers. The discrimination power for the combined loci in this collection was nearly one, which indicates that the likelihood of two genetically different accessions sharing the same alleles at all the loci included in this study would be nearly impossible. Coupled with results from a previous survey of M. × domestica accessions, this finding suggests that with five SSR primer pairs, the majority of the Malus holdings could be assigned a unique fingerprint identity. The average direct count heterozygosity over all loci was 0.620, ranging in value from 0.293 to 0.871 over individual loci. These heterozygosity counts will be compared with a survey of naturally occurring M. sieversii to determine whether current repository holdings are representative of the overall levels of diversity occurring in Malus. Information generated with this study, coupled with passport and horticultural data will inform curatorial decisions regarding deaccessioning of duplicate holdings and plans for future germplasm collections.