Identification of interspecific hybrids among domesticated apple and its wild relatives

Abstract

Potential interspecific hybrids are usually identified in natural populations by their proximity to interbreeding species or their intermediate phenotypes; hybridization can then be confirmed by comparing the genetic makeup of putative hybrids to pure species. In contrast, detecting interspecific hybridization and misclassifications in ex situ collections can be difficult because fine-scale geographic locations and species-specific phenotypic data are generally unavailable. Thus, there is little a priori information available to suggest which individuals might be hybrids. Instead, hybrids or misclassified individuals must be identified based on molecular data via population assignment and admixture detection programs. We have applied a variety of population assignment and admixture detection programs to over 400 samples of four closely related Malus species held in the US Department of Agriculture–Agricultural Research Service National Plant Germplasm System that were genotyped at 19 simple sequence repeat loci. Our findings indicate that over 10 % of the samples of the wild species Malus sieversii and Malus orientalis and nearly 20 % of the samples of the wild species Malus sylvestris may be admixed or misclassified. The percentage of admixed or misclassified samples of the domesticated species, Malus × domestica, was much lower, at <5 %. These findings provide an illustration of how to detect hybridization and misclassification in ex situ collections using molecular data and, ultimately, should help to maximize the utility of the collections. In addition, the presence of wild-collected samples that show admixture with domesticated apple suggests that gene flow may be occurring from the crop into natural populations of the wild species.