Simple sequence repeats (SSRs) are highly polymorphic regions of DNA that can be used for the molecular characterization of apple (Malus) germplasm. SSR markers are sufficiently variable to distinguish between individual plants in wild Malus species. In this study, accessions of Malus hupehensis were screened for fragment length variation in PCR amplified simple sequence repeat regions of DNA. The fragment length phenotype produced by five SSR primer pairs showed no variation between two lineages of M. hupehensis collected in the Changjiang (Yangtse) River valley. One lineage was collected by E.H. Wilson in 1908 near the city of Ichang, Hubei Province. The second lineage was collected by cooperators at China's Southwest Agricultural University (SWAU) in 1997 near the city of Chongqing (Chungking). M. hupehensis Plant Introduction No. 588760 from the National Plant Germplasm System lacks provenance, but displays a fragment length phenotype identical to both the Wilson and SWAU lineages. The spread of a clone may be aided by asexual reproduction through seed, which is not uncommon in polyploid apples. Two seedlings each of 15 maternal trees from the SWAU lineage were assayed for ploidy level by flow cytometry. The DNA content per nucleus for all SWAU progeny fell within the range for triploids, 2.19 to 2.68 pg DNA/nucleus. It appears that plant explorers in China separated by almost 90 years have succeeded in sampling a single clonal lineage of M. hupehensis.
The U.S. National Plant Germplasm System (NPGS) currently holds 36 separate accessions of the `Yichang' clone of Malus hupehensis (Pamp.) Rehd. The `Yichang' clone originally entered the United States in 1908 as seed collected for the Arnold Arboretum by E.H. Wilson near Yichang, Hubei Province, China. The original description of M. hupehensis omits fruit characters, and botanists frequently augment these omissions with descriptions of the `Yichang' clone. Apomixis occurs in Malus, including M. hupehensis, and is strongly associated with elevated ploidy levels. Simple sequence repeats (SSRs) were used to characterize 65 accessions of M. hupehensis. To check for polyploidy, a set of M. hupehensis accessions was evaluated with flow cytometry. The simple sequence repeat phenotypes and ploidy information revealed the `Yichang' clone under various accession names in arboreta. It was neither known nor suspected that the U.S. National Plant Germplasm System held many duplicate accessions of the `Yichang' clone prior to their molecular characterization. Germplasm conservation decisions for Malus species can benefit from an increased knowledge of the genetic variation or lack thereof in naturalized populations and ex situ collections.