Using SSR Markers to Enhance Genetic Variation in the Development of Pawpaw (Asimina triloba) Cultivars

Pawpaw [Asimina triloba (L.) Dunal] is a tree fruit native to eastern North America which is in the early stages of domestication. Maintaining a high level of genetic diversity is important for the long-term genetic improvement of the crop, and in minimizing vulnerability to disease.  Cultivars available in the early 20^th century have been lost, and significant genetic erosion may have occurred. Recent pawpaw breeding efforts by the PawPaw Foundation (PPF) have yielded seven recent cultivar releases. These selections have shown some fungal (Phyllosticta) and pest (leaf roller, etc) susceptibility. Kentucky State University (KSU) has identified eight advanced selections that display high yields and excellent fruit quality. The objective of this study was to compare the genetic variation exhibited among old and new pawpaw cultivars and KSU advanced selections using simple sequence repeat (SSR) markers.   Leaf samples were collected from older pawpaw cultivars (Middletown, NC-1, Overleese, Rebeccas Gold, Sweet Alice, Taylor, Taytwo, and Zimmerman), recently released PPF cultivars (Allegheny, Shenandoah, Susquehanna, Wabash, Rappahannock, and Potomac), and KSU advanced selections (G4-21, G4-25, G5-23, G5-29, G9-109, G9-111, H3-120, and G6-120) for DNA extraction.  DNA samples were amplified and SSR-PCR products separated by fluorescent capillary electrophoresis. Twenty microsatellite loci were examined. Grouping of genotypes in an unweighted pair group method with arithmetic mean dendrogram showed that the PPF cultivars were more closely grouped with the older cultivars and that the KSU advanced selections group was genetically distinct from the older and PPF cultivars. If the KSU selections are released to the public, they would enhance the genetic variation of commercially available pawpaw cultivars.