Flowers are strongly protogynous and are likely self-incompatible (Willson and Schemske, 1980), although some trees may be self-compatible. Pollination may be by flies (Diptera) (Willson and Schemske, 1980) and beetles (Nitidulidae) (Kral, 1960). Fruit set in the wild is usually low and may be pollinator or resource-limited (Lagrange and Tramer, 1985; Willson and Schemske, 1980), but under cultivation in full sun, tremendous fruit loads have been observed. Fruit may be borne singly or in clusters, which resemble the hands on a banana plant (Musa spp.). When ripe, the fruit softens and has a powerful aroma (McGrath and Karahadian, 1994; Shiota, 1991). In some varieties, there is a skin color change from green to green-yellow (e.g., ‘PA-Golden #1’ and ‘10-35’) when the fruit ripens. Flesh color of ripe fruit ranges from creamy white through bright yellow to shades of orange. The flavor of ripe pawpaw fruit resembles a combination of banana (Musa × paradisiaca), mango (Mangifera indica), and pineapple (Ananas comosus); however, flavor varies among varieties, with some fruit displaying more complex flavor profiles. Usually the shelf life of a tree-ripened fruit stored at room temperature is 2 to 3 d, but with refrigeration [4 °C (39.2 °F)], fruit can be held at least 3 weeks while maintaining good eating quality. As early as 1916 it was noted that the rapid perishability of pawpaw fruit was the major factor inhibiting its commercialization, but that this problem might be solved through breeding efforts (Popenoe, 1916, 1917). Whether there is significant variation in keeping ability of fruit in wild pawpaw germ-plasm has yet to be determined.

Efforts to domesticate the paw-paw began early in the 20th century (Peterson, 1991; Zimmerman, 1941). Pawpaw selections from the wild were assembled in extensive collections by various enthusiasts and scientists, including Benjamin Buckman (Farmington Ill., circa 1900 to 1920), George Zimmerman (Linglestown, Pa., 1918 to 1941), and Orland White (Blandy Experimental Farm, Boyce, Va., 1926 to 1955) (Peterson, 1986; Peterson 1991; Zimmerman, 1941). From about 1900 to 1960, at least 56 varieties of pawpaw were selected and named. Fewer than 20 of these selections remain, with many being lost from cultivation through neglect, abandonment of collections, and loss of records necessary for iden-

tification (Peterson, 1991). With the resurgence of interest in pawpaw since 1960, additional pawpaw varieties have been selected in the wild or as a result of breeding efforts of hobbyists. There are currently more than 40 pawpaw varieties commercially available (Jones et al., 1998). In 1994, KSU was approved as the USDA National Clonal Germplasm Repository, or gene bank, for Asimina species. Therefore, germplasm evaluation, preservation, and dissemination have been a high priority for the KSU program since that time. There are presently over 1700 accessions from 17 states and over 40 varieties contained in the repository orchards. Several isozyme and DNA marker techniques have been used to evaluate genetic diversity of paw-paw varieties and material collected from wild populations in the KSU repository.

collection, and also to fingerprint varieties to establish a baseline for future variety authentication and breeding efforts (Huang et al., 1997, 1998, 2000, 2002; Pomper et al., 2003).

In 1981, R. Neal Peterson and Harry Swartz began a long-term breeding project which aimed to develop improved pawpaw varieties (Peterson 1986, 1991). A collection of about 1500 accessions of open-pollinated seedlings was assembled at the University of Maryland Experiment Stations at Queenstown and Keedysville, Md. The seed for this gemplasm collection was obtained from pawpaw trees that remained at the sites of the historic collections of Buckman, Zimmerman, the Blandy Experimental Farm, as well as those of Hershey (Dowington, Pa.), Allard (Arlington, Va.), Ray Schlaanstine