Assessment of Controlled Crossing Success Without Pollinator Exclusion in Pawpaw [Asimina triloba (L.) Dunal] Cultivars

Abstract

The North American pawpaw [Asimina triloba (L.) Dunal] is the largest edible tree fruit native to the United States and is being grown in orchards by small growers in Kentucky. There are currently few university-based pawpaw breeding programs, with most ongoing breeding conducted by private breeders and hobbyists. Pawpaw flowers bloom in early spring and are pollinated by flies, beetles, and ants. Controlled pollination methods vary widely, with some breeders bagging flowers before and after hand pollination to exclude pollinators, and others leaving flowers unprotected. Even when flowers are bagged, pollination may still occur if small insects, such as ants, enter bags and pollinate pawpaw flowers. Since pawpaw pollinators are lethargic in cool spring temperatures at flowering, many private breeders and enthusiasts do not attempt to exclude pollinators during controlled crosses, leading to questions of parentage. The objective of this study was to examine the efficacy of controlled spring pawpaw crosses performed without bagging flowers before or after hand pollination and to verify parentage using simple sequence repeat (SSR) markers. Crosses were made over two years between the flowers of the pawpaw cultivars ‘Sunflower’ and ‘Susquehanna’, as well as selfs, without bagging prior to or after crossing. Seeds were collected from resulting fruit and the seedlings grown out in a greenhouse and then were planted in an orchard. DNA was extracted from young leaves of parents and offspring. PCR was performed using Simple Sequence Repeat markers, for the loci Pp-B3, Pp-B103, Pp-G124, Pp-B129, and Pp-G119.  PCR products were separated by capillary electrophoresis for genotypic fingerprinting. For the cross ‘Sunflower’ × ‘Susquehanna’, 77.4% of the seedlings were of the expected progeny, and for ‘Susquehanna’ × ‘Sunflower’, 65.4% of the seedlings were of the expected progeny. Selfs of ‘Sunflower’ × ‘Sunflower’ and ‘Susquehanna’ × ‘Susquehanna’ did not yield progeny, confirming that pawpaw usually requires out-crossing. Although high levels of (72%) of crossing success can be achieved without bagging protection before and after hand pollination of flowers, a high percentage of crosses will still be of unexpected parentage, suggesting pollinator exclusion is helpful.