Using Isozyme Polymorphisms for Identifying and Assessing Genetic Variation in Cultivated Pawpaw [Asimina triloba (L.) Dunal]

Abstract

The utility of isozyme phenotypes for identifying and determining genetic variation in pawpaw cultivars was studied using isoelectric focusing in thin-layer polyacrylamide gels. Based on a sample of 32 clones (cultivars and advanced selections) and 23 enzyme systems, 7 enzymes were found to be polymorphic, involving 9 polymorphic loci [acid phosphatase (ACP), dihydrolipoamide dehydrogenase (DDH), malic enzyme (ME), phosphoglucoisomerase (PGI), phosphoglucomutase (PGM), peroxidase (PRX), and shikimate dehydrogenase (SKD)]. Altogether these 9 loci and 32 clones yielded 28 multi-locus isozymic phenotypes useful for cultivar identification; 24 of the 32 clones were uniquely identified. The allozyme variation in these clones has the average of other long-lived woody perennials of widespread geographic range in temperate regions with insect-pollinated outcrossing breeding systems, secondary asexual reproduction, and animal-dispersed seed. Genetic differentiation among these pawpaw clones, measured by Nei's distance, D, was substantial: 496 pairwise comparisons of genetic distance among the 32 clones indicated that they differed on average of D = 0.068 ± 0.04 and ranged from 0 to 0.188. Cluster analysis (UPGMA) produced a most likely division of the 32 clones into 7 groups; however, these groups did not conform to known pedigree relations. Additional polymorphic enzymes are needed for accurate allozyme-based genetic discrimination.