Genetic diversity and geographic differentiation in pawpaw [Asimina triloba (L.) Dunal] populations from nine states as revealed by allozyme analysis

As a new National Clonal Germplasm Repository for Asimina species at Kentucky State University (KSU), of major concern to us is the genetic variation within our germplasm collection. The present study investigated the extent of genetic diversity for the pawpaw germplasm in our collection and the geographical pattern of genetic diversity among populations using isozyme markers. Allozyme diversity was high in Asimina triloba (L.) Dunal (Annonaceae) collected from all nine different states, as is typical for temperate woody perennial, widespread and outcrossing plant species. Averaged across populations, mean number of alleles per locus (A), percent polymorphic loci (P), effective number of alleles per locus (A(c)), and expected heterozygosity (H(e)) were 1.54, 43.5, 1.209, and 0.172, respectively. Significant deviations from Hardy-Weinberg equilibrium were found in nine populations at an average of 4.8 loci. Observed heterozygosity was higher than expected. Partitioning of genetic diversity showed that 88.2% resided within populations. The proportion of genetic diversity among populations (G(st) = 0.118; F(ST)= 0.085) was either lower than or within the range of those species with similar ecological and life-history traits. The mean genetic identity among populations was high (I = 0.988). An analysis using UPGMA clustered most populations as one major group, with the southernmost (Georgia) and the westernmost (Illinois) populations readily separated from the main group. The relationships discovered by principal component analysis (PCA) were similar to those revealed by UPGMA. In addition, PCA separated the northernmost population (New York) from the major group. Sampling strategies for future germplasm collection of A. triloba are also discussed.