Table 3. ISSR markers scored in 19 pawpaw (Asimina triloba) cultivars.
Cultivarz
Markery12345678910111213141516171819
UBC812-06401x100101100100000110
UBC825-06961000100010001100000
UBC825-07601111111111111011111
UBC841-o4800101000101010011101
UBC841-10750010000000001110000
UBC853-19300000000000000011000
UBC860-08750010001101100101000
UBC860-09401111111111111111111
UBC873-04800101000111000000100
UBC880-11501101111101111111101
zCultivar numbers correspond to those in Table 1.
ySegregating ISSR markers were identified by the manufacturer primer code corresponding to the primer responsible for their amplification, followed by a four digit number indicating product size in base pairs
x0 = the marker was absent and 1 = marker was present in the cultivar.
however, their isozyme study included 15 advanced selections from the PawPaw Foundation breeding effort and 2 cultivars that were not included in this ISSR study. Huang et al. (2000) used 21 RAPD markers that segregated in a simple Mendelain fashion and used 6 populations (10 to 18 trees each) from 6 states in the KSU repository collection to evaluate genetic diversity. Direct comparisons of the effectiveness of all three marker systems would require the use of these marker systems with the same genotypes. However, it would appear that the 10 ISSR loci had a higher level of polymorphism than the nine isozyme markers of Huang et al. (1997). Previous studies did not support the idea that high levels of genetic variation could be maintained in A. triloba, due to its clonal reproductive habit and low seed set in nature (Rogstad et al., 1991). This study provides additional evidence that moderate to high levels of genetic diversity exist in A. triloba (Huang et al. 1997, 1998, 2000).
The ISSR markers could be useful tools in cultivar identification. Using Nei’s (1978) genetic distance, 11 of the 19 cultivars were uniquely identified (Fig. 1). The ISSR based UPGMA den- program showing genetic similarity among 19 pawpaw cultivars using separation by Dice (1945) was very similar to the separation using Nei’s (1978) genetic distance (data not shown). The ISSR separation confirms the findings of Huang et al. (1997, 2003), in that the cultivar NC-1, which has been thought to be a seedling of a ‘Davis’ x ‘Oversleese’ cross, does not share many markers with these cultivars. A mix up of pollen or seed must have occurred and NC-1 may have a different parentage than that reported. The cultivar IXL, which is thought to be a seedling of an ‘Overleese’ x ‘Davis’ cross, did show some genetic similarity with these parents. Genetic relationships for the remaining cultivars in this study were similar to those found by Huang et al. with isozymes (1997) and RAPD markers (2003). The pawpaw cultivars SAA-Zimmerman and Mitchell, Prolific and Taytwo, and Sweet Alice and SAA-Zim- merman, shared similar grouping in both the dendrograms of Huang et al. (2003) (Fig. 1). Additional ISSR Mendelian markers could further improve genetic diversity estimates in pawpaw cultivars. We
were uniquely identified (Fig. 1). The ISSR based UPGMA den- rogram showing genetic similarity among 19 pawpaw cultivars using separation by Dice (1945) was very similar to the separation using Nei’s (1978) genetic distance (data not shown). The ISSR separation confirms the findings of Huang et al. (1997, 2003), in that the cultivar NC-1, which has been thought to be a seedling of a ‘Davis’ x ‘Oversleese’ cross, does not share many markers with these cultivars. A mix up of pollen or seed must have occurred and NC-1 may have a different parentage than that reported. The cultivar IXL, which is thought to be a seedling of an ‘Overleese’ x ‘Davis’ cross, did show some genetic similarity with these parents. Genetic relationships for the remaining cultivars in this study were similar to those found by Huang et al. with isozymes (1997) and RAPD markers (2003). The pawpaw cultivars SAA-Zimmerman and Mitchell, Prolific and Taytwo, and Sweet Alice and SAA-Zim- merman, shared similar grouping in both the dendrograms of Huang et al. (2003) (Fig. 1). Additional ISSR Mendelian markers could further improve genetic diversity estimates in pawpaw cultivars. We
Table 4. Comparison of genetic variation of pawpaw (Asimina triloba) with plant species having the same characteristics.
Species characteristicsPolymorphic locus (%) (P)yExpected heterozygosity (He)
Life form: long-lived woody perennialz64.7 \pm 2.70.177 \pm 0.010
Regional distribution: widespreadz58.9 \pm 3.10.202 \pm 0.015
Geographic range: temperatez48.5 \pm 1.50.146 \pm 0.000
Breeding system: outcrossing-animalz50.1 \pm 2.00.167 \pm 0.010
Seed dispersal: animal ingestedz45.7 \pm 3.90.176 \pm 0.019
Mode of reproduction: sexual and asexualz43.8 \pm 3.70.138 \pm 0.016
Average of all characteristicsz51.2 \pm 8.20.168 \pm 0.023
California cherimoyax73.30.330 \pm 0.064
California and Spanish cherimoyaw44.80.183 \pm 0.044
Cultivated pawpaw (Isozymes)v44.40.166 \pm 0.048
Pawpaw wild accessions (Isozymes)u43.50.172 \pm 0.013
Pawpaw wild accessions (RAPDs)t640.249 \pm 0.022
Cultivated pawpaw (RAPDs)s---0.285 \pm 0.160
Cultivated pawpaw (ISSRs)800.358 \pm 0.205
zData from Hamrick and Godt, 1989 derived from isozyme studies. ± standard deviation.
yP is the percent polymorphic loci and He the mean gene heterozygosity, respectively.
xCalculated by Huang et al. (1997) from the data published by Elstrand and Lee (1987) and included other three monomorphic loci.
wCalculated by Huang et al. (1997) from data published by Pascual et al. (1993) and included 16 other monomorphic loci.
vFrom Huang et al. (1997).
uFrom Huang et al. (1998).
tFrom Huang et al. (2000).
sFrom Huang et al. (2003).
J. AMER. SOC. HORT. SCI. 128(4):521–525. 2003.