Genetic Diversity of USDA Cucumis Metuliferus Collection Revealed by Cucumber Genomic Microsatellite Markers

Cucumis metuliferus E. Mey. ex Naud (African horned cucumber, AHC) is indigenous to Africa and is a relative of cultivated cucumber (C. sativus L.) and melon (C. melo L.). AHC is being promoted as a specialty fruit crop in a number of countries. In addition, several traits in C. metuliferus such as high level resistance to the southern root knot nematode and papaya ring spot virus, sequential and concentrated fruiting are very attractive for improving cucumber or other cucurbit crops. However, attempts in the past to cross AHC with either cucumber or melon have been unsuccessful. In the present study, genetic diversity among the USDA C. metuliferus collection was evaluated with microsatellite (simple sequence repeat, SSR) markers derived from cucumber genomic DNA sequences. Of the 564 cucumber SSRs tested, over 50% (286/564) were able to amplify PCR products from C. metuliferus genomic DNA templates suggesting high degree DNA sequence homology between the two species. Forty cross-species transferable SSRs were selected to investigate genetic variability among 37 USDA accessions and 2 commercial source C. metuliferus selections. Clustering analysis of marker data revealed unexpectedly low genetic diversity among these C. metuliferus accessions. Each marker, on average, was able to detected only two alleles. Seventeen of the 40 markers were monomorphic among the 39 lines tested. The possible reasons of the very low genetic diversity among C. metuliferus accessions and the implications of this finding with regard to use of C. metuliferus resources in cucurbit improvement will be discussed.