expressed sequence tag (EST)-derived simple sequence repeats (EST-SSRs) were
used to investigate genetic diversity in 139 plants obtained from seeds of 35
watermelon accessions collected from all the geographical provinces of Zimbabwe.
In addition, 15 plants representing three commercial varieties developed in the
United States (USA) were analyzed for comparison. A total of 65 alleles were
detected among all the watermelon accessions. For the 13 polymorphic EST-SSR
loci, number of alleles per locus varied from 2 to 13, with an average of 5
alleles per locus. Values for the polymorphic information content increased as
the number of alleles increased, and varied from 0.15 to 0.77 with an average
of 0.54 suggesting sufficient discriminatory power. Both cluster analysis and
principal coordinate analysis (PCA) produced two major clusters; one with the
22 cow-melon accessions and the other with the 16 sweet watermelon accessions.
Within the sweet watermelon group, two distinct sub-clusters formed, one of
which contained only two of the commercial varieties from USA. Partitioning of genetic
variation in the Zimbabwean material using analysis of molecular variation
(AMOVA) revealed that 64% of the total variation resides between the two major
forms, i.e. sweet watermelons and
cow-melons, 28% between accessions within forms and 8% within accessions. The
EST-SSR markers revealed a somewhat higher diversity in sweet watermelon
accessions compared to that of cow-melons. This finding is contrary to previous
reports using other markers (genomic SSR loci or RAPD) and/or a plant material
that is likely to have experienced more stringent selection procedures compared
to the landraces analyzed in our study.