No abstract

Watermelon (Citrullus lanatus) is one of the most economically important vegetable cucurbits. However, the genetic and taxonomic statuses of its closely related species remain scantly documented, limiting their full use for agronomic purposes. The genetic diversity and structure of 74 accessions covering 47 dessert type (C. lanatus subsp. vulgaris), 21 oilseed type (C. mucosospermus), and 6 citron melon (C. lanatus subsp. lanatus var citroides) collected from 36 countries throughout 4 continents, were analyzed using 18 polymorphic SSR markers. The mean values of proportion of polymorphic loci (P = 29.73), number of alleles per locus (A = 1.243), effective number of alleles per locus (Ae = 1.153) Shannon index (I = 0.191), observed and expected heterozygosities (Ho = 0.124; He = 0.149) confirmed the narrow genetic basis of C. lanatus. According to molecular variance analysis the most important component of the genetic variation was obtained among accessions (70%). On the contrary, lower genetic variation was noted among species (16%), countries (37%), and continents (14%). It is suggested that the cultivated forms of Citrullus spp. originated from or successive selection cycles aimed at few and/or common traits, in few ancestral populations. Clustering based on both Bayesian approach and an unweighted pair group method with arithmetic mean pointed out three groups of accessions corresponding to use types and collecting countries. Based on these results, future collecting missions could be focused mainly on representative ecological sites in Citrullus spp. distribution areas, and increasing the number of accessions and seeds per accession.