Inter- and Intracultivar Variation of Heirloom and Open-pollinated Watermelon Cultivars

in HortScience

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] cultivars exhibit diverse phenotypic traits, yet are derived from a narrow genetic base. Heirloom cultivars, and to a lesser extent modern open-pollinated (OP) cultivars, are perceived to contain vital genetic variation that is critical for biodiversity conservation and crop improvement. The objective of this study was to characterize the diversity of six heirloom and open-pollinated watermelon cultivars that are popular among U.S. organic, direct-market, and home gardeners. An additional evaluation was conducted to determine whether significant phenotypic and genotypic variation existed among seed lots sourced from different commercial seed vendors. Important horticultural traits such as days to germination, days to first flower, yield, and fruit quality were measured over two field seasons. Genetic diversity was estimated using 32 simple sequence repeat (SSR) markers. Significant differences in horticultural traits among seed lots in both years were observed only in days to germination and first male flower, which may be a consequence of vendor differences in seed storage and quality control. Heirloom ‘Moon and Stars’ and modern OP ‘Sugar Baby’ were the most genetically distinct from the other cultivars and heirloom ‘Georgia Rattlesnake’ was determined to be highly related to the modern OP ‘Charleston Gray’. The two heirloom cultivars were observed to have lower average gene diversity than the modern cultivars. Heirloom ‘Moon and Stars’ contained significant genetic variation among seed lots, yet heirloom ‘Georgia Rattlesnake’ contained none. These findings suggest that genetic variation can be more accurately attributed to pedigree and foundation seed maintenance practices than to the “heirloom” designation per se. The variation reported in this study can be used to inform conservation and breeding efforts.

Contributor Notes

This research was partially funded by the University of Georgia (UGA) Department of Horticulture and made possible thanks to the assistance of Ryan McNeill and the UGA Durham Horticultural Farm staff.

Corresponding author. E-mail: cmcgre1@uga.edu.

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Article Figures

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    Neighbor-joining tree using Nei’s standard distance (Nei, 1972) for seed lots from various commercial seed vendors for six heirloom and modern open-pollinated watermelon cultivars. The optimal tree with the sum of branch length = 1.12 is shown. The tree is drawn to scale.

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    The average gene diversity of six heirloom and open-pollinated watermelon cultivars using 32 simple sequence repeat loci. Gene diversity was calculated for each locus using the formula D = 1 − ∑ pi2, where pi is the frequency of ith allele, then averaged over loci. Standard error bars were generated by bootstrapping the data 1000 times over loci.

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    Number of private alleles observed in in six heirloom and modern open-pollinated watermelon cultivars generated from the genotyping of 32 simple sequence repeat loci. Gray bars indicate private alleles that were found in only one individual; black bars indicate private alleles that were found in more than on individual.

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    The average gene diversity of seed lots from various commercial seed vendors for six heirloom and open-pollinated watermelon cultivars using 32 simple sequence repeat loci. Gene diversity was calculated for each loci using the formula D = 1 − pi2, where pi is the frequency of ith allele, then averaged over loci. Standard error bars were generated by bootstrapping the data 1000 times over loci.

  • View in gallery

    Number of private alleles observed in seed lots from various commercial seed vendors generated from the genotyping of 32 simple sequence repeat loci in six heirloom and modern open-pollinated watermelon cultivars. Gray bars indicate private alleles that were found in only one individual; black bars indicate private alleles that were found in more than on individual.

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    A power analysis of proportion of gene diversity estimated in six heirloom and open-pollinated watermelon cultivars vs. number of loci, as generated by 1000 random permutations of the data set of 32 simple sequence repeat loci.

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    A principal coordinate analysis using genetic distance among individuals of six heirloom and modern open-pollinated watermelon cultivars.

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    A principal coordinate analysis using genetic distance among individuals of seed lots from various commercial seed vendors for six heirloom and modern open-pollinated watermelon cultivars.

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