Hortscience | 2019
Inter- and Intracultivar Variation of Heirloom and Open-pollinated Watermelon Cultivars
Abstract
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, directmarket, 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 themost genetically distinct from the other cultivars and heirloom ‘Georgia Rattlesnake’ was determined to be highly related to themodernOP ‘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. Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is a warm-season annual vegetable crop that is grown on 3.5 million hectares worldwide (Food and Agriculture Organization of the United Nations, 2014). Cultivars express a wide range of phenotypes including fruit size, flesh color, rind pattern, disease resistance, and sweetness. Despite geographic and phenotypic diversity, the genetic variation of cultivated watermelon is limited (Levi et al., 2001b). Analysis of genome-wide diversity revealed that cultivars from Asia, Europe, and America are derived from one of three subsets of sweet watermelon accessions from Africa (Nimmakayala et al., 2014b). As such, estimates of genotypic variation among cultivars have been low. The genetic diversity among 130 edible-type accessions sampled throughout the world was estimated at 5% (Nimmakayala et al., 2014a). Levi et al. (2001b) found that 46 American cultivars varied by 0.4% to 8%. East Asian and American cultivar types were found to be genetically similar by some analyses (Nimmakayala et al., 2014a; Reddy et al., 2015) but as distinct ecotypes in others (Guo et al., 2013; Zhang et al., 2012). The resequencing of 20 watermelon accessions shows that watermelon is less genetically diverse than maize, soybean, and rice (Guo et al., 2013). In all, these findings are consistent with a severe genetic bottleneck during domestication. Conservation of genetic variation is critical to crop improvement through plant breeding. Farmer-maintained landraces are favorable sources of genetic variation because they are more adapted to agricultural production than wild relatives (Villa et al., 2005). By their nature, open-pollinated (OP) cultivars maintain greater population-level genetic diversity than hybrid seed types, which are derived from the cross-pollination of two inbred parental lines. A benefit to the grower is that seed of OP cultivars can be saved from year to year, unlike hybrid seed that does not grow true-to-type in subsequent generations and thus must be purchased from seed companies each season. Due to these realized and perceived benefits, organic, direct-market, and home growers have inspired a renewed interest in OP cultivars (Phillips, 2016). Today, farmer-maintained landraces in industrialized countries are rare (Thomas et al., 2011). In the United States, the designation ‘‘heirloom’’ is considered by some as analogous to landrace in that heirlooms are perceived to be locally adapted and genetically diverse. In this study and in present-day seed catalogues, ‘‘heirloom’’ is defined as a cultivar that was introduced before the advent of modern breeding techniques (the year 1942 is a commonly used temporal threshold) by farmers or nonprofessional breeders (DeMuth, 1998). However, the development of the modern seed industry has made the term ambiguous. If one considers that commercially distributed heirloom varieties are maintained and multiplied in a similar fashion to modern OP cultivars within a ‘‘certified seed’’ model (Parlevliet, 2007), rather than maintained through ongoing recurrent selection by end users, then the public perception of heirloom cultivars as more diverse than modern OP cultivars is questionable. As a related but separate issue, the discovery of within-cultivar variation, whether in heirloom or modern cultivars, is of practical relevance to the seed industry and scientific community. Within-cultivar variation is an essential genetic resource in the maintenance and improvement of elite cultivars in a changing climate (Berry et al., 2014). Numerous studies report that significant variation of many agronomic traits were observed within inbred S5 to S20+ lines from different seed stock sources (reviewed in Tokatlidis, 2015), which is invaluable information to breeders. In these cases, cultivars and inbred lines assumed to be pure lines undergo changes when they are regenerated and/or maintained in separate locations; when properly characterized, this variation can be used for cultivar improvement and the conservation breeding of elite cultivars. The OP cultivars featured in this study are not covered by plant variety protection (PVP) and thus foundation seed maintenance is unregulated and likely decentralized (M. Colley, personal communication). The term ‘‘foundation seed’’ in the present study refers to seed stock from which commercial seed is multiplied, but does not imply the formal designation associated with state-certified seed programs. It is expected that seed multiplied from independent foundation seed stocks, particularly in an unregulated model, is more likely to be genetically differentiated than certified seed covered by PVP. Significant variation among seed lots of cultivars sourced from different seed companies should be considered in conservation and breeding efforts. Candole et al. (2012) identified differentiated levels of disease resistance among seed lots from different seed companies in the heirloom pepper ‘California Wonder’, which had long been used as a standard in pathology experiments. This finding helped Received for publication 10 Oct. 2018. Accepted for publication 3 Dec. 2018. 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. 212 HORTSCIENCE VOL. 54(2) FEBRUARY 2019 scientists select a more reliable cultivar against which to judge other cultivars in disease screens. A seed lot with greater genetic diversity or with novel alleles may prove more useful in breeding programs than genetically uniform seed lots. What role, then, does commercial seed production play in the conservation of genetic diversity of heirloom and modern OP cultivars? A balance between the maintenance of cultivar integrity and the conservation of genetic diversity must be sustained. Cultivars must satisfy distinctness, uniformity, and stability (DUS) standards (Union for the Protection of New Varieties of Plants, 2002) and are perceived to be uniform genotypes. Consequently, in commercial seed production, emphasis is placed on rogueing off-type and diseased plants to maintain uniform and high-quality seed (Parlevliet, 2007). Guidelines for isolation distance and minimum population size during multiplication and maintenance of foundation seed vary by crop (George, 2013). These practices safeguard against genetic drift and gene flow between cultivars. The extent to which cultivar purity strategies, during both foundation seed maintenance and multiplication, are practiced by each commercial seed company is unknown. By a somewhat competing natural phenomenon, cultivars are not genetically uniform due to abundant biological mechanisms that ensure adaptability of genomes. Genetic variation is inherent to cultivars via natural processes, including 1) heterogeneity in the progenitor gene pool, 2) de novo mutation, 3) genetic drift, and 4) environmentally triggered alterations to the genome. Artificial forces also affect intracultivar diversity during commercial seed propagation, including 1) unintentional gene flow during seed propagation, 2) bottlenecks during establishment of foundation or multiplication seed stocks, and 3) unintentional selection as a result of environmental conditions or management practices. Although most genetic variation is derived from the progenitor gene pool, de novo variation has been estimated as high as 18% of total variation in single-plant derived soybean lines (Yates et al., 2012). Genetic variation is not necessarily a condition to be avoided, but in fact is an essential mechanism to exploit for long-term maintenance of cultivars and in breeding improved cultivars. Useful intracultivar genetic variation has been documented in maize (Gethi et al., 2002), soybean (Yates et al., 2012), rice (Olufowote et al., 1997), cotton (Hinze et al., 2012), sunflower (Zhang et al., 1995), olive (Caruso et al., 2014), and mango (Singh et al., 2009), and the selection of superior lines