Genetic Diversity, Population Structure, and Formation of a Core Collection of 1197 Citrullus Accessions

Abstract

<jats:p>Watermelon belongs to the genus <jats:italic>Citrullus</jats:italic>. There have been continuing interests in breeding of watermelon for economic benefits, but information on the scope and utilization of genetic variations in <jats:italic>Citrullus</jats:italic> is still limited. The present study was conducted in 2012–13, to evaluate the genetic diversity and population structure of the 1197 line watermelon collection maintained by the Beijing Vegetable Research Center (BVRC), which belongs to seven <jats:italic>Citrullus</jats:italic> species including <jats:italic>Citrullus naudinianus</jats:italic>, <jats:italic>Citrullus colocynthis</jats:italic>, <jats:italic>Citrullus rehmii</jats:italic>, <jats:italic>Citrullus ecirrhosus</jats:italic>, <jats:italic>Citrullus amarus</jats:italic>, <jats:italic>Citrullus mucosospermus</jats:italic>, and <jats:italic>Cirullus lanatus</jats:italic> subsp. <jats:italic>vulgaris</jats:italic>. Twenty-three highly informative microsatellite markers evenly distributed in the watermelon genome were used to assess genetic diversity in this collection. The markers detected on an average of 6.05 alleles per locus with the average value of polymorphism information content (PIC) at 0.49. A high level of gene diversity [Nei’s gene diversity index (Nei) = 0.56] and a low observed heterozygosity (<jats:italic>H</jats:italic><jats:sub>o</jats:sub> = 0.10) were revealed within the collection. Structure analysis grouped the 1197 accessions into two main populations (Pop I and Pop II) and an admixture group. Pop I contained 450 accessions from <jats:italic>C. lanatus</jats:italic> subsp. <jats:italic>vulgaris</jats:italic> (446) and <jats:italic>C. mucosospermus</jats:italic> (4). Pop II comprised 465 accessions, 379 of which belonged to <jats:italic>C. lanatus</jats:italic> subsp. <jats:italic>vulgaris</jats:italic> and 86 to <jats:italic>C. naudinianus</jats:italic> (3), <jats:italic>C. ecirrhosus</jats:italic> (2), <jats:italic>C. rehmii</jats:italic> (2), <jats:italic>C. colocynthis</jats:italic> (11), <jats:italic>C. amarus</jats:italic> (58), and <jats:italic>C. mucosospermus</jats:italic> (10). The remaining 282 accessions were classified as an admixture group. The two main populations were further subdivided into four subgroups. The groupings were consistent with the estimation of F statistics (<jats:italic>F</jats:italic><jats:sub>st</jats:sub>) and Nei’s genetic distances in collections. We confirmed the distinct genetic backgrounds between American and East Asian ecotypes. Subsequently, we defined a core set consisting of 130 accessions including 47 from Pop I, 68 from Pop II, and 15 from the Admixture group. This core set was able to capture all 133 alleles detected by 23 simple sequence repeats (SSRs) in 1197 accessions. These results will facilitate efficient use of genetic variations in <jats:italic>Citrullus</jats:italic> in watermelon breeding and help optimization of accessions in genomewide association studies.</jats:p>

Chromosome numbers, Sudanese wild forms, and classification of the watermelon genus Citrullus, with 50 names allocated to seven biological species

“…Both seed batches have now been removed from the National Plant Germplasm System (NPGS Web Site, http:// www.ars-grin.gov/cgi-bin/npgs/) and are no longer distributed. The so-called core collection of Zhang & al. (2016) also includes germplasms labeled as Citrullus (such as PI 671961, PI 618817, and CXG in their figure 3: all labeled C. naudinianus), but that are Cucumis myriocarpus (see footnotes in our Table 2).…”

“…Conclusions. -The need for an up-dated classification of Citrullus has often been stressed, most recently by Zhang & al. (2016), and the present assignment of 50 available names to seven biological species, along with morphological and ecological traits as well as a tabulation of chromosome counts should help towards that goal.…”

“…The USDA's National Plant Germplasm System maintains a collection of 1800 Citrullus germplasms (see http://www.ars-grin.gov/npgs). Similarly, the Beijing Vegetable Research Center (BVRC) maintains 1197 accessions labeled as Citrullus (Zhang & al., 2016). The National Botanical Research Institute (NBRI), National Plant Genetic Resources Centre (Namibia) and the Zambia Agriculture Research Institute (ZARI) also store many Citrullus seed batches (McGregor, 2012).…”

Simple Sequence Repeat Markers Reveal Genetic Diversity Within and among Landrace Collections of Citron and Dessert Watermelon from South Africa

“…The SSR markers generated a total of 135 putative alleles (different fragment size) in both sampled populations (Table 3). This was higher than 133 alleles detected by Zhang et al (2016) who evaluated 23 SSR markers on 1197 watermelon core collections. Nimmakayala et al (2009) reported a total of 169 alleles amplified in watermelon cultivars.…”

“…2009; Solmaz et al, 2010;Zhang et al, 2016). Watermelon breeding programs in the country establish the genetic relationships of germplasm through phenotypic characterization.…”