An Extended Genetic Linkage Map for Watermelon Based on a Testcross and a BC<sub>2</sub>F<sub>2</sub> Population

Abstract

There is limited genetic mapping data useful for breeding programs of watermelon. Introgression lines should be a use-ful tool for genetic studies and genetic enhancement of watermelon cultivars. In this study, we used an advanced re-combinant population (BC2F2) to identify and map chromosomal segments of the wild watermelon Citrullus lanatus var. citroides that were incorporated in the genome of the watermelon cultivar Crimson Sweet (Citrullus lanatus var. lana-tus). An advanced recombinant population (BC2F2) was constructed using a United States Plant Introduction (PI) 494817 (C. lanatus var. citroides) (known to have moderate resistance to bacterial fruit blotch) as a donor parent, and the elite watermelon cultivar Crimson Sweet (C. lanatus var. lanatus) as the recurrent parent. The genetic linkage map consists of 272 markers, including 89 sequence-related amplified polymorphism (SRAP), 72 targeted region amplifica-tion polymorphism (TRAP), and 111 high frequency oligonucleotide-targeting active gene (HFO-TAG) markers. The 272 markers were assembled into 51 linkage groups, covering a total genetic distance of 2162 cM, with an average genetic distance of 7.9 cM between markers. Also, we expended the genetic linkage map for watermelon derived from a testcross population {Griffin 14113 [C. lanatus var. citroide (L.H. Bailey) Mansf.] x watermelon cultivar New Hamp-shire Midget (C. lanatus var. lanatus)} x PI 386015 [C. colocynthis (L.) Schrad.]. The genetic linkage map based on the test cross population consists of 558 markers that cover a genetic distance of 2760.8 cM. This linkage map consists of 41 linkage group, including 10 large linkage groups (ranging from102-240 cM), nine intermediate size linkage groups (ranging from 62-93 cM), and 22 small linkage groups (ranging from 2-56 cM). Comparative mapping between these two linkage maps identified high consensus in 25 HFO-TAG markers and one TRAP marker that represent 8 linkage groups in the BC2F2 population and 9 linkage groups in the testcross population. These results indicate that HFO-TAG markers should be useful in comparative mapping. The extended genetic maps and the genetic population in this study should be useful in breeding programs using marker assisted selection and should serve as a platform for further de-velopment of introgression lines for watermelon.