A first genetic linkage map of mulberry (Morus spp.) using RAPD, ISSR, and SSR markers and pseudotestcross mapping strategy

To lay the foundation for molecular breeding efforts, the first genetic linkage map of mulberry (2n=2x=28) was constructed with 50 F1 full-sib progeny using randomly amplified polymorphic DNA (RAPD), inter-simple sequence repeat (ISSR), and simple sequence repeat (SSR) markers and two-way pseudotestcross mapping strategy. We selected 100 RAPD, 42 ISSR, and 9 SSR primers that amplified 517 markers, of which 188 (36.36%) showed a test-cross configuration, corresponding to the heterozygous condition in one parent and null in the other. Two separate female and male maps were constructed using 94 each of female- and male-specific testcross markers, containing 12 female linkage groups and 14 male linkage groups. At a minimum logarithm of the odds (LOD) score threshold of 6.0 and at a maximum map distance of 20 cM, the female map covered a 1,196.6-cM distance, with an average distance of 15.75 cM and maximum map distance of 37.9 cM between two loci; the male-specific map covered a 1,351.7-cM distance, with an average distance of 18.78 cM and a maximum map distance between two loci is of 34.7 cM. The markers distributed randomly in all linkage groups without any clustering. All 12 linkage groups in the female-specific map consisted of 4–10 loci ranging in length from 0 to 140.4 cM, and in the male-specific map, the 13 largest linkage groups (except linkage group 12, which contained three loci) consisted of 4–12 loci, ranging in length from 53.9 to 145.9 cM and accounting for 97.22% of the total map distance. When mapping, progeny pass through their juvenile phase and assume their adult characters, mapping morphological markers and identification of quantitative trait loci for adaptive traits will be the primary target. In that sense, our map provides reference information for future molecular breeding work on Morus and its relatives.