Silk fibers are derived from the cocoon of the silkworm Bombyx mori, which was domesticated over the past 5,000 years from the wild progenitor Bombyx mandarina. Cocoon quality is very important because it can influence the yield of sericulture and determines whether a silkworm line can be used in silk production. Through the efforts of silkworm breeders over several thousands of years, many silkworm strains have been collected and conserved. Moreover, the different properties of these conserved silkworm strains, such as filament length, cocoon weight, cocoon shell weight, cocoon shell ratio, and cocoon color, have distinctive applications. Until now, crossbreeding was the only method of enriching loci that control cocoon quality to enhance the yield from a silkworm cocoon. Modern techniques involving gene cloning and marker-assisted breeding are now widely considered to be the most effective way of improving silk properties.
Genetic linkage map is an essential tool for mapping traits of interest and are used in positional cloning and marker-assisted breeding. Some genetic maps for the silkworm have been reported, including various genetic markers such as restriction fragment length polymorphisms (RFLPs; [1
]), random amplified polymorphic DNA (RAPD; [3
]), amplified fragment length polymorphisms (AFLPs; [5
]), simple sequence repeats (SSRs; [6
]), and single nucleotide polymorphisms (SNPs; [8
]). SSRs (also called microsatellites) are generally accepted to be ideal markers because of their sound transferability, high reproducibility, and co-dominant inheritance. SSR markers are especially suitable for high-throughput genotyping, allowing rapid analysis of hereditary monogenetic traits and quantitative trait loci (QTL). Once SSR markers were established, polymorphisms could be detected merely by visualizing PCR products on an agarose gel, and these markers are still important for the meiotic analysis of livestock and agricultural species [10
In our previous SSR linkage map [7
], the 518 robust markers reported accounted for only 20% of all identified SSRs. The number of polymorphisms was low due to reliance on parental combinations between Dazao and C108 and to the minimal number of polymorphisms that occur between silkworm strains. Because the genetic distance between markers can be as large as 6.3 cM, fine mapping and gene cloning remain difficult.
A general approach for increasing the marker density in genetic linkage maps involves the identification of more markers and the integration of several linkage maps. Xia et al. [16
] constructed an integrated, high-density linkage map of soybean using RFLPs, SSRs, sequence-tagged sites (STSs), and AFLP markers. Similarly, Vezzulli et al. [17
] constructed an integrated map of grapevine using SSR and SNP markers. In many cases, the maps from different parental populations and even species have been integrated [18
The choice of using the parental population of Dazao and C108 was based mainly on its internationally consistent use in silkworm genetic research. However, neither strain is applied widely for economic production of silk-related products. In China, more than 70% of silkworm breeders use the Jingsong strain for practical applications. Jingsong has properties that are advantageous for silk production, such as an average filament length of 1,200–1,500 m. In contrast, L10, which has poor silk-producing properties, possesses high stress resistance. Additional matings between strains of different origins may increase the mapping efficiency of markers due to the increased potential for genetic diversity.
Herein, we report an improved method for constructing silkworm SSR genetic maps with more informative loci based on new mapping populations (Figure ). Using this approach, we localized QTL for whole cocoon weight, cocoon shell weight, cocoon shell ratio, and pupal weight. This work underpins the further cloning of genes that control properties advantageous to silk production and will be utilized further to identify molecular markers to assist in the breeding of productive silkworm lines.
Figure 1 Outline of the study. We mapped SSR loci based on the mapping population Fa50B × Nistari and Jingsong × Lan10. Two new SSR linkage maps (NF and JL) were constructed, and the resultant data set was merged with previous data sets to generate (more ...)