Abstract. Cocoon weight and shell weight are the key economic traits ultimately determining silk yield. In order to detect the main quantitative trait loci (QTL) associated with the cocoon traits of the mulberry silkworm, Bombyx mori, the parents of larvae that produced cocoons that differed greatly in weight and shell weight were screened using 240 primer pairs of single nucleotide polymorphic markers (SNPs) representing all the 28 linkage groups in silkworm. Out of the 240 primer pairs, 48 (20%) revealed distinct polymorphism between the parents, which was confirmed by the co-dominant expression of both polymorphic PCR products in the F1 generation. The bulked segregant analysis (BSA) was used to compare the SNP profiles of the parents, F1 and F2 bulks using the 48 informative SNP primers. This revealed that out of 48 primer pairs, only one pair, i.e., No. 04124 of the linkage group 4 showed clear differences in the amplified products between the bulks corresponding to that of the parents with different cocoon traits suggesting that the DNA regions amplified by this primer pair are closely linked to the QTL controlling the cocoon traits. The results were also confirmed by screening the backcross (BC) progeny. This is the first report of the identification of a QTL using SNPs with BSA. The results of the present study indicate that it might be possible to use SNPs for marker assisted selection (MAS) in silkworm breeding programs aimed at improving cocoon traits. 347* Corresponding author. MATERIAL AND METHODS Silkworm strains used and raising of F2 and BC progenyThe silkworm strains Pure Mysore (PM) and CSR2, which differ in cocoon traits, were used in the present study. The parents were reared under standard conditions and the F1 generation of PM × CSR2, F2 by selfing of F1 progeny, backcross progeny, viz., (PM × CSR2) × PM and (PM × CSR2) × CSR2, were produced and reared simultaneously. After cocooning the cocoons were cut open and sexes of the pupae determined by recording the markings on the individual pupae. Cocoon weight represents the total weight of the pupa along with the silken shell. The shell weight is the weight of the cocoon after removing the pupa. Generally, female pupae are heavier than those of males, as they contain 350 to 500 eggs and a geater amount of fat tissue. To determine the mean cocoon weight of parents and progeny, a sample of 100 cocoons consisting of 50 female and 50 male cocoons of the parents, F1, F2 and BC progeny were weighed. Further the shell weight was recorded using the same cocoon samples. The frequency distribution of the cocoon and shell weight of the F2 was analyzed. The cocoon weight and shell weight data were sorted in descending order and the top ten (high weight) and the bottom ten (low weight) in the samples of F2 and BC progeny were selected. DNA extractionHigh molecular weight genomic DNA was extracted from frozen moths of PM females, CSR2 males, and their F1 progeny (PM × CSR2). Frozen pupae were used for the extraction of DNAs from F2 and BC progenies. All DNA...

Digestive amylase has been identified as a useful marker for breeding in the silkwrom, Bombyx mori L (Lepidoptera: Bombycidae), due to its wide genetic divergence, its role in better digestibility and robustness. The low yielding indigenous B. mori breeds of tropics like India are characterized by high activity amylase genes controlled by Amy div or dv alleles, while the high yielding breeds of temperate origin are endowed with ‘null’ type (Amy dn) with low activity. For improving the digestibility and survival of temperate breeds of Japanese origin, Near Isogenic Lines (NILs) were developed introgressing the Amy div and dv alleles from the Donor Parents (DPs) into the genetic background of the Recurrent Parents (RPs) with ‘null’ type of amylase, which showed significant improvement in viability of the NILs. With the objective to know whether the amylase gene itself may confer higher survival by improving digestibility or some other closely linked genes flanking the amylase locus is responsible for better viability of the NILs, RAPD profiles among six B. mori breeds comprising of the DPs, RPs, and NILs developed through introgression of Amy div or dv alleles were analysed using 27 sets of RAPD primers. Out of the 27 primers, six (OPA01, OPA06, OPA09, OPA15, OPAH03, and OPAH05) showed RAPD products linked to the amylase genes of the DPs introgressed in the NILs, which were absent in their respective RPs. Three amplicons of 1584 bp, 1904 bp, and 1232 bp were specific to Amy div allele and one amplified product of 1776 bp was found to be linked with the Amy dv allele. Interestingly, two PCR products of 2628 and 1375 bp were associated with both Amy div and dv alleles. The results are discussed in light of further characterization of these amplified products leading to identification of DNA sequences that may be responsible for better digestibility and higher survival in B. mori.

In silkworms, sex-limited strains are either obtained spontaneously or induced by X-rays or gamma rays. When a fragment of an autosome carrying a dominant allele of those genes responsible for certain characters is translocated onto a W chromosome, the female of the successive generations will express these phenotypic characters and sex discrimination can be facilitated. Gensei-kouken strains are sex-limited strains of silkworms developed by irradiating the pupae with gamma rays, by which a portion of the second chromosome is translocated onto the W chromosome. In these improved strains, the females are yellow-blooded and spin yellow cocoons. By using the EST-cDNA clones mapped on the Z chromosome, we identified the sex according to the polymorphic banding pattern or intensity of the signals. Furthermore, by using the clones on the second chromosome, the region of the second chromosome translocated onto the W chromosome was also defined. In both the A95 and A 96 strains selected for the present study, only the mid-portion of the second chromosome was translocated. The differences in length of the fragments translocated in these strains are discussed.