Genetic analysis of Nid-1 (non-infection with densonucleosis virus type1), showing complete resistance against DNV-1, was carried out. From the linkage analysis, Nid-1 was linked to Bm and bts2, on the seventeenth linkage group. Two point analysis between Bm and Nid-1 showed that the distance between them was 20.8% when calculated from the 53 recombinants from 285 backcross segregants. Next, three point analysis was carried out between the genes, bts, ow and Nid-1, which were located at the midst of the chromosome, and the order and the loci of them were bts (30.1), Nid-1 (31.1) and ow (36.4) from the recombination values of the backcrosses.
The morphological linkage map of Bombyx mori has been made by the combination of linkage analyses of BF1, i. e., heterozygous females crossed with homozygous males, and the three-point analyses of reciprocal BF1. We have advanced the methods of linkage analysis and three-point analysis by using EST-cDNA clones' RFLP as a marker. In the present study, we tried to perform fluorescence in situ hybridization (FISH) vs. chromosomal specimens from the ovaries of larvae on day 1 of the fifth instar in order to compare the genetic distances with the actual positions on a chromosome. The probes used were a mixture of digoxigenin-labeled medium-sized Bombyx genomic fragments corresponding to six cDNA clones located on three sites (4, 18 and 36cM) of the RFLP linkage group 9 (RFLG9). As a result, three groups of specific spot signals with different intensities were concurrently detected on a pachytene bivalent. These three kinds of signals were relatively positioned at 24.9}3.1, 45.2}4.1 and 70.8}3.8%. These values were roughly in agreement with the previously obtained genetic distances of the sites on RFLG9 from which the probes were taken.