Fine Mapping Of Lethal Gene L₁ And L₂ In The Sex-Linked Balanced Lethal Silkworm Strain

The sex-linked balanced lethal silkworm strain (S14), constructed with radiation by Russia scientists V. A. Strunnikov, has two non-allelic recessive lethal genes (l1 and l2). The two lethal genes are trans-heterozygous on two different Z chromosome. The lethal stage of l1 is at the body pigmentation stage; The lethal stage of l2 is at the end reversal embryo stage. There is 0.8% of recombination probability between the two lethal loci (l1 and l2). A fragment of the Z chromosome having wild-type allele of l1 is translocated onto the W chromosome of S14. Females with the genotype of W+l1 Z+l1·l2 and males with the genotype of Zl1·+l2/Zl1·+l2 are eliminated during the embryogenesis. Half females with the genotype of W+l1 Zl1·+l2 and half of male with the genotype of Zl1·+l2 Zl1·l2 can survive in strain S14. On Z-W translocation segment of S14 there is a dominant allele for recessive os (larval skin translucent) linked to wild-type allele of l1·Males of S14 strain hybridized with females of P50 strain and males of F1 generation backcrossed with females of P50 strain. The female moths of first backcross generation (BC1) were divided into two groups BCl-l1 and BCl-l2 according to the genotype of F1 male moths and their genotyes were tested respectively for locating two lethal loci (l1 and l2).The contents and results are as follow:(1) The molecular marker N20.80b (accession no. AB023114) closely linked to os is near 19.2Mb on physical map of Z chromosome. 5.7 Mb genome sequence near the locus of N20.80b were downloaded from whole genome database of silkworm (KAIKObase) and SSR Markers in this region were search out using SSRHunterl.3 software. According to the conserved sequence ends of SSR sequence 563 pairs of specific primers were designed for detecting polymorphic loci among two Z chromosomes with lethal l1 (Zl1·+l2) and l2 (Z+l1·l2) of S14 strain and Z chromosome of P50 strain.(2) 10 polymorphic loci were found between Z1l·+l2 and Z+l1·l2 and one of them (such as Z-14) was used to identify the genotype of F1 male moths.16 polymorphic loci between P50 and Zl1·+l2 and 18 polymorphic loci between P50 and Z+l1·l2were found by detecting the genomic DNA of F1 male moths.(3) 16 polymorphic molecular markers between Z chromosome of P50 and Zl1· +l2 were used to identify the genotype of BCl-l1 and 18 polymorphic molecular markers between Z chromosome of P50 and Z+l1·l2 were used to identify the genotype of BCl-l2. The fine linkage maps of Zl1·+l2 and Z+l1·l2 were constructed separately according to the genotype of BC1 generation female moths. Finally l1 was located in the region of 2.6Mb from 19.79 Mb to the end of Z chromosome and l2 was mapped in the length region of 0.69 Mb from 17.86 Mb to 18.55 Mb of Z chromosome. The distance between two lethal genes l1 and l2 is at least 1.25 Mb. It was found that Zl1·+l2 chromosome is unmoral from the fine linkage map of l1.(4) By detecting the genomic DNA sequence of Zl1·+l2/Zl1·+l2 genotype male eggs we found that Zl1·+l2 chromosome lost a fragment about 0.184 Mb with five genes in chromosome end. The reason of lethal effect of l1 was caused by the loss of fragment. The candidate gene of l1 may be the PdpI (Par domain protein I, BGIBMGA003874) and this need to be tested.