Polymorphism and linkage analysis of the prothoracicotropic hormone gene in the silkmoth, Bombyx mori

We looked for polymorphism of the prothoracicotropic hormone gene locus (Ptth) among inbred strains of the silkmoth, Bombyx mori, by in vitro DNA amplification (polymerase chain reaction), and found three alleles, PtthA, PtthB and PtthC. The PtthA allele contained a third intron consisting of 680 bp and a fourth intron of 350 bp. PtthB contained the same size third intron but a longer fourth intron of 490 bp, while PtthC had a longer third intron of 1080 bp and a shorter fourth intron of 350 bp. In 29 strains which we examined, 9 strains had PtthA, 8 strains had PtthB, and 2 strains had PtthC. The other 10 strains had heterogeneous genotypes with the same 3 alleles. Linkage analyses using these polymorphisms showed that Ptth was linked to the or gene on chromosome 22, and was independent of other chromosome markers. To determine the map position, we obtained an F1 hybrid between the w41 strain (PtthC+or+mw) and the w22 strain (PtthB or mw), and backcrossed the F1 hybrid to females of the w22 strain. From the segregation of or, mw and Ptth in 861 progeny, recombination values were calculated as 18.5% between or and mw, 23.9% between mw and Ptth, and 7.3% between Ptth and or. Because or and mw have already been mapped at positions 22-8.9 cM and 22-25.2 cM, respectively, we mapped Ptth at 22-2.5 cM. The Ptth locus is independent of any loci which are known to control growth and molting.