Bm kettin, homologue of the Drosophila kettin gene, is located on the Z chromosome in Bombyx mori and is not dosage compensated

In Bombyx mori, the female is the heterogametic sex and the sex determining system is referred to as ZZ/ZW. In a previous study, we found that this in sect does not show dosage compensation at the transcriptional level. To con firm the validity of our conclusion, we investigated whether or not another sex-linked gene is dosage compensated. To identify new Z-linked genes, tot al RNA from reciprocal hybrid females between the silkworm strains p50 and C108 was compared using the differential display technique. Nine cDNA fragm ents corresponding to several differentially expressed mRNAs were cloned an d sequenced. The analysis of nucleotide sequence polymorphisms confirmed th at one of these cDNAs, ZDD4, originated from the Z chromosome. The amino ac id sequence deduced from ZDD4 has homology with kettin, a modular protein i n the Z-disc of Drosophila melanogaster muscles. On immunoblots of Bombyx l arval muscle proteins a polypeptide of 380 kDa was labelled with antibody t o the ZDD4 peptide. We considered that the gene corresponding to ZDD4 encod es a kettin homologue in the silkworm, and denote it as Bm kettin. By a thr ee-point cross, Bm kettin was mapped at 40.0 CM<SUP></SUP> on the Z chromos ome. Southern blot analysis revealed that Bm kettin was present at one copy in the genome. Northern blot analysis showed that Bm kettin mRNA was 9.1kb in length, and that the level of the mRNA in males was two times greater t han that of females. Taken together with our previous observations, the pre sent data suggest that lack of dosage compensation is a general rule in B. mori. Moreover, the twofold difference in Bm kettin expression between male s and females may help explain the sexual difference in the wing flapping a ctivity observed in some groups of Lepidoptera.