Casein kinases I of the silkworm, Bombyx mori:: Their possible roles in circadian timing and developmental determination

Doubletime (DBT), a homolog of casein kinase I epsilon (CKI epsilon), is an essential circadian clock component and developmental regulator in Drosophila melanogaster. The authors cloned a dbt homolog from the silkworm, Bombyx mori (Bmdbt), and examined its spatial and temporal expression in comparison to a CKI alpha homolog (BmCKI alpha). Four Bmdbt splice variants and 2 BmCKIa splice variants were detected, and their expression patterns varied in different tissues. The level of Bmdbt transcript in the brain was constant under LD 12:12 while those of BmCKIa transcripts fluctuated with a decrease at ZT12. In situ hybridization showed presumably identical distribution of dbt, CKI alpha, and per transcripts in the putative clock neurons of the head ganglia, as well as in the retina, where CKI- and PER-like immunoreactivities were colocalized, suggesting a possible involvement of both CKIs in the B. mori circadian system. Signals were detected at 4 Ia(1) neurons in each dorsolateral protocerebrum, 6 to 8 cells in the pars intercerebralis, about 6 cells in the suboesophageal ganglion, 2 neurons in the frontal ganglion, and most of the photoreceptors. All these cells contained dbt, CKI alpha, and per antisense transcripts. The Northern analysis of dbt and CKI alpha transcripts at different developmental stages showed that both genes were expressed at relatively high levels during early embryogenesis and in the ovary. The levels of CKI alpha transcripts were also high in the late larval stages until the mid-fifth instar and then suddenly disappeared before larval-pupal ecdysis. In contrast, the transcriptional activity of both genes was low in dia-pausing eggs.