The scaleless wings mutant in Bombyx mori is associated with a lack of scale precursor cell differentiation followed by excessive apoptosis

The scaleless wings mutant in Bombyx mori (scaleless, sl) was previously reported morphologically. In the present study, we give data to clarify the mechanism of the mutation at the developmental level. Programmed cell death participates in the wing scale development during early pupal stage, and there are significant differences between that of sl and the wild type (WT) at each phase. Well-differentiated scale precursor cells do not form in sl when they have formed in WT. The peak of Caspase-3/7 activity in sl occurs 1 day later than and ten times as much as that in WT. Apoptotic bodies and DNA ladder studies also show that there is excessive apoptosis in sl early pupal wing. In addition, we have studied Bm-ASH1, an achaete–scute homolog in B.mori, which is thought to play a key role during the development of wing scales, and have found that the gene structure and expression levels of Bm-ASH1 in sl and WT are identical. All the data indicate that the wing scale precursor differentiation mechanism is abnormal in sl, which causes failing determination of scale cells and the downstream symptom of excessive apoptosis. But some of the elements to the scale differentiation circuit, such as Bm-ASH1, still operate in sl.