A scaleless wings mutant associated with tracheal system developmental deficiency in wing discs in the silkworm, Bombyx mori.

A mutant of Bombyx mori has wings with few scales and is named scaleless. We investigated the morphology of this mutant and found that it had many fewer wing scales than the corresponding wild type (WT) silkworm and that the remaining scales were smaller in shape with fewer furcations. Reciprocal transplantation of wing discs between scaleless and WT revealed that the WT wing disc could develop into a small wing with scales after transplantation into a scaleless larva; however, the scaleless wing disc developed into a small wing without scales in a WT larva. Upon dissection of WT and scaleless wing discs at different stages from the fifth instar larva to adulthood, no obvious differences were found before pupation. However, after pupation, tracheae produced from WT wing veins extended to the lacunae between the veins and formed a network on the second day after pupation, whereas this did not happen in scaleless. At the same time, no marked difference in adult body tracheal development was found between the mutant and wild type. Furthermore, if the surface of a wing disc was cut and its veins injured, the resulting wing also had fewer scales than the corresponding WT. Also, we found that higher partial pressure of O2 could rescue the loss of scales in scaleless. These data suggest that the factors affecting the growth of scales were not produced in the hemolymph, but in the wing disc itself. It is also implied that wing scale development is dependent on the correct organization of the tracheal system in the wing disc.