Chromosomal control of foot pad development in Sarcophaga bullata

The puffing schedule of 71 puffs of chromosome B in Sarcophaga bullata polytene foot pad cells were analyzed during an 8 day period of pupal development. The number of puffs fluctuates, with maxima at about day 7 and day 10. The nucleolar volume reaches two maxima which precede the maxima in the number of puffs by approximately one day. With respect to qualitative changes in the puffing pattern, the present study shows that all puffs appear and disappear in a sequential fashion which is related to development. The pattern of potentially active loci and the sequence in which they form puffs are identical in all foot pad cells. Nevertheless, two neighboring cells may differ characteristically in the timing of the activity of a few individual loci relative to the rest of the puffing sequence. Furthermore, pulvilli cells from different pairs of legs may differ in the timing of the entire puffing sequence. It is concluded that puffing, at least in part, is under the control of intracellular processes rather than of external factors such as hormones.