The expression of "puffy" X chromosomes in pfy^BM2 males is temperature dependent. Puffy X chromosomes are seen in about 25% of polytene nuclei of male larvae reared continuously at 18 +/- 1^oC until the third larval instar. At 24 +/- 1^oC all polytene male X chromosomes show normal morphology. Temperature downshift from 24 +/- 1 to 18 +/- 1^oC at various stages of development can result in the induction of a small number of puffy X chromosomes in male larvae. The structure of polytene female X chromosomes and autosomes remains unaltered. Puffy male X chromosomes are not induced by shifting from 18 +/- 1 to 24 +/- 1^oC at any time during development. Temperature downshifts from 24 +/- 1 to 12 +/- 1^oC also result in induction of puffy male X chromosomes in mutant animals. When animals are shifted from 24 +/- 1^oC to 12 +/- 1^oC and then allowed to recover at 18 +/- 1 or 24 +/- 1^oC, the number of puffy X chromosomes increases during the first 2 hours of recovery but returns to the basal level by the 4th hour of recovery. Identical observations are made for animals shifted from 24 +/- 1^oC to 18 +/- 1^oC and then allowed to recover at 24 +/- 1^oC. Two types of temperature induced alteration in the morphology of the polytene male X chromosome are seen; alteration of the global structure of the chromosome, resulting in typical or highly condensed puffs and random regions of increased width along the chromosome. This latter phenotype is associated with regions of spiralisation of the X chromosome. Extracts purified from mutant males can alter the structure of polytene chromosomes from Oregon R larvae; at low concentrations the structure of the male X chromosome is altered, while at higher concentrations a dramatic sex-nonspecific and chromosome-nonspecific alteration in the structure of the polytene chromosomes is seen.

In third instar male larvae reared at 18^oC, about 25% of polytene nuclei have X chromosomes that appear twice as wide as the X chromosomes of wild-type larvae.

In males at 18^oC the X chromosomes of about 25% of nuclei appear puffy and are twice as wide as normal X chromosomes. X chromosomes may also appear randomly puffy at different regions along the chromosome. Chromosome does not show an enhanced level of transcriptional activity. Puffy Xs can still be scored upon heat shock. X chromosome of females does not manifest any morphological abnormality.

Abnormal over-decondensation of male X chromosome, described as puffy X chromosomes. These puffy chromosomes do not show elevated transcription (Kar and Pal, unpublished observations). At 18^oC 50% of male X chromosomes show complete or partial puffy morphology. Heat shock at 37^oC for 20-30 mins reduces the number of puffy chromosomes observed.