Heteromorphic Puffs

The differences in the characteristics of DNA replication could play an important role in differential transcriptional activity of homologous chromosomes, especially in interspecific hybrids. It was demonstrated that in polytene chromosomes of D. melanogaster there is positive correlation between transcription intensity and the rate of replication of the corresponding X chromosomes (Berendes, 1966a,b Ananiev and Gvozdev, 1974 a, b). 

Differences in DNA syntheses in homologous regions of polytene chromosomes were also demonstrated for Anopheles atroparvus x A. labranchiae hybrids (Tiepolo et al, 1974). 

The correction of the structurally damaged locus was not obtained when vg6 flies were crossed with flies which carried the pericentric inversion, in (3LR) Cl65, with breakage points in the 64C region. Almost total inhibition of puffing in the 64C region of vg6 chromosomes was demonstrated in D. melanogaster x D, simu- 

Ians hybrids when conjugation of homologous chromosomes had occurred. However, in those nuclei where conjugation was not present, the vg6 chromosomes always showed a puff at 64C. 

Puff 22B is characteristic for Pacific but not for Oregon strains of D. melano-gaster. Total reparation of activity of the 22B locus of the Oregon strain was obtained, in Pacific x Oregon interstrain hybrids. This event was demonstrated regardless of conjugation of the homologous chromosomes. 

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The best arguments for possible differential activity of autosomal genes are cases of heteromorphism of chromosomal regions for which the relation between cytological and functional expressions have been clearly demonstrated. These include puffs of polytene chromosomes, lateral loops of lampbrush chromosomes and nucleoli. 

The lateral loops of lampbrush chromosomes are active chromosomal regions, similar to puffs. These chromosomes have been studied less than polytene chromosomes from thp point of view of the differential activity of homologous chromosomes. However, there are examples of the functional heteromorphism in the behavior of homologous loops (Callan, 1963, 1965). 

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