Cytogenetic and molecular aspects of position effect variegation in Drosophila melanogaster

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Abstract
The peculiarities of compact blocks appearing as a consequence of position effect variegation were studied in male polytene chromosomes. In T(1;2)dor var7 /Y males the frequency of nuclei with a block in the 2B region was lower at all temperature and the chromosome regioninvolved in compaction was shorter than in T(1;2)dor var7 /FM6 females. The fraction of nuclei with blocks was considerably increased indor var7 /0 males, especially at 18° C when the viability of these males is sharply reduced. The following features distinguish theblocks in males from those in females: (i) compaction of the 2B region in the males results in genetic inactivation only to a very small extent; (ii) the structure of the blocks in males is diffuse; and(iii) the male blocks still maintain some transcriptional activity as indicated by 3H-uridine incorporation. The temperaturesensitive period of both block formation and geneticinactivation was found to be during the first 3 h of embryonic development.