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The genetic and molecular analysis of the mfs(2)31 locus in Drosophila melanogaster : a novel suppressor of position-effect variegation Whitehead, Ian P.


Position-effect variegation (PEV) is the variable inactivation of a euchromatic gene which has been moved, by way of a chromosomal rearrangement, into a heterochromatic environment. The transcriptional repression at the variegating locus is thought to be a consequence of inappropriate packaging of the euchromatin asheterochromatin. Second site mutations which modify the PEV phenotype (Su(var)s and E(var)s), identify loci which encode non-histone chromosomal proteins. Although the majority of mutations which modify PEV exhibit a dominant phenotype, rare recessives u(var) mutations have been reported. This study describes a locus which is identified by one such recessive mutation, mfs(2)31. A cytogenetic analysis of subdivisions 31D-E was undertaken to determine the precise location of the mfs(2)31 locus and to isolate additional alleles. Five new deficiencies and 123 new lethal mutations were induced, allowing for the partitioning of 31D-E intosix cytological subintervals. The mfs(2)31 gene was localized todistal 31E in an interval containing nine lethal complementation groups. Three new mfs(2)31 alleles were recovered, one of whichwas isolated in a screen for P element insertions. The new alleles of mfs(2)31 were used for a phenotypic analysis of the locus. Strong alleles that were lethal as homozygotes died in the larval phase, while weaker alleles exhibited the previously described bristle, sterility and su(var)phenotypes. Larval, pupal and adult functions were defined for the locus. Although no dominant phenotypes were observed, surviving heteroallelic combinations suppressed PEV in a variety of variegating backgrounds. When the weak, P-induced, allele was outcrossed in a dysgenic background, all the mfs(2)31 phenotypes, including suppression of PEV, were co-reverted. An in situhybridization to salivary gland polytene chromosomes revealed a P element in distal 31E of this strain. The P element, located in distal 31E, was cloned from thedysgenic mfs(2)31 allele. This element is responsible for the reduced transcription of a 1.2 kb message which is expressed throughout development. Wild-type levels of transcription are restored at this locus in revertants of the mfs(2)31 phenotype. The predicted amino acid sequence, as determined from a cDNA analysis, reveals similarities with a mouse microtubule-associated protein and mammalian histone H1.

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