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Related Experiment Videos

Towards an understanding of position effect variegation.

K D Tartof1, C Bishop, M Jones

  • 1Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, PA 19111.

Developmental Genetics
|January 1, 1989
PubMed
Summary

Variegating position effects in Drosophila melanogaster result from euchromatic genes near heterochromatin. Modifiers of variegation, often duplications or deficiencies, influence gene expression by altering gene copy number.

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Area of Science:

  • Genetics
  • Molecular Biology
  • Developmental Biology

Background:

  • Variegating position effects arise when euchromatic genes are placed near alpha-heterochromatin, leading to mosaic expression.
  • These effects are known to be influenced by trans-acting mutations that enhance or suppress variegation.

Purpose of the Study:

  • To investigate the molecular basis of variegating position effects in Drosophila melanogaster.
  • To identify and characterize dominant modifiers of variegation.

Main Methods:

  • Detailed molecular analysis of three variegating white mottled mutations (inversions of the X chromosome).
  • Analysis of revertants of the wm4 mutation to understand variegation control.
  • P-element mutagenesis to induce and examine dominant enhancers of variegation.

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Main Results:

  • Euchromatic breakpoints of variegating mutations are clustered downstream of the white promoter, with heterochromatic sequences being transposons.
  • Revertants of wm4 retained some heterochromatic sequences, suggesting variegation control is internal to heterochromatic domains.
  • Twelve dominant enhancers of variegation were identified, representing four loci and often appearing as duplications or deficiencies.
  • These modifiers function by altering gene copy number and fall into two reciprocal classes: Class I (enhance when duplicated, suppress when deficient) and Class II (suppress when duplicated, enhance when deficient).

Conclusions:

  • A boundary model for heterochromatin domain formation is proposed based on variegation control mechanisms.
  • In Drosophila, approximately 20-30 loci can dominantly modify variegation, with Class I modifiers being more common than Class II.