Two modes of transvection: Enhancer action in trans and bypass of a chromatin insulator in cis

Abstract

Ed Lewis introduced the term 'transvection' in 1954 to describe mechanisms that can cause the expression of a gene to be sensitive to the proximity of its homologue. Transvection since has been reported at an increasing number of loci in Drosphila, where homologous chromosomes are paired in somatic tissues, as well as at loci in other organisms. At the Drosphila yellow gene, transvection can explain intragenic complementation involving the yellow2 allele (y2). Here, transvection was proposed to occur by enhancer of one allele acting in trans on the promoter of a paired homologue. In this, report, we describe two yellow alleles that strengthen this model and reveal an unexpected, second mechanisms for transvection. Data suggest that, in addition action in trans, transvection can occur by enhancer bypass of a chromatin insulator in cis. We propose that bypass results from the topology of paired genes. Finally, transvection at yellow can occur in genotypes not involving y2, implying that it is a feature of yellow itself and not an attribute of one particular allele.