Jr. Morris et al., 2 MODES OF TRANSVECTION - ENHANCER ACTION IN TRANS AND BYPASS OF A CHROMATIN INSULATOR IN CIS, Proceedings of the National Academy of Sciences of the United Statesof America, 95(18), 1998, pp. 10740-10745
Ed Lewis introduced the term ''transvection'' in 1954 to describe mech
anisms 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 Drosophila, where homologous chromosomes
are paired in somatic tissues, as well as at loci in other organisms.
At the Drosophila yellow gene, transvection can explain intragenic co
mplementation involving the yellow(2) allele (y(2)). Here, transvectio
n was proposed to occur by enhancers of one allele acting in trans on
the promoter of a paired homologue. In this report, we describe two ye
llow alleles that strengthen this model and reveal an unexpected, seco
nd mechanism for transvection. Data suggest that, in addition to enhan
cer action in trans, transvection can occur by enhancer bypass of a ch
romatin insulator in cis. We propose that bypass results from the topo
logy of paired genes. Finally, transvection at yellow can occur in gen
otypes not involving y(2), implying that it is a feature of yellow its
elf and not an attribute of one particular allele.