Genes include cis-regulatory regions that contain transcriptional enhancers. 
Recent reports have shown that developmental genes often possess multiple 
discrete enhancer modules that drive transcription in similar spatio-temporal 
patterns: primary enhancers located near the basal promoter and secondary, or 
'shadow', enhancers located at more remote positions. It has been proposed that 
the seemingly redundant activity of primary and secondary enhancers contributes 
to phenotypic robustness. We tested this hypothesis by generating a deficiency 
that removes two newly discovered enhancers of shavenbaby (svb, a transcript of 
the ovo locus), a gene encoding a transcription factor that directs development 
of Drosophila larval trichomes. At optimal temperatures for embryonic 
development, this deficiency causes minor defects in trichome patterning. In 
embryos that develop at both low and high extreme temperatures, however, absence 
of these secondary enhancers leads to extensive loss of trichomes. These 
temperature-dependent defects can be rescued by a transgene carrying a secondary 
enhancer driving transcription of the svb cDNA. Finally, removal of one copy of 
wingless, a gene required for normal trichome patterning, causes a similar loss 
of trichomes only in flies lacking the secondary enhancers. These results 
support the hypothesis that secondary enhancers contribute to phenotypic 
robustness in the face of environmental and genetic variability.