Insect body color is an easily assessed and visually engaging trait that is informative on a broad range of topics including speciation, biomaterial science, and ecdysis. Mutants of the fruit fly Drosophila melanogaster have been an integral part of body color research for more than a century. As a result of this long tenure, backlogs of body color mutations have remained unmapped to their genes, all while their strains have been dutifully maintained, used for recombination mapping, and part of genetics education. Stemming from a lesson plan in our undergraduate genetics class, we have mapped sable1, a dark body mutation originally described by Morgan and Bridges, to Yippee, a gene encoding a predicted member of the E3 ubiquitin ligase complex. Deficiency/duplication mapping, genetic rescue, DNA and cDNA sequencing, RT-qPCR, and two new CRISPR alleles indicated that sable1 is a hypomorphic Yippee mutation due to a mdg4 element insertion in the Yippee 5’-UTR. Further analysis revealed additional Yippee mutant phenotypes including curved wings, ectopic/missing bristles, delayed development, and failed adult emergence. RNAi of Yippee in the ectoderm phenocopied sable body color and most other Yippee phenotypes. Although Yippee remains functionally uncharacterized, the results presented here suggest possible connections between melanin biosynthesis, copper homeostasis, and Notch/Delta signaling; in addition, they provide insight into past studies of sable cell nonautonomy and of the genetic modifier suppressor of sable.

Tandem duplicated genes are common features of genomes, but the phenotypic consequences of their origins are not well understood. It is not known whether a simple doubling of gene expression should be expected, or else some other expression outcome. This study describes an experimental framework using engineered deletions to assess any contribution of locally-acting cis- and globally-acting trans-regulatory factors to expression interactions of particular tandem duplicated genes. Acsx1L (CG6300) and Acsx1R (CG11659) are tandem duplicates of a putative acyl-CoA synthetase gene found in D. melanogaster. Experimental deletions of the duplicated segments were used to investigate whether the presence of one tandem duplicated block influences the expression of its neighbor. Acsx1L, the gene in the left block, shows much higher expression than either its duplicate Acsx1R or the single Acsx1 in D. simulans. Acsx1L expression decreases drastically upon deleting the right-hand duplicated block. Crosses among wildtype and deletion strains show that high tandem expression is primarily due to cis-acting interactions between the duplicated blocks. No effect of these genes on cuticular hydrocarbons was detected. Sequence and phylogenetic analysis suggest that the duplication rose to fixation in D. melanogaster and has been subject to extensive gene conversion. Some strains actually carry three tandem copies, yet strains with three Acsx1s do not have higher expression levels than strains with two. Surveys of tandem duplicate expression have typically not found the expected twofold increase in expression. This study suggests that cis-regulatory interactions between duplicated blocks could be responsible for this trend.