Description

The adult Drosophila wing blade contains sense organs known as campaniform sensilla.  These cellular structures sense pressure or strain in the wing during flight and provide neural feedback required for coordinated wing movements (Bartussek and Lehmann, 2016).  The dorsal surface of the wing blade includes three campaniform sensilla along wing vein L3, two on the first longitudinal vein near the tip of the costa, and a single sensillum on the anterior cross vein.  Other campaniform sensilla are found at the base of the wing and on the ventral wing surface (Huang et al., 1991).  In experiments utilizing GAL4/UAS inducible expression of the Zinc finger transcription factor hindsight (hnt), also known as pebbled (peb) (Yip et al., 1997), we found that campaniform sensilla are frequently transformed to mechanosenory external sense organs, known as microchaetae.  The GAL4 driver used was scaGAL4, a reporter driving expression of GAL4 in the pattern of the proneural gene scabrous (sca), which is expressed in sensory organ precursor cells as is hnt (Buffin and Gho, 2010).  We found campaniform to microchaetae transformation occurred in the context of scaGAL4 driving expression of a particular transgene insertion, UAS-GFP-hnt[J18].  In order to rule out the possibility that this transformation was somehow specific to UAS-GFP-hnt[J18], we also tested scaGAL4 > peb[EP55] and found the same transformation.  The penetrance of this phenotype was not complete, as not all campaniform sensilla were transformed to microchaetae in all individuals.  Approximately half of progeny carrying scaGAL4 and either peb[EP55] or UAS-GFP-hnt[J18] display at least one transformation.  To the best of our knowledge, there are two instances of this particular phenotype reported in the literature.  The first is a specific allelic combination of loss-of-function mutants of the gene absent, small, or homeotic discs 2 (ash2[7]/ash2[18]) (Adamson and Shearn, 1996).  The second involves the expression throughout the wing imaginal disc of a human SMAD tumour allele (UAS-SMAD4[100T]) (Takaesu et al., 2005). The former instance involving ash2 loss-of-function mutants is of particular interest because ash2 is implicated in the negative regulation of EGFR/Ras/MAPK signalling, through the negative regulation of rhomboid (Angulo et al., 2004), which is required for the production of active EGFR ligand (Wasserman et al., 2000).  Thus, overexpression of hnt is consistent with overactivation of EGFR/Ras/MAPK signalling in the context of campaniform sensilla SOPs.  Interestingly, hnt is the Drosophila homologue of human Ras Responsive Element Binding protein-1 (RREB-1) (Melani et al., 2008; Ming et al., 2013).