The Early Developmental Gene Semaphorin 5c Contributes to Olfactory Behavior in Adult Drosophila

Abstract

Behaviors are complex traits influenced by multiple pleiotropic genes. Understanding the mechanisms that give rise to complex behaviors requires an understanding of how variation in transcriptional regulation shapes nervous system development and how variation in brain structure influences an organism's ability to respond to its environment. To begin to address this problem, we used olfactory behavior in Drosophila melanogaster as a model and showed that a hypomorphic transposon-mediated mutation of the early developmental gene Semaphorin-5c (Sema-5c) results in aberrant behavioral responses to the repellant odorant benzaldehyde. We fine mapped this effect to the Sema-5c locus using deficiency mapping, phenotypic reversion through P-element excision, and transgenic rescue. Morphometric analysis of this Sema5c allele reveals subtle neuroanatomical changes in the brain with a reduction in the size of the ellipsoid body. High-density oligonucleotide expression microarrays identified 50 probe sets with altered transcriptional regulation in the Sema-5c background and quantitative complementation tests identified epistatic interactions between nine of these coregulated genes and the transposon-disrupted Sema-5c gene. Our results demonstrate how hypomorphic mutation of an early developmental gene results in genomewide transcriptional consequences and alterations in brain structure accompanied by profound impairment of adult behavior.

Neurogenetic networks for startle-induced locomotion in Drosophila melanogaster

“…Only three of these genes (Beadex, Sema-5c, and neur) had previously been documented to affect locomotion (19)(20)(21). Many of these candidate genes affect nervous system development and function, as well as other aspects of development, the cell cycle, and oogenesis and spermatogenesis, consistent with previous observations that genes affecting startle-induced locomotion are highly pleiotropic (5,11,12).…”

“…Given the low penetrance, we next asked whether subtler, quantitative effects on brain structures could be seen. We used a previously established standardized method for determining morphometric parameters for these neuropils by immunohistochemistry using an anti-fasciclin 2 monoclonal antibody (21). This procedure revealed several mutations with more subtle, quantitative effects on morphology of brain structures: lola [length of the ␤ lobe (P ϭ 0.024) and area of the Enhance P < 0.05 P < 0.01 P < 0.001 Suppress P < 0.05 P < 0.01 P < 0.001…”

“…Previous studies also suggested that a high fraction of the genome is associated with startle-induced locomotion (5). Only three of the candidate genes affecting startle-induced locomotion had been previously linked to effects on locomotor behavior (19)(20)(21). Although many of the candidate genes affect nervous system development and function, others are in known genes that have not been found to affect locomotion, or indeed any behavior, in computationally predicted genes or in regions of the genome with no nearby annotated gene.…”

“…Adult brains from female flies were dissected and processed for immunohistochemistry with a mouse monoclonal antifasciclin 2 antibody to visualize mushroom body ␣ and ␤ lobes and the ellipsoid body, as described previously (21). The immunostaining was documented by using an Olympus BX61 epifluorescence microscope equipped with a DP70 digital camera controlled with analySIS FIVE software.…”

Genetic architecture of quantitative traits in mice, flies, and humans

“…Mutations in the developmental loci neuralized, Semaphorin-5c, and Calreticulin affect numbers of sensory bristles as well as olfactory behavior (Sambandan et al 2006;Rollmann et al 2007). Mutations in the intergenic region between Tre1 (which affects transepithelial migration of germ cells) and Tre (a trehalose taste receptor also known as Gr5a) affect not only trehalose sensitivity, but also life span and resistance to starvation and heat stress .…”