An Allele of Sequoia Dominantly Enhances a Trio Mutant Phenotype to Influence Drosophila Larval Behavior

Abstract

The transition of Drosophila third instar larvae from feeding, photo-phobic foragers to non-feeding, photo-neutral wanderers is a classic behavioral switch that precedes pupariation. The neuronal network responsible for this behavior has recently begun to be defined. Previous genetic analyses have identified signaling components for food and light sensory inputs and neuropeptide hormonal outputs as being critical for the forager to wanderer transition. Trio is a Rho-Guanine Nucleotide Exchange Factor integrated into a variety of signaling networks including those governing axon pathfinding in early development. Sequoia is a pan-neuronally expressed zinc-finger transcription factor that governs dendrite and axon outgrowth. Using pre-pupal lethality as an endpoint, we have screened for dominant second-site enhancers of a weakly lethal trio mutant background. In these screens, an allele of sequoia has been identified. While these mutants have no obvious disruption of embryonic central nervous system architecture and survive to third instar larvae similar to controls, they retain forager behavior and thus fail to pupariate at high frequency.

The function of Drosophila larval class IV dendritic arborization sensory neurons in the larval-pupal transition is separable from their function in mechanical nociception responses

“…We had previously shown that the seq 9 . 17 allele acted as a dominant enhancer of a sensitized trio mutant background, blocking the larval-pupal transition [ 15 ]. This observation has allowed us to identify the tissue responsible for this phenotype by rescuing pupation via driving trio in various tissue-specific patterns with the GAL4-UAS system [ 29 ].…”

“…In contrast, our simple trio mutant genotype ( trio M89 / trio P1 ) did not show a reduction in dendrite density nor an alteration in dendritic branch frequency ( Fig 3A and 3B ) presumably because we utilized the trio hypomorphic allele trio P1 . This hypomorphic allele was originally used to provide a sensitized genetic background in our screen for dominant second-site modifiers of the trio mutant phenotype [ 15 ]. The increased dendritic branch frequency observed in the larvae that generally failed to undergo the larval-pupal transition ( seq 9 .…”

“…seq 9 . 17 and trio M89 were generated in our laboratory and have been described previously [ 15 , 23 ].…”

“…Previous work from our laboratory identified a dosage-sensitive genetic interaction between the genes encoding the Rho-GEF Trio and the zinc-finger transcription factor Sequoia that interfered with the larval-pupal transition [ 15 ]. In genetic terms, we reported that a heterozygous mutation in sequoia ( seq ) dominantly enhanced the trio mutant phenotype, as animals simply heterozygous mutant for seq or simply homozygous mutant for trio were well-represented as pupae, but animals both heterozygous mutant for seq and homozygous mutant for trio were dramatically under-represented as pupae.…”

“…However, those heterozygous seq , homozygous trio mutant animals were fully represented as third-instar larvae. Those larvae failed to pupate because they failed to undergo the transition from forager larvae to wanderer larvae [ 15 ].…”