Pattern formation and development of the visual system in Drosophila
Our laboratory focuses on two major developmental questions: The evolution of early embryonic development and the establishment of retinal and brain circuitry that underlies color vision. These two distinct systems represent paradigms for understanding how pattern formation is genetically controlled.
- The first project is an Evolution and Development (Evo-Devo) approach to the earliest steps of insect development, the formation of the antero-posterior embryonic axis. In flies, the morphogenetic gradient of the Bicoid protein is essential for anterior development. However, bicoid is not conserved and is a newly evolved gene that has taken over the ancestral function performed by another more general system in other insects. We are using the wasp Nasonia as an alternate model system to study how a very distant species patterns its axis. Nasonia is a hymenopteran that diverged from dipterans over 200MY ago and does not have a bicoid gene. It exhibits a long germband mode of development that is very similar to that of flies: All segments form at the same time and the embryo occupies the entire length of the egg. In contrast, short germband insects such as Tribolium only generate a few head and thoracic segments that form in a small posterior part of the egg: all other segments are added later in a growth zone. This similarity with flies allows us to directly compare expression patterns with those of flies. Furthermore, the sequence of the Nasonia genome was recently completed and there are many early developmental mutants. Finally, parental RNAi is a very powerful technique to knock down gene function. We have shown that Nasonia, like flies, utilizes an anterior morphogenetic center to generate the patterning information required for the formation of all segments: flies have bcd mRNA localized at the anterior of the egg while Nasonia has localized otd mRNA, at both poles. This represents a convergence of two distinct strategies to create the needed patterning information required for long germband development. Our ongoing investigations attempt to understand the entire segmentation pattern in Nasonia, with the goal of reaching the same level of mechanistic detail as Drosophila in order to compare the two systems.
- The other system that we study is the determination of the retinal mosaic and neural network that underlie color vision in flies. Color vision is achieved through the comparison in the brain of inputs coming from photoreceptors containing photopigments (rhodopsins) with different wavelength specificity. Different rhodopsins are expressed in stochastic but mutually exclusive patterns in the compound eye of Drosophila as in cones of the vertebrate retina. This implies that there is a process for choosing a given rhodopsin gene and to transcriptionally repress all others. From this choice, another mechanism must then inform the brain of its connection to a photoreceptor with a specific color sensitivity. We have used molecular and genetic approaches to identify most of the functions involved in the establishment of the retinal mosaic and the signaling pathways responsible for the elaboration of this sensory system. We are also imaging the target neurons of photoreceptors in the optic lobes and we are manipulating (ablation or silencing) these neurons to obtain a functional map the neural network underlying color vision. We have developed a color behavior assay to assay how changes in the retina or optic lobe neurons affect the visual process. We hope to understand how color vision has evolved and how brain processing allows the fly to detect its color environment.
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