It is an especially exciting time to be a developmental biologist. New tools are allowing us to ask questions about gene function and developmental mechanism in an ever-expanding array of species. With CRISPR/Cas9, if we can rear a species in the lab we can test the role of specific genes. In addition to new functional tools, a new approach called single cell sequencing or scSeq allows us to obtain gene expression profiles of all the cell types in a tissue in one shot. This approach works well in the Drosophila visual system, in both developing and adult brains, and we are now using this approach to compare neural cell types within and across species. This allows us to ask questions about the evolution of neural complexity, such as where do new types of neurons come from? What kinds of genetic changes produce novel cell types? How do new neurons “plug in” to existing circuitry in a way that is useful to the animal? Using a comparative approach is allowing us to understand the rules that build and modify neural systems.
Butterflies: Butterflies have improved color vision in large part due to their more complex retinas, which are made up of three stochastically distributed types of unit eyes (ommatidia) instead of two (as in most other insects). Our work has shown that this evolutionary innovation relies on the recruitment of two R7-type photoreceptors during development. Ongoing work is aimed at understanding how the second R7 photoreceptor is recruited and how the brain accommodates the additional color input.
Mosquitoes: we are interested in the genes that control mosquito retinal development, and how the brain responds to different distributions of photoreceptors and ommatidial types across the eye.
House Flies (Musca domestica): Male house flies have a region of their eye dedicated to just one thing: catching sight of a female and locking on during a fast-paced pursuit. This region of the eye, sometimes referred to as the “Love Spot”, has sacrificed color discrimination for improved motion vision, with changes in morphology (shape), Rhodopsin expression, and axonal wiring, sending additional input to the motion vision circuits of the brain. In fact, species in over fifteen families of Diptera have similar male-specific eye regions. We are working to understand the genetic network used in development to produce this complex feature and how it evolved. This is a great test case for the use of single cell sequencing in Evo Devo: we can compare developing male vs. female eyes and look for novel cell types and changes to existing cells. And, because of decades of work in Drosophila, we know a huge amount about cell fate specification in retina development, making it an ideal place to understand neural cell type evolution.
Further reading: we have written several reviews on insect eye Evo Devo and the evolution of neural diversity where you can find more detail.
Perry, M.W., Konstantinides, N., Pinto-Teixeira, F., and Desplan, C.
2017. Generation and evolution of neural cell types and circuits: insights from the Drosophila visual system.
Annual Reviews of Genetics, 51: 501-527. PMID: 28961025
Wernet, M.F, Perry, M.W., and C. Desplan.
2015. The evolutionary diversity of insect retinal mosaics: common design principles and emerging molecular logic.Trends Genetics, 31(6): 316-28. PMID: 26025917