Conserved role of the Vsx genes supports a monophyletic origin for bilaterian visual systems.
BACKGROUND
Components of the genetic network specifying eye development are conserved from flies to humans, but homologies between individual neuronal cell types have been difficult to identify. In the vertebrate retina, the homeodomain-containing transcription factor Chx10 is required for both progenitor cell proliferation and the development of the bipolar interneurons, which transmit visual signals from photoreceptors to ganglion cells.


RESULTS
We show that dVsx1 and dVsx2, the two Drosophila homologs of Chx10, play a conserved role in visual-system development. DVSX1 is expressed in optic-lobe progenitor cells, and, in dVsx1 mutants, progenitor cell proliferation is defective, leading to hypocellularity. Subsequently, DVSX1 and DVSX2 are coexpressed in a subset of neurons in the medulla, including the transmedullary neurons that transmit visual information from photoreceptors to deeper layers of the visual system. In dVsx mutant adults, the optic lobe is reduced in size, and the medulla is small or absent. These results suggest that the progenitor cells and photoreceptor target neurons of the vertebrate retina and fly optic lobe are ancestrally related. Genetic and functional homology may extend to the neurons directly downstream of the bipolar and transmedullary neurons, the vertebrate ganglion cells and fly lobula projection neurons. Both cell types project to visual-processing centers in the brain, and both sequentially express the Math5/ATO and Brn3b/ACJ6 transcription factors during their development.


CONCLUSIONS
Our findings support a monophyletic origin for the bilaterian visual system in which the last common ancestor of flies and vertebrates already contained a primordial visual system with photoreceptors, interneurons, and projection neurons.
Erclik
Ted
T
Program in Developmental and Stem Cell Biology, Research Institute, The Hospital for Sick Children, TMDT Building, 101 College Street, Toronto, Ontario M5G 1L7, Canada.
Hartenstein
Volker
V
Lipshitz
Howard D
HD
McInnes
Roderick R
RR
D000818:Animals; D002454:Cell Differentiation / Q000235:genetics; D049109:Cell Proliferation; D004330:Drosophila / Q000166:cytology / Q000196:embryology / Q000235:genetics*; D004625:Embryo, Nonmammalian / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D047108:Embryonic Development / Q000235:genetics; D005123:Eye / Q000196:embryology; D007814:Larva / Q000737:chemistry / Q000166:cytology / Q000235:genetics; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000032:analysis / Q000235:genetics / Q000502:physiology*; D046569:Neuroepithelial Cells / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D009899:Optic Lobe, Nonmammalian / Q000737:chemistry / Q000166:cytology / Q000196:embryology; D017956:Photoreceptor Cells, Invertebrate / Q000378:metabolism; D010802:Phylogeny; D012160:Retina / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013234:Stem Cells / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D014785:Vision, Ocular / Q000235:genetics*
D009419:Nerve Tissue Proteins; D014157:Transcription Factors