Error rate of axons at the owl's optic chiasm.
BACKGROUND
In owls, the visual pathways from the retina are totally crossed. Attempts to find ganglion cells with uncrossed axons have failed consistently, when retrograde labeling with HRP is used for their identification. In the present investigation we have used retrograde fluorescent tracers of complementary colour in each optic tectum to demonstrate a tiny population of ipsilaterally-projecting retinal ganglion cells in the owl.


METHODS
We studied two species, one from each of the two important owl families: the Southern boobook owl, Ninox boobook: (Family Strigidae); and the Barn Owl, Tyto alba: (Family Tytonidae).


RESULTS
The small numbers, random distribution and heterogeneity of the mis-projecting ganglion cells, taken together, argue against a functional role for them. Instead, they appear to be the result of developmental errors in the specification of laterality. At a number of different eccentricities and ganglion cell densities, the error rate was roughly a constant fraction of the neurons involved, at around 10-4 for the tytonid owl (lacking a fovea) and around 10-5 for the strigid owl (which has a fovea and a higher overall density of ganglion cells).


CONCLUSIONS
These values are close to the error rates of replicating enzymes, such as nucleic acid polymerases. The evolution of a higher retinal ganglion cell density and a fovea in the Strigidae appears to be accompanied by an improvement in the error rate.
Pettigrew
John D
JD
Vision Touch and Hearing Research Centre, The University of Queensland, Australia.
Sand
Olaf
O
Gynther
Ian
I
D000818:Animals; D001369:Axons / Q000502:physiology; D005456:Fluorescent Dyes; D009897:Optic Chiasm / Q000033:anatomy & histology* / Q000502:physiology; D012107:Research Design; D012165:Retinal Ganglion Cells / Q000502:physiology*; D020305:Strigiformes / Q000033:anatomy & histology* / Q000502:physiology; D014795:Visual Pathways / Q000033:anatomy & histology* / Q000502:physiology
D005456:Fluorescent Dyes