Journal of Neuroscience, Vol 13, 208-228, Copyright © 1993 by Society for Neuroscience
Rapid evolution of the visual system: a cellular assay of the retina and dorsal lateral geniculate nucleus of the Spanish wildcat and the domestic cat
RW Williams, C Cavada and F Reinoso-Suarez
Department of Anatomy and Neurobiology, College of Medicine, University of Tennessee, Memphis 38163.
The large Spanish wildcat, Felis silvestris tartessia, has retained
features of the Pleistocene ancestor of the modern domestic cat, F. catus.
To gauge the direction and magnitude of short-term evolutionary change in
this lineage, we have compared the retina, the optic nerve, and the dorsal
lateral geniculate nucleus (LGN) of Spanish wildcats and their domestic
relatives. Retinas of the two species have the same area. However,
densities of cone photoreceptors are higher in wildcat-- over 100% higher
in the area centralis--whereas rod densities are as high, or higher, in the
domestic lineage. Densities of retinal ganglion cells are typically 20-100%
higher across the wildcat retina, and the total ganglion cell population is
nearly 70% higher than in the domestic cat. These differences are confined
to the populations of beta and gamma retinal ganglion cells. In contrast,
the population of alpha cells is almost precisely the same in both species.
The wildcat LGN is much larger than that of the domestic cat and contains
approximately 50% more neurons. However, cell size does not differ
appreciably in either the retina or LGN of these species. The differences
in total numbers of ganglion cells and LGN neurons correspond neatly to the
overall decline in brain size in the domestic lineage and to allometric
predictions based on average species differences in body size. We suggest
that an increase in the severity of naturally occurring cell death is the
most plausible mechanism that can account for the rapid evolutionary
reduction in cell populations in this feline lineage.