Primate-like retinotectal decussation in an echolocating megabat, Rousettus aegyptiacus

The current study was designed to reveal the retinotectal pathway in the brain of the echolocating megabat Rousettus aegyptiacus. The retinotectal pathway of other species of megabats shows the primate-like pattern of decussation in the retina; however, it has been reported that the echolocating Rousettus did not share this feature. To test this prior result we injected fluorescent dextran tract tracers into the right (fluororuby) and left (fluoroemerald) superior colliculi of three adult Rousettus. After a 2-week survival period the animals were killed, fixed via transcardial perfusion, and the retinas whole mounted and examined under fluorescent excitation to reveal the pattern of retrograde transport. Red and green labeled retinotectal ganglion cells were found in side-by-side patches on either side of a vertical decussation line in the temporal retina of all six retinas. The Rousettus examined thus exhibited the same pattern of retinal decussation as reported previously for other megabats and primates, but unlike that seen in other mammals. The current result indicates that the prior study appears to have suffered technical problems leading to an incorrect conclusion. The results of our study indicate that, as may be expected, all megabats share the derived retinotectal pathway once thought to be the exclusive domain of primates. The current study provides additional support for the diphyletic origin of the Chiroptera and aligns the megabats phylogenetically as a sister group to primates.