Marsupial retinocollicular system shows differential expression of messenger RNA encoding EphA receptors and their...

The protracted development of the wallaby (Macropus eugenii) has allowed study of messenger RNAs encoding Eph receptors EphA3 and EphA7 and ligands ephrin-A2 and -A5 in the retina and superior colliculus at intervals throughout the development of the retinocollicular projection: from birth, before retinal innervation, to postnatal day 95, when the projection is mature. Reverse transcription-polymerase chain reaction showed messenger RNAs for both receptors and ligands were expressed at all ages. EphA7 was expressed more highly in the rostral superior colliculus. Ephrin-A2 and -A5 were expressed more highly in the caudal colliculus. EphA3 was expressed in a complementary manner, more highly in temporal than in nasal retina. There are higher levels of expression of the ligands when the projection is only coarsely topographically organised. This suggests a role for them and their receptor EphA3 in this stage, by repulsive interactions which restrict temporal axons to rostral superior colliculus. This is the first account in a marsupial mammal of the appearance of this molecular family, substantiating its ubiquitous role in topographically organised neuronal connections. Nevertheless, expression is not the same as in the mouse, suggesting differences in the details of topographic coding between species.