Control of pattern duplication in the retinotectal system of Xenopus. Suppression of duplication by eye-fragment interactions.

Recombining right nasal half-eyes with left temporal half-eyes at embryonic stage 32 in Xenopus produces a double or “twinned” pattern of functional connections between retina and midbrain optic tectum. The left temporal half-eye is reprogrammed such that it projects to the tectum as a mirror-image duplicate pattern of the nasal right half-eye; both half-eyes project across the entire tectum. However, recombining a right nasal half-eye (in situ) with a right dorsal half-eye (grafted into the temporal position; N_RD_R eye) produces a single normal retinotectal projection. Where interactions between N_R and T_L involve both axial reprogramming and duplication of N_R positional values in T_L, N_RD_R interactions involve axial reprogramming in D_R without duplication of N_R values. In a second approach to interactions which suppress pattern duplication, both nasal and temporal one-third-sized eye fragments form approximate “NN” or “TT” duplicate pattern maps, respectively, when either is allowed to round up and form a whole eye. Allowing nasal and temporal thirds to permanently fuse (after removal of a one-third-sized vertical center strip of retina at stage 32) produces a normal projection; allowing the nasal and temporal thirds to interact (fuse) for 35–40 hr, followed by removal of one or the other third, suppresses pattern duplication (produces normal maps) in the remaining third in a majority of cases. Allowing the thirds to interact for 18–30 hr before removal of the temporal third produces a majority result of partial duplication in the remaining nasal third. Partial duplicates are apparent spatial intermediates with regard to interactions which suppress duplication in either fragment type.