The retinal fate of Xenopus cleavage stage progenitors is dependent upon blastomere position and competence: studies of normal and regulated clones.

The clonal origin of the stage 43-44 Xenopus retina from cleavage stage precursors was quantitatively assessed with lineage tracing techniques. The retina descends from a specific subset of those blastomeres that form forebrain. The most animal dorsal midline cell (D1.1.1) produced about half of the retina, the three other dorsal ipsilateral blastomeres each produce about an eighth of the retina, and the four contralateral dorsal blastomeres and an ipsilateral ventral-animal cell together produce the remaining eighth of the retina. There was no significant spatial segregation of the clones derived from different progenitors in either the anterior-posterior or dorsal-ventral axes of the retina and no boundaries between clones were observed. Instead, the clones intermixed to form multiple radial subclones that were equivalent to those demonstrated by marking optic vesicle progenitor cells (Holt et al., 1988; Wetts and Fraser, 1988). This mosaic pattern was initiated by the beginning of gastrulation, advanced in the neural plate, and virtually complete in the optic vesicle. At optic vesicle stages cell movement within subclones was restricted, resulting in the formation of lineally related columns of cells in the mature retina. To determine if the blastomere progenitors are determined to produce these retinal lineage patterns, the major retinal progenitor (D1.1.1) was deleted bilaterally. About 60% of the tadpoles developed normal-appearing eyes; of these the retinas in two-thirds were normal in size and the rest were smaller. The blastomeres surrounding the deleted D1.1.1 progenitors changed their contributions to retina in different ways to effect a complete or partial restoration. Ventral blastomeres, which normally contribute mainly to the tail, produced substantial amounts of the retina while dorsal blastomeres, which normally contribute mainly to the head, decreased their contribution to the retina. To determine whether these changes in retinal lineage were due to changes in blastomere position after the surgery, various other blastomeres were deleted prior to lineage mapping. Dorsal-animal blastomeres took over the retinal fate of their dorsal-vegetal neighbors after those neighbors were deleted, but did not change fate after the deletion of their ventral-animal neighbors. This result suggests that dorsal-animal blastomeres change positional values in only one direction (dorsal to vegetal) after neighbor cell deletion, and that retinal fate is dictated by blastomere position. To test this hypothesis directly, different ventral and vegetal blastomeres, which normally do not produce retina, were transplanted to the position of D1.1.1.(ABSTRACT TRUNCATED AT 400 WORDS)