Three types of serotonin-containing amacrine cells in tadpole retina have distinct clonal origins

In the Xenopus tadpole there are three different serotonin-containing amacrine cells: large, brightly fluorescent (LB), and small, dimly fluorescent (SD) cells in the inner nuclear layer and displaced (DIS) cells in the ganglion cell layer. To reveal the potential roles of regional cues and lineage factors in their determination, quantitative maps were made of the spatial distribution and blastomere origin of each cell type. LB and SD cells were evenly distributed across the four retinal quadrants, arguing against a hypothesis that these cells are induced differentially by quadrant-specific cues. Blastomere progenitors of the 32-cell embryo are biased to produce only subsets of serotonin amacrine cells: 1) all nine progenitors of one retina produced some SD cells, but only eight produced LB, and only five produced DIS cells; and 2) there are overlapping but distinct subsets of blastomere progenitors for each serotonin subtype. This bias is not simply a reflection of the size of a clone in the retina; significant quantitative differences were observed between the proportion of serotonin progeny and the proportion of the entire retina produced by six of the nine retinal progenitors. This bias also is not simply a reflection of the spatial distribution of a blastomere clone in the retina; the number of LB descendants in each retinal quadrant was statistically different from its progenitor's total contribution to the quadrant. These results indicate that the development of the three different serotonin-containing amacrine cells in the retina is biased by membership in specific blastomere clones.