Rhombomeres in the developing hindbrain are characterized by a unique pattern of gene expression. As shown in the schematic, gene expression in mature rhombomeres r1 through r7 follows a segmental pattern.
For example, odd numbered r3 and r5 express Krox20. r4 is the only one that expresses Hoxb1, and r2 expresses Hoxa2, but no other Hox genes.
A genetic technique allows permanent labeling of all cells that have ever expressed a particular gene. Whether a cell continues to express the gene, or expressed it sometime in the past, but no longer expresses it, the technique allows us to visualize such cells in the tissue.
Using this technique, researchers labelled cells that at one point or another turned on Krox20 transcription factor expression. Surprisingly, at a time when segment borders and identity are already well established, some of the labelled cells turned up in even numbered rhombomeres, where, as shown in the figure, Krox20 expression is no longer present.
What are possible interpretations of these data?
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