I have three C. elegans lines with the same transcriptional RFP reporter inserted in the same site via MosSCI. Two of them show an expression pattern that match what I would expect based WormBase, gene function, etc. The third line shows the generally the same expression pattern in juvenile and adult worms, except that it is not expressed in the eggs -- they are completely dark while the other two lines produce bright red eggs. The expression pattern of all three lines is stable over generations and I have observed no other differences in phenotype.
The only explanation I can think of is a chance mutation in a transcription factor required for embryonic expression of this gene. Assuming this would affect the gene itself and not just the reporter, I would expect such a mutation to be lethal, but I am outcrossing the line to check reporter expression in the offspring.
Is there a more mundane explanation that I have overlooked?
Spur of the moment impression seems that maybe insertion of the reporter was slightly different in one of the lines, preventing its expression in the eggs due to a disrupted transcription factor binding site, or something altering epigenetics/nucleosome positioning in the eggs, or potentially splicing/alternative start codon, etc. Unless of course you've had the reporter insertion and surrounding sites sequenced to verify fidelity in all 3 lines. Or, a slightly different insertion could cause issues at the mRNA level, stability etc. Do you get the same level of transcripts for the reporter in the 3 lines (in the eggs)?
Either way, sounds interesting, like you've possibly found something that controls gene expression in different developmental stages when perturbed.
Erik Martin, thank you for the insights! A disruption in a transcription factor binding site of the reporter does sound most likely and better explains the phenotype than a background mutation and would match what I have seen crossing the line. I'd had set in my mind that the transgene must be the same in all of them since they were made at the same time, but it makes sense that some error could be introduced during insertion. How fortuitous! I will definitely have the insertion and surrounding region sequenced and should look at transcript levels in the eggs as well. Thank you! :)
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