What causes the aphid 28S rRNA to lack the hidden break?

In order to determine why the aphid 28S rRNA lacks the hidden break otherwise found in insects, the structure of the region of the aphid ribosomal DNA (rDNA) corresponding to the gap region, which in other insect rDNA transcripts is excised posttranscriptionally, was studied. Sequence comparison suggested that, in contradistinction to what is found in rDNA transcripts of other insects, a stem-loop structure formed in this region of the aphid rDNA transcript is not AU-rich. Nor did the loop of the aphid molecule contain the UAAU tract that can be a signal for the introduction of the hidden break, suggesting that in this particular region the aphid 28S rRNA resembles 28S rRNAs of deuterostomes, which do not contain the hidden break.