Same rule, different genes: <i>Blimp1</i> is a pair-rule gene in the milkweed bug <i>Oncopeltus fasciatus</i>

Morphological features of organismal body plans are often highly conserved within large taxa. For example, segmentation is a shared and defining feature of all insects. Screens in <i>Drosophila</i> identified genes responsible for the development of body segments, including the "pair-rule" genes (PRGs), which subdivide embryos into double-segment units in a previously unexpected pre-patterning step. Here we show that the milkweed bug <i>Oncopeltus fasciatus</i> also uses a pair rule for embryo subdivision but <i>Oncopeltus</i> employs different genes for this process. We identified the gene <i>Blimp1</i> as an <i>Oncopeltus</i> PRG based on its expression pattern, tested its function with RNA interference and CRISPR-Cas9, and generated the first PR mutant in this species. Although it does not have PR function in <i>Drosophila</i>, like <i>Drosophila</i> PRGs, <i>Blimp1</i> encodes a transcription factor required for embryonic viability. Thus, pair-rule subdivision of the insect body plan is more highly conserved than the factors mediating this process, suggesting a developmental constraint on this pre-patterning step.