Analyses of interactions among pair-rule genes and the gap gene Krüppel in Bombyx segmentation

Abstract

In the short-germ insect Tribolium, a pair-rule gene circuit consisting of the Tribolium homologs of even-skipped, runt, and odd-skipped (Tc-eve, Tc-run and Tc-odd, respectively) has been implicated in segment formation. To examine the application of the model to other taxa, I studied the expression and function of pair-rule genes in Bombyx mori, together with a Bombyx homolog of Krüppel (Bm-Kr), a known gap gene. Knockdown embryos of Bombyx homologs of eve, run and odd (Bm-eve, Bm-run and Bm-odd) exhibited asegmental phenotypes similar to those of Tribolium knockdowns. However, pair-rule gene interactions were similar to those of both Tribolium and Drosophila, which, different from Tribolium, shows a hierarchical segmentation mode. Additionally, the Bm-odd expression pattern shares characteristics with those of Drosophila pair-rule genes that receive upstream regulatory input. On the other hand, Bm-Kr knockdowns exhibited a large posterior segment deletion as observed in short-germ insects. However, a detailed analysis of these embryos indicated that Bm-Kr modulates expression of pair-rule genes like in Drosophila, although the mechanisms appear to be different. This suggested hierarchical interactions between Bm-Kr and pair-rule genes. Based on these results, I concluded that the pair-rule gene circuit model that describes Tribolium development is not applicable to Bombyx.

Arthropod segmentation

“…The main evidence for this is that knocking down primary pair-rule genes can block segmentation and truncate the body axis, as has been found in Tribolium (Choe et al, 2006), the silkmoth Bombyx (Nakao, 2015), a second beetle species Dermestes (Xiang et al, 2017) and the hemipteran bug Oncopeltus (Auman and Chipman, 2018;Liu and Kaufman, 2005). It can also cause the expression of other primary pair-rule genes to become aperiodic (Choe et al, 2006;Nakao, 2015), suggesting that at least some of the oscillations are mutually interdependent. This observation distinguishes these knockdowns from those of downstream patterning genes, which may also yield asegmental phenotypes but do not perturb expression dynamics in the SAZ (Choe and Brown, 2007;Farzana and Brown, 2008).…”

“…In contrast to the earlier network, which drives dynamic expression, this later one behaves like a multistable switch, 'locking in' specific segment-polarity fates (Clark, 2017). Interestingly, different primary pair-rule genes undergo frequency doubling in each of Drosophila, Bombyx, Tribolium and Nasonia (Choe et al, 2006;Clark and Akam, 2016;Nakao, 2015;Rosenberg et al, 2014), contrasting with the conserved expression of the segment-polarity and secondary pair-rule genes.…”

“…Finally, the specific regulatory interactions proposed for the circuit seem unlikely. In holometabolous insects (and also Strigamia), eve, runt and odd are expressed sequentially within each pattern repeat (Choe et al, 2006;Clark, 2017;Green and Akam, 2013;Nakao, 2015;Rosenberg et al, 2014). In both Tribolium and Bombyx, Eve is necessary for runt expression, and Runt is necessary for odd expression (Choe et al, 2006;Nakao, 2015).…”

“…(B) Notch signalling might indirectly synchronise intracellular oscillations of eve, runt and odd across cells, by acting through hairy. This figure shows a hypothetical regulatory network, which synthesises genetic interactions documented from various different arthropod species (Clark, 2017;Eriksson et al, 2013;Nakao, 2015;Pueyo et al, 2008;Stollewerk et al, 2003). The left half of the network ('oscillator 1') would synchronise oscillations of hairy across neighbouring cells, by coupling hairy expression to Notch signalling (1).…”

Dynamic patterning by theDrosophilapair-rule network reconciles long-germ and short-germ segmentation

“…First, because it seems likely that the role of the gap system is effectively to mimic the output and dynamics of a segmentation clock, there is no reason why gap-based patterning and clockbased patterning shouldn't work partially redundantly to pattern a given set of stripes during the transition from one mechanism to the other. Indeed, it is possible that the two mechanisms coexist today in insects such as Nasonia and Bombyx, which exhibit some evidence of both patterning modes [129][130][131].…”