Caudal Regulates the Spatiotemporal Dynamics of Pair-Rule Waves in Tribolium

Abstract

In the short-germ beetle Tribolium castaneum, waves of pair-rule gene expression propagate from the posterior end of the embryo towards the anterior and eventually freeze into stable stripes, partitioning the anterior-posterior axis into segments. Similar waves in vertebrates are assumed to arise due to the modulation of a molecular clock by a posterior-to-anterior frequency gradient. However, neither a molecular candidate nor a functional role has been identified to date for such a frequency gradient, either in vertebrates or elsewhere. Here we provide evidence that the posterior gradient of Tc-caudal expression regulates the oscillation frequency of pair-rule gene expression in Tribolium. We show this by analyzing the spatiotemporal dynamics of Tc-even-skipped expression in strong and mild knockdown of Tc-caudal, and by correlating the extension, level and slope of the Tc-caudal expression gradient to the spatiotemporal dynamics of Tc-even-skipped expression in wild type as well as in different RNAi knockdowns of Tc-caudal regulators. Further, we show that besides its absolute importance for stripe generation in the static phase of the Tribolium blastoderm, a frequency gradient might serve as a buffer against noise during axis elongation phase in Tribolium as well as vertebrates. Our results highlight the role of frequency gradients in pattern formation.

The Wnt and Delta-Notch signalling pathways interact to direct pair-rule gene expression via caudal during segment addition in the spider Parasteatoda tepidariorum

“…Firstly, there is evidence that hunchback (hb) and Distal-less (Dll) perform gap gene-like functions during formation of the prosomal segments of spiders Pechmann et al, 2011). Secondly, the orthologues of Drosophila pair-rule genes are also expressed in the SAZ and segments of short-germ arthropod embryos, which is consistent with roles in segmentation in these animals, although it is likely that these genes were expressed in single rather than double segmental periodicity ancestrally in arthropods (Frasch et al, 1987;Sommer and Tautz, 1993;Patel et al, 1994;Damen et al, 2000Damen et al, , 2005Davis et al, 2001;Dearden et al, 2002;Chipman et al, 2004b;Schoppmeier and Damen, 2005b;Choe et al, 2006;Damen, 2007;Mito et al, 2007;Chipman and Akam, 2008;Janssen et al, 2011;Sarrazin et al, 2012;Brena and Akam, 2013;Green and Akam, 2013). Finally, the expression and function of segment polarity genes are highly similar across arthropods (Damen, 2002;Hughes and Kaufman, 2002).…”

“…Our data provide further evidence for differences in the regulation of prosomal and opisthosomal segments in spiders, whereby gap and pair-rule gene orthologues respectively direct the formation of segments in these tagmata (Damen et al, 2000(Damen et al, , 2005Pechmann et al, 2009Pechmann et al, , 2011Schwager et al, 2009). Interestingly, this also indicates that the roles of eve and run-1 in spiders is restricted to formation of more posterior segments than, for example, in the insects D. melanogaster and Tribolium, and the myriapods Strigamia maritima and Glomeris marginata, in which eve is expressed in a segmental pattern in four segments more anterior to O1/T2 (Frasch et al, 1987;Brown et al, 1997;Janssen et al, 2011;Brena and Akam, 2013). Moreover, while there is a hierarchy of primary and secondary pair-rule gene orthologues in arthropods (Damen, 2007), our study further exemplifies that the register of the expression of these genes has diverged among these animals: expression of eve and run overlaps in forming segments in Glomeris as in Parasteatoda, but they are out of phase in Strigamia and Drosophila (Green and Akam, 2013).…”

“…Unlike in Parasteatoda, however, cad represses Dl in the Periplaneta SAZ (Chesebro et al, 2013). Furthermore, it is likely that Wnt1 is required for cad expression during posterior development in Tribolium as in Gryllus (Shinmyo et al, 2005;McGregor, 2006;Oberhofer et al, 2014) and a recent study has shown that the graded expression of Tc-cad is required for the dynamic expression of Tc-eve (El-Sherif et al, 2014). This suggests that although there are differences in the regulation of segment addition among short-germ arthropods, the regulation of eve by cad, probably directed by upstream signalling pathways, may have been used ancestrally in arthropods.…”

“…Furthermore, it is not known how the expression of putatively downstream segmentation genes, such as even-skipped (eve), is regulated compared with other arthropods. In Drosophila, eve is regulated by a combination of maternal and gap factors, whereas in Tribolium, eve expression is regulated by cad and other pairrule genes that are likely to operate in a circuit (Small et al, 1991(Small et al, , 1992Fujioka et al, 1996;Choe et al, 2006;El-Sherif et al, 2014), probably downstream of Wnt signalling (Oberhofer et al, 2014). Therefore, to better understand the regulation of segment formation in short-germ arthropods, we further investigated the regulatory interactions between Pt-Dl, Pt-Notch (Pt-N), Pt-Wnt8 and Pt-cad and studied the expression and regulation of the Parasteatoda orthologues of the Drosophila pair-rule genes eve (Pt-eve) and runt (Pt-run-1) during early embryogenesis in this spider.…”

Speed regulation of genetic cascades allows for evolvability in the body plan specification of insects

“…For example, a molecular clock mediates stripes of gene expression that delimit vertebrate somites (1)(2)(3), segments in short-germ arthropods (4)(5)(6)(7)(8)(9)(10), and lateral roots in plants (11,12). Aperiodic sequential activation of genes regulates the spatial patterning of Drosophila neuroblasts (13,14) and the vertebrate neural tube (15).…”