Novel functions for Dorsocross in epithelial morphogenesis in the beetle Tribolium castaneum

Abstract

Epithelial morphogenesis, the progressive restructuring of tissue sheets, is fundamental to embryogenesis. In insects, not only embryonic tissues but also extraembryonic (EE) epithelia play a crucial role in shaping the embryo. In Drosophila, the T-box transcription factor Dorsocross (Doc) is essential for EE tissue maintenance and therefore embryo survival. However, Drosophila possesses a single amnioserosa, whereas most insects have a distinct amnion and serosa. How does this derived situation compare with Doc function in the ancestral context of two EE epithelia? Here, we investigate the Doc orthologue in the red flour beetle, Tribolium castaneum, which is an excellent model for EE tissue complement and for functional, fluorescent live imaging approaches. Surprisingly, we find that Tc-Doc controls all major events in Tribolium EE morphogenesis without affecting EE tissue specification or maintenance. These macroevolutionary changes in function between Tribolium and Drosophila are accompanied by regulatory network changes, where BMP signaling and possibly the transcription factor Hindsight are downstream mediators. We propose that the ancestral role of Doc was to control morphogenesis and discuss how Tc-Doc could provide spatial precision for remodeling the amnion-serosa border.

Amniotic ectoderm expansion occurs via distinct modes and requires SMAD5-mediated signalling

“…Animals classified as amniotes, including reptiles, birds and mammals, develop extra-embryonic tissues and organs that perform respiration-related, nutritional and protective functions during embryogenesis: the chorion, yolk sac, amnion and allantois. Alongside their supportive role, extra-embryonic tissues take part in the development of the early embryo proper by providing signalling cues and spatial continuity (Horn and Panfilio, 2016;Mesnard and Constam, 2010;Shao et al, 2016). The amnion is the innermost extra-embryonic tissue that delineates the fluid-filled amniotic cavity, providing space for the embryo to move freely.…”

Functional evolution of a morphogenetic gradient

“…While the identity of regulatory factors of the positive feedback circuit may be evolutionarily labile (in Tribolium doc and hnt appear to be dispensable for amnion specification [Horn and Panfilio, 2016]), we propose that the mechanism of amnion specification through feedback-driven BMP signaling dynamics applies to a wide range of insects, because in Tribolium the pMad domain also gradually shifts to become elevated in the presumptive amnion during early gastrula stages (van der Zee et al, 2006; Nunes da Fonseca et al, 2008). The principle of evolving tissue complexity through changes in positive feedback circuits of morphogen gradients has not yet been documented in other developmental contexts, but it may also apply to unrelated traits, such as eyespots on butterfly wings (Monteiro, 2015).…”

Unexpected mutual regulation underlies paralogue functional diversification and promotes maturation of a protective epithelial tissue

“…Consistent with this, we identified a homologue of the endocycle factor fizzy-related among DE genes upregulated by Tc-Zen1 (Table S1A) 33,34 . From known targets of Tc-Zen1, we also recovered Dorsocross and hindsight, involved in EEM formation 35 , and chitin synthase 1, required for production of the protective cuticle 10 . Additionally, we hypothesized that the slight offset whereby Tc-zen1 expression precedes Tc-zen2 is consistent with Tc-zen1 activating Tc-zen2.…”

“…Note that for Tc-zen2 more consistent results were obtained using SYBR Green Master Mix. "Relative abundance" was calculated for each sample as the ratio relative to a pooled template control with cDNA from all depicted samples (method as in 35 ). Samples were measured for the Tc-zen paralogues' wild type expression profiles (four biological replicates: Figs.…”