Tc-knirps plays different roles in the specification of antennal and mandibular parasegment boundaries and is regulated by a pair-rule gene in the beetle Tribolium castaneum

Abstract

BackgroundThe Drosophila larval head is evolutionarily derived at the genetic and morphological level. In the beetle Tribolium castaneum, development of the larval head more closely resembles the ancestral arthropod condition. Unlike in Drosophila, a knirps homologue (Tc-kni) is required for development of the antennae and mandibles. However, published Tc-kni data are restricted to cuticle phenotypes and Tc-even-skipped and Tc-wingless stainings in knockdown embryos. Hence, it has remained unclear whether the entire antennal and mandibular segments depend on Tc-kni function, and whether the intervening intercalary segment is formed completely. We address these questions with a detailed examination of Tc-kni function.ResultsBy examining the expression of marker genes in RNAi embryos, we show that Tc-kni is required only for the formation of the posterior parts of the antennal and mandibular segments (i.e. the parasegmental boundaries). Moreover, we find that the role of Tc-kni is distinct in these segments: Tc-kni is required for the initiation of the antennal parasegment boundary, but only for the maintenance of the mandibular parasegmental boundary. Surprisingly, Tc-kni controls the timing of expression of the Hox gene Tc-labial in the intercalary segment, although this segment does form in the absence of Tc-kni function. Unexpectedly, we find that the pair-rule gene Tc-even-skipped helps set the posterior boundary of Tc-kni expression in the mandible. Using the mutant antennaless, a likely regulatory Null mutation at the Tc-kni locus, we provide evidence that our RNAi studies represent a Null situation.ConclusionsTc-kni is required for the initiation of the antennal and the maintenance of the mandibular parasegmental boundaries. Tc-kni is not required for specification of the anterior regions of these segments, nor the intervening intercalary segment, confirming that Tc-kni is not a canonical ‘gap-gene’. Our finding that a gap gene orthologue is regulated by a pair rule gene adds to the view that the segmentation gene hierarchies differ between Tribolium and Drosophila upstream of the pair rule gene level. In Tribolium, as in Drosophila, head and trunk segmentation gene networks cooperate to pattern the mandibular segment, albeit involving Tc-kni as novel component.

The insect central complex as model for heterochronic brain development—background, concepts, and tools

“…RNAi-mediated gene knockdown is strong and is either environmental or systemic. Hence, when dsRNA is injected into the hemolymph, the knockdown spreads to reach all cells of the injected animal and is even transmitted to the offspring of injected females (Brown et al 1999 ; Curtis et al 2001 ; Bucher et al 2002 ; Tomoyasu and Denell 2004 ; Miller et al 2012 ; Peel et al 2013 ). Mutant phenotypes were described for Tc - knirps , Tc - Distal - less , and Tc - sex combs reduced and in all these cases, RNAi phenocopied the null phenotype.…”

Enhanced genome assembly and a new official gene set for Tribolium castaneum

“…However, the main strength of the model system lies in its reverse genetics via RNAi. First, the RNAi response is very strong, reaching the null phenotype in those cases where a genetic mutant was available for comparison [25][26][27][28]. In addition, RNAi is environmental, i.e.…”

The iBeetle large-scale RNAi screen reveals gene functions for insect development and physiology

“…Finally, Tribolium research builds on an expanding transgenic toolkit 6 30 31 32 and a particularly strong systemic RNAi response. Knockdown phenotypes can be induced at all life stages through dsRNA injection into the body cavity, and the RNAi effect spreads throughout the animal and is transferred to the offspring, often phenocopying null mutants 33 34 35 36 .…”