Developmental coupling of larval and adult stages in a complex life cycle: insights from limb regeneration in the flour beetle, Tribolium castaneum

Alison K Lee,Christie C Sze,Elaine R Kim,Yuichiro Suzuki

doi:10.1186/2041-9139-4-20

Abstract

BackgroundA complex life cycle, such as complete metamorphosis, is a key innovation that can promote diversification of species. The evolution of a morphologically distinct larval stage is thought to have enabled insects to occupy broader ecological niches and become the most diverse metazoan taxon, yet the extent to which larval and adult morphologies can evolve independently remains unknown. Perturbation of larval limb regeneration allows us to generate larval legs and antennae with altered limb morphologies, which may be used to explore the developmental continuity that might exist between larval and adult appendages. In this study, we determined the roles of several appendage patterning transcription factors, abrupt (ab), dachshund (dac), Distal-less (Dll), and spineless (ss), in the red flour beetle, Tribolium castaneum, during larval appendage regeneration. The functions of these genes in regenerating and non-regenerating limbs were compared using RNA interference.ResultsDuring limb regeneration, dac and ss were necessary to re-pattern the same larval structures as those patterned during embryogenesis. Removal of these two genes led to larval appendage patterning defects that were carried over to the adult legs. Surprisingly, even though maternal knockdown of ab had minimal effects on limb allocation and patterning in the embryo, it was necessary for blastema growth, an earlier phase of regeneration. Finally, knockdown of Dll prevented the blastema-like bumps from re-differentiating into appendages.ConclusionsOur results suggest that, similar to vertebrates, the re-patterning phase of Tribolium larval limb regeneration relies on the same genes that are used during embryonic limb patterning. Thus, the re-patterning phase of regeneration is likely to be regulated by taxon-specific patterning mechanisms. Furthermore, Ab and Dll appear to play important roles during blastema proliferation and re-differentiation, respectively. Finally, our results show that continuity exists between larval and adult limb patterning, and that larval and adult leg morphologies may be developmentally coupled. Thus, the evolution of imaginal discs may have been a key step towards completely removing any developmental constraints that existed between larval and adult phenotypes.

Highlights

A complex life cycle, such as complete metamorphosis, is a key innovation that can promote diversification of species
We examined the consequence of genetic perturbations during larval appendage regeneration, to examine the degree of developmental continuity that might exist between larval and adult morphologies
We determined the phenotypes of embryonic appendages resulting from silencing Dac and Ab, and the effects of dac, Dll, ab and ss knockdowns during larval limb regeneration in Tribolium. We found that both Ab and Dll are necessary for limb regeneration prior to patterning, suggesting regeneration-specific functions of these genes

Summary

Introduction

A complex life cycle, such as complete metamorphosis, is a key innovation that can promote diversification of species. Complex life cycles, which contain two or more morphologically discrete postembryonic phases, are found ubiquitously among many metazoan taxa [1] Such complex life cycles can serve as adaptive strategies that allow different developmental stages to evolve independently to occupy larger ecological niches [1]. For example, imaginal cells embedded within the larval tissues replace much of the larval-specific cells to generate the adult structures [4,5] In other insects, such as beetles, polymorphic larval cells are thought to make direct contribution to the developing adult body [5,6], more studies are needed before definitive conclusions can be drawn

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