Exploring the Evolution of the Torso Activation Module Components in the Arthropods
The Arthropods are diverse and vary extensively in their form, function and life history strategies. Some of the apparent variation is likely to be due to how different species control their development through the evolution of differing developmental pathways. Insect species such as Drosophila melanogaster and Tribolium castaneum have evolved mechanisms for controlling the important developmental pathways, canonical terminal patterning, pupation and moulting, through the use of one common receptor, Torso. These pathways are controlled by two different ligands with trunk controlling canonical terminal patterning and prothoracicotropic hormone (PTTH) controlling pupation and moulting. Previous analysis has revealed that trunk and PTTH are orthologs of one another. Torso, its ligands as well as the spatially restrictive component of the terminal patterning system, torso-like, compose what has been termed the Torso Activation Module. Initial inquiries into the employment of these ligands for the control of these pathways have shown that there is variability in whether insects employ these methods of controlling canonical terminal patterning, pupation and moulting. Unlike D. melanogaster and T. castaneum, the silk moth (Bombyx mori) uses PTTH to control pupation and moulting but does not use trunk to control terminal patterning, and in fact lack a copy of trunk altogether. Other insect species have also been shown to control these pathways in a trunk and PTTH independent manner, with these components being seemingly absent in their genomes. Other components of the Torso Activation Module, like torso-like have proven to be more universally conserved. Previous analysis of the evolution of the conservation of the components of the Torso Activation Module have been limited in their scope due to the comparatively fewer number of fully sequenced arthropod genomes available. The recent establishment of the i5k project, an initiative to sequence the genomes of 5000 arthropod species, has resulted in a much broader availability of arthropod genomes. Seventy or more of a diverse range of arthropod genomes were assayed for the presence of the Torso Activation Module components through the employment of rigorous bioinformatic techniques as well as phylogenetic analysis. Assaying for the presence of the components of the Torso Activation Module allowed for the inference of possible evolutionary explanations into how this pathway evolved in the arthropods. These analyses revealed that there is significant lability in the loss of some components of the Torso Activation Module, namely trunk, PTTH and torso implying that new developmental pathways have evolved, multiple times, in order to control terminal patterning, pupation and moulting in the arthropods. Torso-like, on the other hand, has been broadly conserved in the arthropods, further evidence that it has been historically co-opted into its current role in Drosophila melanogaster and Tribolium castaneum. A more thorough understanding of where the gaps in our knowledge lie allowed for the proposition of additional experiments which could further our understanding of this invaluable pathway in arthropods and may serve as an example of the usefulness of bioinformatics and phylogenetic techniques in regards to the study of integral developmental pathways in animals as a whole.
Advisor: Dearden, Peter; Duncan, Elizabeth
Degree Name: Master of Science
Degree Discipline: Biochemistry
Publisher: University of Otago
Keywords: trunk; PTTH; prothoracicotropic hormone; evolution; development; Arthropods; insects; phylogenetics; bioinformatics; canonical terminal patterning; pupation; moulting; torso; Torso Activation Module
Research Type: Thesis