Hoops, Daniel
Description
This thesis is about the evolution of brain structure in lizards,
with a particular focus on the agamid lizards of the genus
Ctenophorus. Achieving this required advancing the methods with
which to study lizard brains. Therefore, this thesis is split
about equally between developing the tools to study lizard brain
structure and the evolution of brain structure in Ctenophorus.
Below I present short summaries of each chapter. Full abstracts
are presented with...[Show more] each chapter.
Section 1: Development of the framework and tools necessary for
the study of the dragon brain
Chapter 1
Among vertebrates, reptiles have lagged far behind birds,
mammals, fishes and amphibians in neurobiological research.
Nonetheless, in the past twenty years there have been significant
advances in our understanding of the neurobiology of reptiles,
particularly among squamates (lizards and snakes). The first
chapter of this thesis presents a broad literature review of
lizard brain research. All peer-reviewed publications since the
last major review in 1998 are summarized to give a complete
overview of the state of the published literature on squamate
neurobiology. I use this overview to highlight what is unique
about squamate brains and identify gaps that remain in our
understanding of these systems. Finally, I provide a framework
for future studies that includes exciting new and unanswered
questions about squamate brain evolution, structure and function.
Chapter 2
Perfusion is the most common technique for preserving brains for
neuroscience research. Standard perfusion techniques were
developed primarily for application in mammals, which are
traditional neuroscience research models. A perfusion method has
never been published for lizards and following mammalian
perfusion protocols for lizards results in failed perfusions. In
this chapter, I present a modified perfusion protocol suitable
for lizards.
Chapter 3
In this chapter we present a magnetic resonance-based atlas for
the brain of an agamid lizard, the tawny dragon (Ctenophorus
decresii). We use literature sources as well as histological
sections to identify and delineate, in three dimensions, the cell
regions and fiber tracts visible in this model. This atlas has
acted as a guide for measuring and analyzing brains in the
subsequent chapters, and as a template with which to automate
brain measurements across many individuals from multiple
species.
Section 2: Evolutionary patterns in dragon brain structure
Chapter 4
Two models have been proposed to explain the patterns observed in
evolutionary changes in brain morphology: the concerted model and
the mosaic model of brain evolution. It is now well understood
that both models are relevant in explaining brain evolution but
the relative influence of each mode on brain structure varies
between vertebrate groups. It remains unclear what factors favour
concerted or mosaic brain evolution. In this chapter, we found
evidence for both mosaic and concerted brain evolution in dragon
lizards. Brains showed a pattern of concerted brain evolution
with respect to the morphological characters. In contrast, they
showed a pattern of mosaic brain evolution with respect to
ecological and life history characters.
Chapter 5
The role of sexual selection in altering brain organisation and
structure over evolutionary time is poorly understood. In this
chapter we compare the brains of species under strong and weak
sexual selection. Males belonging to species that experience
strong sexual selection had a larger medial preoptic nucleus and
a smaller ventromedial hypothalamic nucleus. Conversely, females
did not show any obvious variation in these brain regions. The
medial preoptic nucleus controls male reproductive behaviour
while the ventromedial hypothalamic nucleus controls female
reproductive behaviour and is also involved in male aggression.
Therefore, the primary brain nuclei underlying reproductive
behavior evolve in a mosaic fashion in dragons, differently
between males and females, likely in response to the strength of
sexual selection.
Collectively, these findings describe in detail the structure of
an agamid brain and some of the ways in which that structure has
changed through evolution. In doing so, these results have
highlighted both how labile the brain can be in response to
evolution, and how conserved brain structure is in general.
Lizards, and reptiles in general, are the most understudied
vertebrate group in neuroscience, but there is huge potential for
discovery in this field.
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