Here, I describe 14 regions of interest, or neuropils, of the N. vitripennis brain: the optic lobes (comprised of the lobula, medulla, and lamina), the anterior optic tubercle, the antennal lobe, the lateral horn, the mushroom body (comprised of the calyces and the lobes), and the central complex (comprised of the fan-shaped body, the ellipsoid body, the noduli, and the protocerebral bridge). I compare the shape and (relative) volumes of these neuropils with the other model organisms the honey bee Apis mellifera and
the fruit fly Drosophila melanogaster, as well as a diverse group of paper wasps.
Furthermore, this chapter introduces the Jewel Wasp Standard Brain (JWSB), an average brain obtained by iterative shape averaging of 10 brains from recently eclosed female N. vitripennis (AsymCx strain). Standard brains such as the JWSB
can be used as reference frameworks for integration of multidisciplinary results, such as protein expression data, single neuron recordings, and tracer injections. In contrast to most other standard brains, the JWSB is not based on dehydrated and shrunken tissue, thereby representing a brain closer to the in vivo shape and size, which may aid future stereological studies.
Volumetric descriptions and an interactive 3D model of the Jewel Wasp Standard Brain have been deposited in the online Insect Brain Database (insectbraindb. org), which will serve as a resource for comparative studies, as well as an excellent tool for demonstrations and education.
In Chapter 3 I expand on this basic knowledge of the N. vitripennis brain by seeking answers to the question “How does the brain of N. vitripennis scale over
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candidate for the study of Haller’s rule, which states that larger animals have a relatively smaller brain (allometric scaling). Until now, the “breaking” of this rule
has only been observed in the miniscule egg parasitoid Trichogramma evanescens,
which had the same relative brain size regardless of its body size (isometric
scaling). Unexpectedly, my data show that the slightly larger N. vitripennis wasps
had diphasic, or bimodal, brain scaling: the larger individuals did adhere to Haller’s rule, but under a body weight of 183 µg these wasps showed isometric brain scaling as in T. evanescens. This may indicate that isometric brain scaling
might be a scaling “mode” that is related to very small absolute body sizes. Regardless of its cause, the basic knowledge I describe in Chapter 2 allows me to see how the brain of N. vitripennis is affected by brain scaling. Is the brain
of a small individual a one-to-one copy of that of a large individual, although of smaller size), or do specific neuropils scale at different rates (a notion best described as brain mosaicism)? I show that the brains of the smallest individuals had large differences in the relative neuropil distribution when compared to the larger wasps. The smallest wasps had relatively smaller optic lobes, indicating that
N. vitripennis, which can easily find hosts without using sight, can easily sacrifice
visual tissues to maintain other neuropils such as the antennal lobes, which had the same relative size. The mushroom bodies, which are often described as memory centers in insect brains, also were relatively smaller in the smallest wasps. A parallel study by colleagues showed that small N. vitripennis performed
worse in memory tests, which may be explained by this difference in neuropil volume. Finally, the relative volume of the central complex was larger in the smallest wasps, indicating that this neuropil is probably of large importance. A possible reason for this remains unknown, as the functions of the central complex are very diverse.
The neuropil is only one part of the insect brain, what about the outer layer of cell bodies? In Chapter 4, I explore the question “Are there cell type-specific adaptations in brains of different sizes, and does this relate to behavior?” Specifically, I describe investigations of the dopaminergic network in N. vitripennis of different sizes, because of the role of this neurotransmitter in learning (and the changes therein that were observed in these wasps). In addition, I expanded on a previous study that compared the octopaminergic network of N. vitripennis with Nasonia giraulti,
a related species that shows different memory dynamics than N. vitripennis.
Although a measurement of the total number of all neurons in the N. vitripennis
brain proved to be unattainable, I do show variation in the dopaminergic network. First, I provide a description of the location and average number of cells for
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nine different dopaminergic clusters located in the cell body rind of the Nasonia
brain. Most of these clusters could be compared, based on their location, with the clusters in the honey bee, the fruit fly, and the egg parasitoid Trichogramma evanescens. Based on this comparison, I identified clusters D2, D3, and D5 as clusters that are likely innervate the mushroom body, thereby influencing the memory dynamics of Nasonia. Comparing N. vitripennis of different size, I found
fewer dopaminergic cells in clusters D5 and D7 in small N. vitripennis; comparing N. vitripennis with N. giraulti showed that N. giraulti had fewer cells in clusters D2
and D4a, but more in D4b. More analyses are needed to confirm that differences in cluster D2 and D5 play a causal role in the different memory dynamics of the studied wasps, but these results are a good indication that dopamine is important in the memory dynamics of N. vitripennis.
Finally, I sought to answer the question “Is relative brain size a trait that can be selected for, and what are the effects of such a selection?” in Chapter 5. In the previous chapters, I specifically address brain scaling in an isogenic line (AsymCx), meaning that all wasps were genetically identical. Any variance in brain structure would be due to plasticity, not due to some wasps simply having a genetic propensity for a specific larger or smaller neuropil. To investigate a potential genetic basis of relative brain size I started with a population of genetically diverse N. vitripennis wasps (the HVRx strain). Every generation, I measured and selected wasps with the largest or smallest relative brain size to proceed to the next generation. After 25 generations of 3 weeks, I found a robust difference in relative brain size between the resulting selection lines. In absolute terms, large-brained lines had 16% larger brains, despite being smaller on average. Although Haller’s rule predicts a relative larger brain for smaller individuals, I showed (in a limited size range) that the brain scaling relationship is grade shifted. In this size range, large-brained lines always had a larger brain than a small-brained wasp of equal size. I expected to find cognitive benefits for the large-brained wasps, but they performed as well as the small-brained lines in a memory performance test. An analysis of the neuropil distribution showed that all lines had equal relative mushroom body volumes, which might be related to the lack of differences in memory performance. In contrast to my study on neuropil differences between extremely different size groups in Chapter 3 (230% difference in total neuropil volume), the only neuropil that was affected in this study was the antennal lobe, which was relatively larger in the large- brained wasps. As the antennal lobe is apparently important enough to maintain its relative volume at the small end of an extreme size range (Chapter 3), it could make sense if the antennal lobe would grow larger when brains are selected to grow beyond their normal relative size. A larger antennal lobe may improve the
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were worse off. On average, they had a shorter longevity to start with (which may be explained by their overall smaller body size), but the large-brained wasps also died sooner after a conditioning experience. The small-brained lines did not suffer from this effect. As the title of this chapter already mentions: there appear to be no gains for bigger brains.
In conclusion, this thesis provides a wealth of information on the genotypic and phenotypic aspects of brain scaling, as well as many new questions and reference points for further studies. With the development of the Jewel Wasp Standard Brain a new tool is available for the overall neuroscience community and the Nasonia community in particular, which will likely benefit from its use as
a framework to consolidate results from past and future studies, but also as an interactive tool for educational purposes.