Effects of Isometric Brain-Body Size Scaling on the Complexity of Monoaminergic Neurons in a Minute Parasitic Wasp

Abstract

Trichogramma evanescens parasitic wasps show large phenotypic plasticity in brain and body size, resulting in a 5-fold difference in brain volume among genetically identical sister wasps. Brain volume scales linearly with body volume in these wasps. This isometric brain scaling forms an exception to Haller's rule, which states that small animals have relatively larger brains than large animals. The large plasticity in brain size may be facilitated by plasticity in neuron size, in the number of neurons, or both. Here, we investigated whether brain isometry requires plasticity in the number and size of monoaminergic neurons that express serotonin (5HT), octopamine (OA), and dopamine (DA). Genetically identical small and large T. evanescens appear to have the same number of 5HT-, OA-, and DA-like immunoreactive cell bodies in their brains, but these cell bodies differ in diameter. This indicates that brain isometry can be facilitated by plasticity in the size of monoaminergic neurons, rather than plasticity in numbers of monoaminergic neurons. Selection pressures on body miniaturization may have resulted in the evolution of miniaturized neural pathways that allow even the smallest wasps to find suitable hosts. Plasticity in the size of neural components may be among the mechanisms that underlie isometric brain scaling while maintaining cognitive abilities in the smallest individuals.

Brain morphology predicts social intelligence in wild cleaner fish

“…Importantly, these findings show that forebrain cell counts do not predict cleaners’ performance in the ephemeral food task, neither directly nor as a function of population density. The fact that forebrains of socially competent individuals were larger without an increase in cell numbers, suggests a neuropil volume expansion, instead 56 , 57 . Unfortunately, the information on neuron numbers was missing (see ‘Methods’ section), making it difficult to estimate the variation in the proportion of neurons to glia.…”

Nasonia Parasitic Wasps Escape from Haller's Rule by Diphasic, Partially Isometric Brain-Body Size Scaling and Selective Neuropil Adaptations

“…The arthropod brain, especially one of this minute size, may pose some unique challenges in uncovering how it responds to the variation we described. For instance, specific neuron subtypes in brains of small and larger T. evanescens do not differ in number, but do differ in size [van der Woude and Smid, 2017a]. For N. vitripennis, more detailed studies are required to ascribe the difference in cell body rind volume to a variation in neuron numbers, neuron/glia ratio, cell body size, or a combination of these factors.…”

“…Such compensatory changes may be seen as the outcome of a trade-off between the energy requirements of neural tissue, which become larger in smaller individuals as relative brain size increases, and the requirement to maintain cognitive functions in the smallest wasps determined by their ecological relevance [Riveros and Gronenberg, 2010;Muscedere and Traniello, 2012;Stöckl et al, 2016;van der Woude and Smid, 2017a]. Examples of absolute brain size influencing neuropil composition include Drosophila melanogaster, for which the optic lobe size is relatively smaller in smaller individuals [Lanet et al, 2013].…”

Comparative genomics of the miniature wasp and pest control agent Trichogramma pretiosum

“…The same genetic line of Trichogramma can produce wasps anywhere from less than 0.2 mm to greater than 0.5 mm long, indicating just how plastic their body size is [ 26 ]. This plasticity in size is also seen at the level of the cell: aminergic neuron somata in Trichogramma evanescens range from 1.7 μm in small adults to 4.4 μm in genetically identical large adults [ 27 ]. As a result of their small size, Trichogramma have a number of unique morphological features in addition to the aforementioned changes often associated with miniaturization.…”