Abstract
The parasitic wasp, Trichogramma evanescens, is an extremely small insect, with a body length as small as 0.3 mm. To facilitate this miniaturization, their brains may have evolved to contain smaller neural components and/or reduced neural complexity than larger insects. Here, we study whether the size and number of neurons are reduced in the miniaturized brain of T. evanescens, focusing on neurons that express serotonin (5HT), octopamine (OA) and dopamine (DA). We provide the first description of the distribution, projection patterns and number of 5HT-, OA- and DA-like immunoreactive cell bodies in T. evanescens and compare our observations with descriptions of much larger insects. The brains of T. evanescens contain comparable numbers of monoaminergic neurons to those of larger insects. Serotonergic neurons appear to be especially conserved; most of the clusters contain a similar number of neurons to those described in Apis mellifera and Drosophila melanogaster. This maintained complexity may have been facilitated by miniaturization of neuron size. However, many dopaminergic and some octopaminergic neuron clusters in T. evanescens contain fewer neurons than in larger insects. Modification of the complexity of these monoaminergic systems may have been necessary to maintain neuron functionality during brain miniaturization in T. evanescens. Our results reveal some of the evolutionary adaptations that may enable behavioural and cognitive complexity with respect to miniaturized brains.
Highlights
The parasitic wasp,Trichogramma evanescens (Hymenoptera: Trichogrammatidae), is an extremely small gregarious parasitoid that lays its eggs inside the eggs of butterflies and moths
We show that these miniaturized brains contain comparable numbers of monoaminergic neurons to those of much larger insects, namely A. mellifera, D. melanogaster and larger parasitic wasps of the genera Nasonia and Cotesia
Additional differences have been observed between the OA-like immunoreactive (OA-L-IR) neuron clusters of T. evanescens and those of the larger insects, the distribution and number of OA-LIR neurons is very similar to descriptions of the related parasitic wasps of the genus Nasonia
Summary
The parasitic wasp,Trichogramma evanescens (Hymenoptera: Trichogrammatidae), is an extremely small gregarious parasitoid that lays its eggs inside the eggs of butterflies and moths. Trichogramma evanescens show isometric brain scaling, exhibiting a linear relationship between brain and body volume. This deviates from the situation described by Haller’s rule, which states that small animals have relatively larger brains. As a result of brain isometry, the smallest T. evanescens have brains that are even smaller than that predicted by Haller’s rule. Their brain volume can be as small as 0.26*106 μm (Van der Woude et al 2013), which is almost 2500× times smaller than the brain of a honeybee (Mares et al 2005)
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