van der Woude Emma, Smid Hans M
Laboratory of Entomology, Wageningen University, Wageningen, The Netherlands.
Brain Behav Evol. 2017;89(3):185-194. doi: 10.1159/000468974. Epub 2017 May 6.
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.
微小赤眼蜂寄生蜂在大脑和身体大小方面表现出很大的表型可塑性，导致基因相同的姐妹蜂之间脑容量相差5倍。在这些黄蜂中，脑容量与身体体积呈线性比例关系。这种等比例的脑缩放是哈勒法则的一个例外，该法则指出小动物的大脑相对比大动物的大脑更大。大脑大小的巨大可塑性可能是由神经元大小、神经元数量或两者的可塑性促成的。在这里，我们研究了脑等比例缩放是否需要表达5-羟色胺（5HT）、章鱼胺（OA）和多巴胺（DA）的单胺能神经元在数量和大小上具有可塑性。基因相同的小体型和大体型微小赤眼蜂在大脑中似乎具有相同数量的5HT、OA和DA样免疫反应性细胞体，但这些细胞体的直径不同。这表明脑等比例缩放可以通过单胺能神经元大小的可塑性来实现，而不是通过单胺能神经元数量的可塑性。对身体小型化的选择压力可能导致了小型化神经通路的进化，使即使是最小的黄蜂也能找到合适的宿主。神经成分大小的可塑性可能是在最小个体中维持认知能力的同时实现脑等比例缩放的机制之一。
