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Abstract：
Brain evolution has primarily been studied at the macroscopic level by comparing the relative size of homologous brain centers between species. How neuronal circuits change at the cellular level over evolutionary time remains largely unanswered. Here, using a phylogenetically informed framework, we compare the olfactory circuits of three closely related Drosophila species that differ in their chemical ecology: the generalists Drosophila melanogaster and Drosophila simulans and Drosophila sechellia that specializes on ripe noni fruit. We examine a central part of the olfactory circuit that, to our knowledge, has not been investigated in these species-the connections between projection neurons and the Kenyon cells of the mushroom body-and identify species-specific connectivity patterns. We found that neurons encoding food odors connect more frequently with Kenyon cells, giving rise to species-specific biases in connectivity. These species-specific connectivity differences reflect two distinct neuronal phenotypes: in the number of projection neurons or in the number of presynaptic boutons formed by individual projection neurons. Finally, behavioral analyses suggest that such increased connectivity enhances learning performance in an associative task. Our study shows how fine-grained aspects of connectivity architecture in an associative brain center can change during evolution to reflect the chemical ecology of a species.
摘要：
通过比较物种之间同源脑中心的相对大小，主要在宏观水平上研究了脑进化。在进化过程中，神经元回路在细胞水平上的变化在很大程度上还没有答案。这里,使用系统发育知情框架，我们比较了三种密切相关的果蝇的嗅觉回路，这些果蝇的化学生态学有所不同：通才果蝇果蝇和果蝇，以及专门研究成熟诺丽果实的果蝇。我们检查嗅觉回路的中心部分，根据我们的知识，尚未在这些物种中进行研究-投射神经元和蘑菇体的Kenyon细胞之间的连接-并确定特定物种的连接模式。我们发现编码食物气味的神经元更频繁地与Kenyon细胞连接，在连通性中产生物种特异性偏见。这些物种特异性的连通性差异反映了两种不同的神经元表型：投射神经元的数量或由单个投射神经元形成的突触前结的数量。最后,行为分析表明，这种增加的连通性增强了联想任务中的学习表现。我们的研究表明，在进化过程中，联想大脑中心的连通性结构的细粒度方面如何变化，以反映物种的化学生态学。
