Data from: Selection for brain size impairs innate, but not adaptive immune responses

The brain and the immune system are both energetically demanding organs, and when natural selection favors increased investment into one, then the size or performance of the other should be reduced. While comparative analyses have attempted to test this potential evolutionary trade-off, the results remain inconclusive. To test this hypothesis, we compared the tissue graft rejection (an assay for measuring innate and acquired immune responses) in guppies (Poecilia reticulata) artificially selected for large and small relative brain size. Individual scales were transplanted between pairs of fish, creating reciprocal allografts, and the rejection reaction was scored over eight days (before acquired immunity develops). Acquired immune responses were tested two weeks later, when the same pairs of fish received a second set of allografts and were scored again. Compared to large-brained animals, small-brained animals of both sexes mounted a significantly stronger rejection response to the first allograft. The rejection response to the second set of allografts did not differ between large- and small-brained fish. Our results show that selection for large brain size reduced innate immune responses to an allograft, which supports the hypothesis that there is a selective trade-off between investing into brain size and innate immunity.

大脑与免疫系统均为高能量需求器官，当自然选择倾向于向其中一方增加资源投入时，另一方的体积或功能理应出现退化。此前已有比较分析尝试验证这一潜在的进化权衡（evolutionary trade-off），但相关结果仍无定论。为验证该假说，我们对人工选育得到相对大脑体积较大与较小的孔雀鱼（Poecilia reticulata），比较其组织移植排斥反应（tissue graft rejection，一种检测先天免疫应答与获得性免疫应答的实验方法）。实验中，我们将成对个体间的鳞片进行移植，构建相互异体移植（reciprocal allografts）模型，并在移植后8天（获得性免疫激活前）对排斥反应进行评分。两周后，我们开展获得性免疫应答检测：将同一组鱼再次接受第二轮异体移植并重新评分。结果显示，与大脑体积较大的个体相比，两种性别的大脑体积较小的孔雀鱼对首次异体移植的排斥反应显著更强；而第二轮异体移植的排斥反应在大、小脑体积的鱼群间无显著差异。本研究结果表明，针对大脑体积增大的选育会降低机体对异体移植的先天免疫应答，这支持了"大脑体积发育投入与先天免疫间存在选择性进化权衡"的假说。