Data from: Reversed brain size sexual dimorphism accompanies loss of parental care in white sticklebacks

Uncovering factors that shape variation in brain morphology remains a major challenge in evolutionary biology. Recently, it has been shown that brain size is positively associated with level of parental care behavior in various taxa. One explanation for this pattern is that the cognitive demands of performing complex parental care may require increased brain size. This idea is known as the parental brain hypothesis (PBH). We set out to test the predictions of this hypothesis in wild populations of threespine stickleback (Gasterosteus aculeatus). These fish are commonly known to exhibit (1) uniparental male care and (2) sexual dimorphism in brain size (males>females). To test the PBH, we took advantage of the existence of closely related populations of stickleback that display variation in parental care behavior: common marine threespine sticklebacks (uniparental male care) and white threespine sticklebacks (no care). To begin, we quantified genetic differentiation among two common populations and three white populations from Nova Scotia. We found overall low differentiation among populations, although FST was increased in between-type comparisons. We then measured the brain weights of males and females from all five populations along with two additional common populations from British Columbia. We found that sexual dimorphism in brain size is reversed in white stickleback populations: males have smaller brains than females. Thus, while several alternatives need to be ruled out, the PBH appears to be a reasonable explanation for sexual dimorphism in brain size in threespine sticklebacks.阐明塑造脑形态变异的关键因素，始终是进化生物学领域的重大挑战之一。近期已有研究表明，在诸多生物类群中，脑容量与亲本抚育行为的强度呈正相关关系。针对这一现象的一种解释是：开展复杂亲本抚育行为所需的认知负荷，可能会促使脑容量增大。这一理论被称为亲本脑假说（parental brain hypothesis, PBH）。本研究旨在以野生三刺鱼（Gasterosteus aculeatus）种群为对象，验证该假说的相关预测。该物种已知存在两大特征：（1）由雄性单独承担亲本抚育职责；（2）脑容量存在性别二态性（雄性脑容量大于雌性）。为验证亲本脑假说，我们利用了亲缘关系相近但亲本抚育行为存在差异的两类刺鱼种群：一类为普通海洋三刺鱼（由雄性单独抚育后代），另一类为白色三刺鱼（无任何亲本抚育行为）。首先，我们对来自加拿大新斯科舍省的2个普通三刺鱼种群与3个白色三刺鱼种群的遗传分化水平进行了量化分析。研究结果显示，各类群间整体遗传分化程度较低，但不同生态型种群间的FST值有所升高。随后，我们对上述5个种群以及另外2个来自加拿大不列颠哥伦比亚省的普通三刺鱼种群的雌雄个体脑重量进行了测定。我们发现，白色三刺鱼种群的脑容量性别二态性发生了逆转：雄性脑容量小于雌性。因此，尽管仍有若干替代假说有待排除，但亲本脑假说似乎能够合理解释三刺鱼脑容量的性别二态性现象。