Data from: Constraint around quarter-power allometric scaling in wild tomatoes (Solanum sect. Lycopersicon; Solanaceae)

The West-Brown-Enquist (WBE) metabolic scaling theory posits that many organismal features scale predictably with body size because of selection to minimize transport costs in resource distribution networks. Many scaling exponents are quarter-powers, as predicted by WBE, but there are also biologically significant deviations that could reflect divergent adaptation. A central but untested prediction of the WBE model is that wide deviation from optimal scaling is penalized, leading to a pattern of constraint on scaling exponents. Here we demonstrate using phylogenetic comparative methods that variation in allometric scaling between mass and leaf area across 17 wild tomato taxa is weakly constrained around a value indistinguishable from that predicted by WBE, but significantly greater than 2/3 (geometric similarity model). The allometric scaling exponent was highly correlated with fecundity, water use, and drought response, suggesting that it is functionally significant and therefore could be under selective constraints. However, scaling was not strictly log-log linear, but rather declined during ontogeny in all species, as has been observed in many plant species. We caution that although our results supported one prediction of the WBE model, it did not strongly test the model in other important respects. Nevertheless, phylogenetic comparative methods like those used here are powerful, but underutilized tools for metabolic ecology that complement existing methods to adjudicate between models.

West-Brown-Enquist（WBE）代谢缩放理论（metabolic scaling theory）提出，诸多生物体特征会随体型大小呈现可预测的缩放关系，这是因为自然选择倾向于最小化资源分配网络中的运输成本。诸多缩放指数均为四分之一幂次，与WBE理论的预测相符，但也存在具有生物学意义的偏差，这些偏差可能反映了趋异适应。WBE模型的一个核心且尚未被验证的预测是，与最优缩放模式的显著偏差会受到自然选择的负向作用，进而形成缩放指数受约束的演化格局。本研究利用系统发育比较分析方法（phylogenetic comparative methods），对17个野生番茄类群的叶面积与体重间的异速缩放（allometric scaling）变异进行分析，结果显示其变异围绕一个与WBE理论预测值无显著差异的数值呈现弱约束状态，且该数值显著高于2/3（即几何相似模型的预测值）。异速缩放指数（allometric scaling exponent）与繁殖力、水分利用效率及干旱响应性状显著相关，表明其具有功能重要性，因此可能处于选择约束之下。不过，缩放关系并非严格的对数-对数线性（log-log linear）关系，而是在所有物种中均随个体发育（ontogeny）过程逐渐降低，这一现象在诸多植物类群中已有报道。本研究需说明：尽管结果支持了WBE模型的一项预测，但并未在其他关键维度上对该模型开展严格检验。尽管如此，本研究所用的这类系统发育比较分析方法，是代谢生态学领域兼具高效性却未被充分利用的研究工具，可与现有方法互补，用于不同模型间的判定与筛选。