Scombroid Fishes Provide Novel Insights into the Trait/Rate Associations of Molecular Evolution

The study of which life history traits primarily affect molecular evolutionary rates is often confounded by the covariance of these traits. Scombroid fishes (billfishes, tunas, barracudas, and their relatives) are unusual in that their mass-specific metabolic rate is positively associated with body size. This study exploits this atypical pattern of trait variation, which allows for direct tests of whether mass-specific metabolic rate or body size is the more important factor of molecular evolutionary rates. We inferred a phylogeny for scombroids from a supermatrix of molecular and morphological characters and used new phylogenetic comparative approaches to assess the associations of body size and mass-specific metabolic rate with substitution rate. As predicted by the body size hypothesis, there is a negative correlation between body size and substitution rate. However, unexpectedly, we also find a negative association between mass-specific metabolic and substitution rates. These relationships are supported by analyses of the total molecular data, separate mitochondrial and nuclear genes, and individual loci, and they are robust to phylogenetic uncertainty. The molecular evolutionary rates of scombroids are primarily tied to body size. This study demonstrates that groups with novel patterns of trait variation can be particularly informative for identifying which life history traits are the primary factors of molecular evolutionary rates.

探究何种生活史性状对分子进化速率起主导影响的相关研究，常因各类性状间的协方差而难以厘清因果关系。鲭类群鱼类（包括旗鱼、金枪鱼、梭鱼及其近缘类群）存在一项非典型特征：其单位质量代谢速率与个体体型呈正相关。本研究借助这一特殊的性状变异模式，得以直接检验单位质量代谢速率与体型，究竟何者为分子进化速率的更关键驱动因素。我们基于整合分子与形态学特征的超级矩阵，重建了鲭类群的系统发育关系，并采用新型系统发育比较分析方法，评估体型与单位质量代谢速率同核苷酸替换速率之间的关联。正如体型大小假说所预测，个体体型与替换速率呈负相关。但令人意外的是，我们同时发现单位质量代谢速率与替换速率亦存在负相关关联。上述结论在全部分子数据集、线粒体与核基因的单独分析以及单个基因座的分析中均得到支持，且不受系统发育不确定性的影响。鲭类群的分子进化速率主要受个体体型调控。本研究证实，具备新颖性状变异模式的类群，在识别主导分子进化速率的生活史性状方面，具有极高的研究价值。