BACKGROUND Hybridization has been widely practiced in plant and animal breeding as a means to enhance the quality and fitness of the organisms. In domestic equids, this hybrid vigor takes the form of improved physical and physiological characteristics, notably for strength or endurance. Because the offspring of horse and donkey is generally sterile, this widely recognized vigor is expressed in the first generation (F1). However, in the absence of recombination between the two parental genomes, F1 hybrids can be expected to be phenotypically intermediate between their parents which could potentially restrict the possibilities of an increase in overall fitness. In this study, we examine the morphology of the main limb bones of domestic horses, donkeys and their hybrids to investigate the phenotypic impact of hybridization on the locomotor system. We explore bone shape variation and covariation to gain insights into the morphological and functional expressions of the hybrid vigor commonly described in domestic equids. RESULTS Our data reveal the occurrence of transgressive effects on several bones in the F1 generation. The patterns of morphological integration further demonstrate that the developmental processes producing covariation are not disrupted by hybridization, contrary to functional ones. CONCLUSIONS These results suggest that an increase in overall fitness could be related to more flexibility in shape change in hybrids, except for the main forelimb long bones of which the morphology is strongly driven by muscle interactions. More broadly, this study illustrates the interest of investigating not only bone shape variation but also underlying processes, in order to contribute to better understanding how developmental and functional mechanisms are affected by hybridization.

中文翻译：

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揭示家养马科动物的杂交活力：杂交对骨形状变异和共变的影响。

背景技术杂交作为提高生物体质量和适应性的手段已广泛应用于植物和动物育种中。在家养的马科动物中，这种混合活力表现为身体和生理特征的改善，特别是力量或耐力。由于马和驴的后代一般是不育的，这种被广泛认可的活力在第一代（F1）中得到体现。然而，在两个亲本基因组之间不存在重组的情况下，预计 F1 杂种的表型介于其亲本之间，这可能会限制整体适应性增加的可能性。在这项研究中，我们检查了家养马、驴及其杂交种的主要肢体骨骼的形态，以研究杂交对运动系统的表型影响。我们探索骨骼形状的变化和共变，以深入了解家养马科动物中常见的混合活力的形态和功能表达。结果我们的数据揭示了 F1 代中一些骨骼发生了海侵效应。形态整合的模式进一步表明，与功能性发育过程相反，杂交不会破坏产生共变的发育过程。结论 这些结果表明，整体健康度的提高可能与混合体形状变化的更大灵活性有关，但主要前肢长骨除外，其形态由肌肉相互作用强烈驱动。更广泛地说，这项研究不仅表明了研究骨骼形状变化的兴趣，而且还阐明了研究潜在过程的兴趣，以便有助于更好地理解杂交如何影响发育和功能机制。