Cranial shape diversification in horses: variation and covariation patterns under the impact of artificial selection.

The potential of artificial selection to dramatically impact phenotypic diversity is well known. Large-scale morphological changes in domestic species, emerging over short timescales, offer an accelerated perspective on evolutionary processes. The domestic horse (Equus caballus) provides a striking example of rapid evolution, with major changes in morphology and size likely stemming from artificial selection. However, the microevolutionary mechanisms allowing to generate this variation in a short time interval remain little known. Here, we use 3D geometric morphometrics to quantify skull morphological diversity in the horse, and investigate modularity and integration patterns to understand how morphological associations contribute to cranial evolvability in this taxon. We find that changes in the magnitude of cranial integration contribute to the diversification of the skull morphology in horse breeds. Our results demonstrate that a conserved pattern of modularity does not constrain large-scale morphological variations in horses and that artificial selection has impacted mechanisms underlying phenotypic diversity to facilitate rapid shape changes. More broadly, this study demonstrates that studying microevolutionary processes in domestic species produces important insights into extant phenotypic diversity.

Unravelling the hybrid vigor in domestic equids: the effect of hybridization on bone shape variation and covariation.

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.

The impact of artificial selection on morphological integration in the appendicular skeleton of domestic horses.

The relationships between the different component parts of organisms, such as the sharing of common development or function, produce a coordinated variation between the different traits. This morphological integration contributes to drive or constrain morphological variation and thus impacts phenotypic diversification. Artificial selection is known to contribute significantly to phenotypic diversification of domestic species. However, little attention has been paid to its potential impact on integration patterns. This study explores the patterns of integration in the limb bones of different horse breeds, using 3D geometric morphometrics. The domestic horse is known to have been strongly impacted by artificial selection, and was often selected for functional traits. Our results confirm that morphological integration among bones within the same limb is strong and apparently partly produced by functional factors. Most importantly, they reveal that artificial selection, which led to the diversification of domestic horses, impacts covariation patterns. The influence of selection on the patterns of covariation varies along the limbs and modulates bone shape, likely due to a differential ligament or muscle development. These results highlight that, in addition to not being constrained by a strong morphological integration, artificial selection has modulated the covariation patterns according to the locomotor specificities of the breeds. More broadly, it illustrates the interest in studying how micro-evolutionary processes impact covariation patterns and consequently contribute to morphological diversification of domestic species.

Morphological integration in the appendicular skeleton of two domestic taxa: the horse and donkey.

Organisms are organized into suites of anatomical structures that typically covary when developmentally or functionally related, and this morphological integration plays a determinant role in evolutionary processes. Artificial selection on domestic species causes strong morphological changes over short time spans, frequently resulting in a wide and exaggerated phenotypic diversity. This raises the question of whether integration constrains the morphological diversification of domestic species and how natural and artificial selection may impact integration patterns. Here, we study the morphological integration in the appendicular skeleton of domestic horses and donkeys, using three-dimensional geometric morphometrics on 75 skeletons. Our results indicate that a strong integration is inherited from developmental mechanisms which interact with functional factors. This strong integration reveals a specialization in the locomotion of domestic equids, partly for running abilities. We show that the integration is stronger in horses than in donkeys, probably because of a greater degree of specialization and predictability of their locomotion. Thus, the constraints imposed by integration are weak enough to allow important morphological changes and the phenotypic diversification of domestic species.