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

Pauline Hanot,Claude Guintard,Anthony Herrel,Raphaël Cornette

doi:10.1186/s12862-019-1520-2

Abstract

BackgroundHybridization 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.ResultsOur 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.ConclusionsThese 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.

Highlights

Hybridization has been widely practiced in plant and animal breeding as a means to enhance the quality and fitness of the organisms
The two-way Multivariate Analyses of Variance (MANOVA) performed on the shape data does not indicate an interaction between species-specific and sexual differences in our sample except for the coxal bone
Coxal shape variation was explored in males and females independently plotting first Principal Component (PC)

Summary

Introduction

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. Hybridization may lead to the production of less competitive phenotypes [1,2,3], cases of an increase in fitness and a selective advantage over the parents have been documented in many taxa [4,5,6] This phenomenon, generally considered as the result of heterozygosis in hybrids [7, 8], is known as hybrid vigor, or heterosis, and is usually measured by the capacity of hybrids to expand their ecological range and outperform their parent species under natural conditions [9,10,11]. No macroscopic feature specific to hybrid bones has been, for identified (in a context in which horse and donkey bones are themselves hard to distinguish) [25], which would suggest the absence of strongly transgressive morphologies related to hybridization in domestic equids

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