Homeotic transformations reflect departure from the mammalian ‘rule of seven’ cervical vertebrae in sloths: inferences on the Hox code and morphological modularity of the mammalian neck

BMC Evolutionary Biology

Eli Amson¹  Patrick Arnold²  John A. Nyakatura³  Anneke H. van Heteren⁴  Christine Bhmer⁵ 

[1] AG Morphologie und Formengeschichte, Institut fr Biologie, Humboldt Universitt zu Berlin, Berlin, Germany;Image Knowledge Gestaltung: An Interdisciplinary Laboratory, Humboldt University, Berlin, Germany;Institut fr Zoologie und Evolutionsforschung mit Phyletischem Museum, Ernst-Haeckel-Haus und Biologiedidaktik, Friedrich-Schiller-Universitt Jena, Jena, Germany;Museum fr Naturkunde, Leibniz-Institut fr Evolutions- und Biodiversittsforschung, Berlin, Germany;UMR 7179 CNRS/MNHN, Musum National dHistoire Naturelle, Paris, France

关键词: Axial patterning;    Hox genes;    Evolution;    Constraint;    Xenarthra;    Cervical vertebrae;   

DOI  :  10.1186/s12862-018-1202-5

学科分类：生物科学（综合）

来源: BioMed Central

【 摘 要 】

Sloths are one of only two exceptions to the mammalian ‘rule of seven’ vertebrae in the neck. As a striking case of breaking the evolutionary constraint, the explanation for the exceptional number of cervical vertebrae in sloths is still under debate. Two diverging hypotheses, both ultimately linked to the low metabolic rate of sloths, have been proposed: hypothesis 1 involves morphological transformation of vertebrae due to changes in the Hox gene expression pattern and hypothesis 2 assumes that the Hox gene expression pattern is not altered and the identity of the vertebrae is not changed. Direct evidence supporting either hypothesis would involve knowledge of the vertebral Hox code in sloths, but the realization of such studies is extremely limited. Here, on the basis of the previously established correlation between anterior Hox gene expression and the quantifiable vertebral shape, we present the morphological regionalization of the neck in three different species of sloths with aberrant cervical count providing indirect insight into the vertebral Hox code. Shape differences within the cervical vertebral column suggest a mouse-like Hox code in the neck of sloths. We infer an anterior shift of HoxC-6 expression in association with the first thoracic vertebra in short-necked sloths with decreased cervical count, and a posterior shift of HoxC-5 and HoxC-6 expression in long-necked sloths with increased cervical count. Although only future developmental analyses in non-model organisms, such as sloths, will yield direct evidence for the evolutionary mechanism responsible for the aberrant number of cervical vertebrae, our observations lend support to hypothesis 1 indicating that the number of modules is retained but their boundaries are displaced. Our approach based on quantified morphological differences also provides a reliable basis for further research including fossil taxa such as extinct ‘ground sloths’ in order to trace the pattern and the underlying genetic mechanisms in the evolution of the vertebral column in mammals.

【 授权许可】

CC BY