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Fig. 1 | BMC Evolutionary Biology

Fig. 1

From: 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

Fig. 1

Two evolutionary hypotheses of mechanisms to break the seven cervical vertebrae (CV) constraint in sloths. The Hox code is a key determinant of vertebral identity and the color coding represents modules with the same Hox code. A three-subunit pattern within the postatlantal CV has been found (green, axis; yellow, anterior; red, posterior). Thoracic vertebrae shown in grey. A: The first hypothesis predicts that the number of CV is changed due to an altered Hox code. Therefore, the modular pattern in the neck of sloths with an aberrant number of CV should differ from that of living mammals (represented by the mouse), e.g., due to expansion of one of the subunits. B: The second hypothesis predicts that the first seven vertebrae retain a cervical identity and the Hox code remains unchanged. According to this hypothesis the modular pattern in sloths corresponds to the general pattern of living mammals and CV that are originally thoracic vertebrae are added (blue)

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