Variation in vertebrae shape across small‐bodied newts reveals functional and developmental constraints acting upon the trunk region

Abstract

The vertebral column forms the central axis of the vertebrate body and has a crucial role in body support and locomotion. It is composed of a number of repetitive, serially homologous skeletal elementsthe vertebrae. The homeobox (Hox) genes are known to be directly involved in somite and vertebrae formation and play a key role in the attainment of vertebral identity (Krumlauf, 1994;Mallo et al., 2010;Woltering, 2012). It has been proposed that different vertebral morphologies are governed by unique combinations of Hox genes expressed in the somites (Kessel & Gruss, 1991;Wellik, 2007) with the corollary that few Hox genes expression boundaries can be used as markers for regional differentiation of the axial skeleton (Böhmer et al., 2015;Burke et al., 1995;Narita & Kuratani, 2005). For example, the cervico-thoracic transition in amniotes is determined by Hox-5 and Hox-6 genes and the formation of the lumbo-sacral boundary is under control of the Hox-10 and Hox-11 genes (Wellik & Capecchi, 2003). Recent morphometric studies have provided new insight into presacral vertebral regionalization in amniotes (Head &

Regionalization and morphological integration in the vertebral column of Eurasian small‐bodied newts (Salamandridae: Lissotriton)

“…I G U R E 1 Morphological differentiation and regionalization of the presacral vertebral column in Lissotriton newts showing (a) different morphologies of trunk vertebrae, indicated by different color shadings, and (b) the configuration of the landmarks used to describe the shape of the atlas and the trunk and sacral vertebrae.T A B L E 1 Concepts of regional differentiation of the presacral vertebral column in tailed amphibians. (1972),Govedarica et al (2017),Scholtes et al (2021), present study…”

“…The first and second trunk vertebrae are connected with the cranial skeleton with the muscles involved in the coordinated head movement and spine flection (Francis, 1934). This could explain the covariation between the first to fifth trunk vertebrae, notwithstanding their differences in shape (Scholtes et al, 2021). The posterior region (from the 6th to the 11th or the 12th vertebra) consists of vertebrae with similar shape and the same arrangement of muscles driving the lateral bending of the trunk during swimming and walking.…”

“…They also have a single sacral vertebra, several postsacral, and numerous caudal vertebrae, depending on the species (Duellman & Trueb, 1994; Lanza et al, 2010; Litvinchuk & Borkin, 2003) (Figure 1a). Recent studies on vertebral regionalization (Jones et al, 2018) and morphological differentiation (Scholtes et al, 2021) indicate that the trunk region in salamanders is not as uniform as previously thought. A three‐region pattern of the presacral vertebral column was found in Ambystoma (Jones et al, 2018).…”

“…A three‐region pattern of the presacral vertebral column was found in Ambystoma (Jones et al, 2018). In small‐bodied newts of the genus Lissotriton substantial morphological differentiation in the vertebrae shape along the trunk region was documented (Figure 1a; Scholtes et al, 2021). We here provide a brief description of three alternative concepts of the salamander's presacral vertebral column regionalization (see Table 1).…”

“…The first region associates with the posterior branch of the brachial plexus, implying homology of the anterior trunk vertebrae with the cervical region, whereas the middle and posterior regions correspond to regions of short and long ribs in the Amniotes (Jones et al, 2018). Alternatively, studies on morphometric variation in vertebrae size (Govedarica et al, 2017; Worthington & Wake, 1972) and shape (Scholtes et al, 2021) revealed morphological differentiation (substantial disparity in size and shape) of the presacral vertebral column in salamanders, particularly in the anterior trunk vertebrae. A similar heterogeneity among short and bulky anterior and elongated posterior vertebrae was documented in caecilians (Lowie et al, 2022).…”