Homeotic transformations and number changes in the vertebral column of Triturus newts

Abstract

We explored intraspecific variation in vertebral formulae, more specifically the variation in the number of thoracic vertebrae and frequencies of transitional sacral vertebrae in Triturus newts (Caudata: Salamandridae). Within salamandrid salamanders this monophyletic group shows the highest disparity in the number of thoracic vertebrae and considerable intraspecific variation in the number of thoracic vertebrae. Triturus species also differ in their ecological preferences, from predominantly terrestrial to largely aquatic. Following Geoffroy St. Hilaire’s and Darwin’s rule which states that structures with a large number of serially homologous repetitive elements are more variable than structures with smaller numbers, we hypothesized that the variation in vertebral formulae increases in more elongated species with a larger number of thoracic vertebrae. We furthermore hypothesized that the frequency of transitional vertebrae will be correlated with the variation in the number of thoracic vertebrae within the species. We also investigated potential effects of species hybridization on the vertebral formula. The proportion of individuals with a number of thoracic vertebrae different from the modal number and the range of variation in number of vertebrae significantly increased in species with a larger number of thoracic vertebrae. Contrary to our expectation, the frequencies of transitional vertebrae were not correlated with frequencies of change in the complete vertebrae number. The frequency of transitional sacral vertebra in hybrids did not significantly differ from that of the parental species. Such a pattern could be a result of selection pressure against transitional vertebrae and/or a bias towards the development of full vertebrae numbers. Although our data indicate relaxed selection for vertebral count changes in more elongated, aquatic species, more data on different selective pressures in species with different numbers of vertebrae in the two contrasting, terrestrial and aquatic environments are needed to test for causality.

Testing the evolutionary constraints of metamorphosis: The ontogeny of head shape inTriturusnewts

“…9-7 Ma) (Ivanović and Arntzen 2014;Wielstra et al 2019). The two species differ in adult morphology (e.g., body shape and coloration (Arntzen 2003;Vukov et al 2011) and the modal number of trunk vertebrae (Arntzen 2003;Slijepčević et al 2015)); some differences in larval tail morphology were also recorded (Vučić et al 2018). Triturus ivanbureschi and T. macedonicus reproduce in similar aquatic habitats (Džukić et al 2016).…”

“…In eastern Serbia, a confirmed unimodal natural hybrid population is found, with Fn individuals that derived from multiple mutual crossings and backcrosses with parental species (Wielstra and Arntzen 2014;Wielstra et al 2014a;Arntzen et al 2018). Studies of Triturus newt hybrids showed that they have intermediate morphological traits relative to their parental species (e.g., Francillon-Vieillot et al 1990;Brede et al 2000;Vinšálková and Gvoždík 2007;Litvinchuk and Borkin 2009;Slijepčević et al 2015), including T. macedonicus × T. ivanbureschi hybrids (Vučić et al 2018;Arntzen et al 2018).…”

“…Body form and head shape in salamanders can be characterized as traits that undergo a different amount of changes during metamorphosis. The axial skeleton is defined during early embryonic development and it is not affected by metamorphic processes (Slijepčević et al 2015;Govedarica et al 2017). Limbs that fully develop at late larval stages do not undergo major structural changes, although contrasting functional demands produce differences in the correlation pattern of limb skeletal elements between larval and metamorphosed stages (Tomašević Kolarov et al 2011).…”

“…Crested newt species diverge in larval shape Vučić et al 2018) and the shape of metamorphs, juveniles, and adults (Cvijanović et al 2014;Ivanović and Arntzen 2014). In natural populations of crested newts, hybridization is an important force that shapes phenotypic variability at species' contact zones (Crnobrnja-Isailović et al 1997;Arntzen et al 2009;Slijepčević et al 2015;Arntzen et al 2018). Studies of hybrid phenotypes could provide an insight into evolutionary mechanisms that lead to divergence from common ancestral developmental program and evolution of ontogenies (Voss and Shaffer 1996).…”

Mechanical strain can increase segment number in live chick embryos

“…vertebrae with morphological characteristics of two adjacent regions (for example trunk and sacrum) are statistically underrepresented in these species. This suggests a developmental mechanism favouring complete numbers of trunk vertebrae, rather than transitional vertebrae that might hamper mechanical function (46,47). If additional trunk somites would form bilaterally via daughter somite formation, both newly created somites might have the same axial identity as their mother somite, given that the Hox identity is determined mainly before the somite is formed, during the earliest stages of somite formation (48)(49)(50).…”