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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.
Comparing to the previous version of the preprint, the higher resolution of the Figure 1 has been provided, an error in the Supplementary Figure 2 has been corrected and the File (Supplementary Table 3) consisting of raw data has been added.
Species, geographic populations, their position relative to the species contact zone and number of specimens analysed
Supplementary Table S1. Overview of the analyzed samples of Triturus: species, geographic populations, their position relative to the species contact zone and number of specimens. # stands for ambiguous species allocation. Hybrid zones are generally narrow ( Arntzen , Wielstra & Wallis, 2014). However, spatial and genetic data suggest that species displaced one another, and hybrid zones have been moving (Wielstra & Arntzen, 2012; Wielstra et al., 2013), which – in the context of this study – widens them. Populations were assigned as “central” or “fringe” based upon their geographical position away (>= 50km) or close to (< 50 km) congeneric species.
Overview of the analyzed samples of Triturus: species, geographic populations, number of specimens, and variation in the number of vertebrae. For each species standard vertebral formula is given next to the species name. # stands for ambiguous species allocation.
Three dimensional view of regular sacral vertebra in Triturus dobrogicus
Supplementary data 1: Three dimensional view of regular sacral vertebra in Triturus dobrogicus (ZMA.RenA.9120_845 from Naturalis Biodiversity Center, Leiden, Netherlands). Specimen was scanned with Skyscan 1171 100kV computed microtomograph [micro computed tomography (CT)-scanner] under settings that were optimized for the material (59 kV, 0.7 rotation step, 145ms exposure time). 3D surface model of newt vertebrae was produced using CTvox, version 3.0 software.
Three dimensional view of transitional sacral vertebra in Triturus dobrogicus
Supplementary data 2: Three dimensional view of transitional sacral vertebra in Triturus dobrogicus (ZMA.RenA.9120_850 from Naturalis Biodiversity Center, Leiden, Netherlands). Specimen was scanned with Skyscan 1171 100kV computed microtomograph [micro computed tomography (CT)-scanner] under settings that were optimized for the material (59 kV, 0.7 rotation step, 145ms exposure time). 3D surface model of newt vertebrae was produced using CTvox, version 3.0 software.
Raw data file: The vertebral formulae of of 1436 adult newts that originate from 126 populations of all eight Triturus species
The vertebral formulae of 1436 adult newts that originate from 126 populations of all eight species of Triturus newts. We determined the vertebral formula by counting the number of cervical (C), thoracic (T) and sacral vertebrae (S). The caudosacral and caudal regions (Cd) are excluded from our formula. Homeotic transformations of thoracic vertebra into sacral vertebra, or vice versa (transitional sacral vertebra having half of the identity of thoracic vertebra and half of the identity of sacral vertebra) were assigned 0.5 and this score was added to the number of complete thoracic vertebrae. Only complete changes of identity on one side of the vertebrae (on one side thoracic and on one side sacral) were declared transitional and scored. Collection abbreviations: IBISS - Institute for Biological Research "Siniša Stanković" Belgrade; ZMA.RenA - Naturalis Biodiversity Center, Leiden.
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