Relationships between phylogenetic relatedness, hybrid zone spatial structure, the amount of interspecific gene flow and population demography were investigated, with the newt genus Triturus as a model system. In earlier work, a bimodal hybrid zone of two distantly related species combined low interspecific gene flow with hybrid sterility and heterosis was documented. Apart from that, a suite of unimodal hybrid zones in closely related Triturus showed more or less extensive introgressive hybridization with no evidence for heterosis. We here report on population demography and interspecific gene flow in two Triturus species (T. macedonicus and T. ivanbureschi in Serbia). These are two that are moderately related, engage in a heterogeneous uni-/bimodal hybrid zone and hence represent an intermediate situation. This study used 13 diagnostic nuclear genetic markers in a population at the species contact zone. This showed that all individuals were hybrids, with no parentals detected. Age, size and longevity and the estimated growth curves are not exceeding that of the parental species, so that we conclude the absence of heterosis in T. macedonicus–T. ivanbureschi. Observations across the genus support the hypothesis that fertile hybrids allocate resources to reproduction and infertile hybrids allocate resources to growth. Several Triturus species hybrid zones not yet studied allow the testing of this hypothesis.
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 &
Background In amphibians, thyroid hormone (TH) has a profound role in cranial development, especially in ossification of the late-appearing bones and remodeling of the skull. In the present study, we explored the influence of TH deficiency on bone ossification and resulting skull shape during the ontogeny of Triturus newt hybrid larvae obtained from interspecific crosses between T. ivanbureschi and T. macedonicus. Methods Larvae were treated with two concentrations of thiourea (an endocrine disruptor that chemically inhibits synthesis of TH) during the midlarval and late larval periods. Morphological differences of the cranium were assessed at the end of the midlarval period (ontogenetic stage 62) and the metamorphic stage after treatment during the late larval period. Results There was no difference in the ossification level and shape of the skull between the experimental groups (control and two treatment concentrations) at stage 62. During the late larval period and metamorphosis, TH deficit had a significant impact on the level of bone ossification and skull shape with no differences between the two treatment concentrations of thiourea. The most pronounced differences in bone development were: the palatopterygoid failed to disintegrate into the palatal and pterygoid portions, retardation was observed in development of the maxilla, nasal and prefrontal bones and larval organization of the vomer was retained in thiourea-treated larvae. Conclusions This implies that deficiency of TH caused retardation in development and arrested metamorphic cranium skeletal reorganization, which resulted in divergent cranial shape compared to the control group. Our results confirmed that skull remodeling and ossification of late-appearing bones is TH–dependent, as in other studied Urodela species. Also, our results indicate that TH plays an important role in the establishment of skull shape during the ontogeny of Triturus newts, especially during the late larval period and metamorphosis, when TH concentrations reach their maximum.
Serial homology or the repetition of equivalent developmental units and their derivatives is a phenomenon encountered in a variety of organisms, with the vertebrate axial skeleton as one of the most notable examples. Serially homologous structures can be viewed as an appropriate model system for studying morphological integration and modularity, due to the strong impact of development on their covariation. Here, we explored the pattern of morphological integration of the cranium and the first three serially homologous structures (atlas, first, and second trunk vertebrae) in salamandrid salamanders, using micro‐CT scanning and three‐dimensional geometric morphometrics. We explored the integration between structures at static and evolutionary levels. Effects of allometry on patterns of modularity were also taken into account. At the static level (within species), we analyzed inter‐individual variation in shape to detect functional modules and intra‐individual variation to detect developmental modules. Significant integration (based on inter‐individual variation) among all structures was detected and allometry is shown to be an important integrating factor. The pattern of intra‐individual, asymmetric variation indicates statistically significant developmental integration between the cranium and the atlas and between the first two trunk vertebrae. At the evolutionary level (among species), the cranium, atlas, and trunk vertebrae separate as different modules. Our results show that morphological integration at the evolutionary level coincides with morphological and functional differentiation of the axial skeleton, allowing the more or less independent evolutionary changes of the cranial skeleton and the vertebral column, regardless of the relatively strong integration at the static level. The observed patterns of morphological integration differ across levels, indicating different impacts of developmental and phylogenetic constraints and functional demands.
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