Evolucija ekstremiteta velikih mrmoljaka (Triturus cristatus complex, Salamandridae, Caudata)
Limb evolution in crested newts (Triturus cristatus complex, Salamandridae, Caudata)
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Integration, modularity and developmental constraints are three related components of phenotypic variability. Their impacts on morphological evolution on limbs in five species of crested newts were analyzed in this thesis. Crested newts are valuable model organisms in studies of morphological integration and developmental constraints, especially when concern limbs. They are closely related group of species with known phylogenetic relations, and each characterized by a specific body-to-limb conformation associated with their respective ecologies. Aquatic species of newts have small, elongate bodies and small limbs while more terrestrial species are with large, stocky bodies and robust limbs. Also, as group with complex life cycle, where larval, juvenile and adult stage have different morphologies, ecologies and different diet and locomotor preferences, crested newts have special place in studies of changes in ontogenetic trajectories. Ontogenetic niche change and transition between aquatic and terrestrial environment is connected with process of metamorphosis. Although limbs in newts do not undergo drastic changes in structure during that process, functional demands for high efficient locomotion in both environments are quit different. It is assumed that the differences between species in degree of aquatic and terrestrial locomotion may be the factor which can attribute to changes in length of skeletal elements and different patterns of correlations between limb elements, reflecting adaptations on different environments and life styles of analyzed species of crested newts. Analysis of morphological integration and modularity has found different aspects of variation patterns in limbs of crested newts. Equivalent ontogenetic stages of different species of crested newts show higher concordance in the correlation pattern than successive ontogenetic stages within species, with highest level of integration in juvenile stage. High integration in juveniles is function of the rapid, coordinated growth of the limb skeleton during this time period, in which systematic hormones are thought to play a major role. While, low integration in adult stage are probably due to many locally and globally-systematic factors which change effect one to another in time and space, although biphasic life cycle with different locomotion modes have decisive role in ontogenetic changes of morphological integration and modularity. Differences in patterns of integrations between aquatic and more terrestrial species of newts were observed, with lower correlation between homologous limb elements and higher correlation within-limb elements in more terrestrial species, the reverse pattern occurs in more aquatic species. This study showed important role of developmental constraints in creating phenotypic variability in limbs of crested newts. Comparative analyses revealed some synapomorphic heterochronic shifts specific to crested newts, including delay of the ossification in the second finger and accelerations in metacarpal III and metatarsal V. These shifts involve a change from pre-axial to post-axial dominance in a developmental sequence uncommon to caudate salamanders. No adaptive explanation of these shifts is apparent. Allometric analyses showed two important results: specific growth patterns in T.dobrogicus and similar growth pattern after metamorphosis in all species of crested newts. These results are reflecting impact of natural selection as well as internal developmental mechanisms which regulated variation in length of limbs during morphological diversification of crested newts. All studied species shared the same basipodial pattern, within which the carpus exhibited much more variation than the tarsus. Important point in this study is lost of one carpal element (centrale 1). Specific pattern observed in T. dobrogicus is followed with low ossification and loose package of carpale elements. It is suggested that the uniqueness of T. dobrogicus originates from heterochronic process. Results of this study showed important influence of morphological integration, modularity and developmental constraints in creating phenotypic variability in limbs of crested newts.
Keywords:Limbs; Crested newts; Morphological integration and modularity; Sequence heterochrony; Allometric heterochrony
Source:University of Belgrade, Faculty of Biology, 2013, 1-111
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