Sergei V. Smirnov

Thyroid responsiveness of the large African barb Labeobarbus intermedius (Teleostei; Cyprinidae): Individual variability and morphological consequences

Thyroid hormones (THs), produced by the thyroidgland, play an important role in the regulation of ontog-eny in bony fishes. They influence the rate of individualdevelopment of the body as a whole, as well as the ratesof growth and differentiation of its particular structures[1]. Definitive morphology of fishes is a result of inter-action between the developing body and a hormone,and depending on the TH level and an ability of thebody to respond to TH, i.e., TH-responsiveness.The TH level depends on the functional activity ofthe thyroid gland. A low activity and, as a result, lowTH levels lead to slowing down the ontogeny. On thecontrary, high TH levels accelerate the ontogeny [1, 2].Differences in the activity of the thyroid gland canresult in variability of the expression of morphologicalfeatures [2]. Recently, indirect data on individual vari-ability of TH-responsiveness in the large African barbLabeobarbus intermedius were reported. They demon-strated that juveniles of this fish differently respondedto similar doses of exogenous THs under experimentalconditions [3, 4]. Taking into account an important roleof TH-responsiveness in the ontogeny control, we stud-ied individual variability of TH-responsiveness and itsinvolvement in intraspecific morphological diversifica-tion in fishes.We compared morphological changes inL. intermedius siblings grown at a high level of exoge-nous TH. We focused our attention on the serial struc-tures that usually decrease in number when theL. intermedius siblings are growing under a high THlevel [3, 4].The eggs were obtained from the crossing ofL. intermedius spawners taken from a Lake Tana tribu-tary, the Gumara River (Ethiopia) and divided into twogroups. The first experimental group was put into amedium with a high TH content, namely, 2 × 10

Frontoparietal Bone in Extinct Palaeobatrachidae (Anura): Its Variation and Taxonomic Value.

Palaeobatrachidae are extinct frogs from Europe closely related to the Gondwanan Pipidae, which includes Xenopus. Their frontoparietal is a distinctive skeletal element which has served as a basis for establishing the genus Albionbatrachus. Because little was known about developmental and individual variation of the frontoparietal, and its usefulness in delimiting genera and species has sometimes been doubted, we investigate its structure in Palaeobatrachus and Albionbatrachus by means of X‐ray high resolution computer tomography (micro‐CT). To infer the scope of variation present in the fossil specimens, we also examined developmental and interspecific variation in extant Xenopus. In adults of extinct taxa, the internal structure of the frontoparietal bone consists of a superficial and a basal layer of compact bone, with a middle layer of cancellous bone between them, much as in early amphibians. In Albionbatrachus, the layer of cancellous bone, consisting of small and large cavities, was connected with the dorsal, sculptured surface of the bone by a system of narrow canals; in Palaeobatrachus, the layer of cancellous bone and the canals connecting this layer with the dorsal surface of the frontoparietal were reduced. The situation in Palaeobatrachus robustus from the lower Miocene of France is intermediate—while external features support assignment to Palaeobatrachus, the inner structure is similar to that in Albionbatrachus. It may be hypothesized that sculptured frontoparietals with a well‐developed layer of cancellous (i.e., vascularized) bone may indicate adaptation to a more terrestrial way of life, whereas a reduced cancellous layer might indicate a permanent water dweller. Anat Rec, 298:1848–1863, 2015.

Influence of thyroid hormone on the sequence of cranial bones appearance in early ontogeny of the large African barb ( Labeobarbus intermedius ; Cyprinidae; Teleostei)

224 The sequence of bone appearance in fish cranio genesis is a species specific characteristic. Deviations from the species specific sequence are believed to be expression of individual variability during ontogeny [1–3]. These fluctuations [2] are supposed to be caused by internal (genetic) and external (tempera ture, diet, etc.) factors; nevertheless, their mechanism and sources of origin are still obscure.

Thyroid hormones in the skeletogenesis and accessory sources of endogenous hormones in Xenopus laevis (Amphibia; Anura) ontogeny: Experimental evidence

Skeletal development was studied in normal and goitrogen-treated Xenopus laevis tadpoles reared under thyroid hormone (TH) deficiency. Early stages of skeletal development proceed similarly in both groups. Later stages are retarded or completely arrested in goitrogen-treated tadpoles. After goitrogen-treated tadpoles were transferred into pure water or into a medium containing both goitrogen and exogenous TH, tadpoles resumed development. Consequently, late stages of skeletogenesis are TH-dependent and TH-induced. Athyroid X. laevis “giant tadpoles” described in literature differ from goitrogen-arrested tadpoles in that they have features which require TH to appear. The appearance of TH-depended features in giant tadpoles indicates the occurrence of the additional sources of TH other than thyroid gland.

Role of heterochronies in the morphogenesis of amphibian skull bones: An experimental study

Heterochronies, i.e., changes in the relative timingof the development of different morphological struc-tures, are widespread in amphibians, which makesthese animals a suitable object for studying both naturaland experimental heterochronies. Temporal parametersof skull morphogenesis, including the timing and rateof the development of cranial elements, are affected bymany factors, including endocrine ones, the main ofwhich are hormones synthesized by the thyroid glandor thyroid hormones (THs). The use of exogenous THsand goitrogens (substances suppressing the thyroidsecretion, thereby decreasing the plasma content ofendogenous THs) allows artificial simulation of hetero-chronies.Experimental heterochronies are expressed in alter-ations of the rate and order of the formation of skullbones depending on the hormonal background of theanimal development. For example, exogenous THsaccelerate bone growth, whereas goitrogens, con-versely, retard it. In addition, high doses of THscause premature development of the skull ossifica-tions that normally occur at the ontogenetic stagesclose to metamorphosis, and goitrogens retard thedevelopment of these bones or prevent their appear-ance altogether [1–3].The study of the role of THs in the control of cran-iogenesis in various species of Urodela and Anura asdescribed in [1–3] demonstrated that THs and goitro-gens could alter not only the rate and timing of thedevelopment of skull bones, but also the morphogeneticpattern of some bones.For example, a dermal bone that is located in thenasal region of the skull of the Siberian salamanderSalamandrella keyserlingi (Urodela, Hynobiidae) andis traditionally termed the nasale develops from twoossification centers, one of which appears in the tectumnasale, and the other, on the tectum internasale cartilagemedial to former. According to Lebedkina [4], they arehomologous to the nasale and postrostrale of Crossop-terygii, respectively. Both ossification centers soon fusebehind the apex of the ascending process of the prae-maxillare to form a single bone, the naso-postrostrale.However, the naso-postrostrale of animals growingunder the conditions of an enhanced hormonal back-ground (treated with exogenous THs) [1] is morpholog-ically typical but develops from a single ossificationcenter.The frontoparietale of the common frog Rana tem-poraria (Anura, Ranidae) normally develops from oneossification center. Thiourea (TU, a goitrogen) retardsits appearance, the retardation being longer, the higherthe goitrogen concentration. In some of animals kept ina 0.02% TU solution, two ossification centers appear atthe site of the future frontoparietale. In animals kept in0.04% TU, the frontoparietale always develops fromtwo separate centers, which partly fuse later in ontog-eny [3].The dermal bone angulospleniale, part of the mandi-ble of

Comparative analysis of skull development in African barbs (Labeobarbus (=Barbus); Cyprinidae; Teleostei) from Lake Tana (Ethiopia)

14 Large African barbs of Labeobarbus (=Barbus) genus inhabiting Lake Tana (Ethiopia) comprise a flock of 15 species differing greatly in external mor phology. Comparative analysis of craniogenesis was carried out in three barbs’ species: in putative ancestral L. intermedius, in small sized L. brevicephalus, and in large sized L. megastoma. No interspecific differences were revealed in the development of chondrocranium and in the sequence of events in the development of osteocranium. However, differences in the temporal parameters of osteocranial development were revealed: acceleration in L. brevicephalus and retarda tion in L. megastoma relative to the ancestral L. inter medius. Temporal shifts revealed in the development of osteocranium lead to chronological changes in the cartilage–bone intertissue inductive interaction and changes in the spatial organization of developing skull as a whole; changes which affect the further skull development and its definitive morphology.