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This review is dedicated to comparative analysis of the early stages of thymus ontogeny in fish, amphibians, and mammals. Morphological and molecular-genetic aspects of the formation of thymic stroma, colonization of this organ with T-cell progenitors, and interaction of different cell populations in the course of organogenesis are considered. Particular attention is given to the hematopoietic role of the thymus during embryogenesis and new data on the origin of T-cell progenitors. The hypothesis about the possible presence in the organ of a self-sustaining population of stem cells, formed regardless of fetal hematopoiesis areas, is discussed.

The review provides modern classification of evolutionarily conserved coilin-containing nuclear bodies of somatic and germ cells that is based on the characteristic features of their molecular composition and the nature of their functions. The main differences between Cajal bodies and histone locus bodies, which are involved in the biogenesis of small nuclear spliceosomal and nucleolar RNAs and in the 3'-end processing of histone precursor messenger RNA, respectively, are considered. It is shown that a significant contribution to the investigation of the diversity of coilin-containing bodies was made by the studies on the architecture of the RNA processing machinery in oocyte nuclei in a number of model organisms. The characteristics features of the molecular composition of coilin-containing bodies in the nuclei of growing oocytes (the so-called germinal vesicles) of vertebrates, including amphibians and birds, are described.

Based on the PAAG disc-electrophoretic spectra of water-soluble total proteins (Pr) and non-specific esterases (Est) from several types of tissues in representatives of Urodela (Triturus vulgaris vulgaris, T. v. lantzi, T. cristatus, T. montandoni, T. dobrogicus) and Anura amphibians (Rana ridibunda, R. lessonae, R. temporaria, R. arvalis; Bufo bufo, B. viridis, Bombina bombina; Xenopus laevis, X. borealis; Hymenochirus boettgeri), mean genetic distances (D = -ln[2i/(2i + d)], where i is the number of fractions identical and d is the number of fractions different by Rf-bands, were estimated within and between taxa of intra-, inter-, and superspecific ranks. The most stable estimates were obtained for Pr and Est spectra of skeletal muscles and eyes. In most cases, the spectra of tissues of the digestive system gave higher D values than those of the other organs. The levels of intraspecific differentiation by D (both Pr and Est) were higher in the studied representatives of the order Urodela, probably due to their lower migration ability and more conservative choice of water-bodies for spawning than in the representatives of the other order; the same trend is true for the interspecific differentiation by Pr spectra. Est, on the contrary, gave lighter interspecific differentiation level in Anura than in Urodela, evidently due to the prevalence of sympatric species pairs able to hybridize among the studied representatives of the latter order.

Molecular mechanisms controlling the most important biological functions are highly conservative. This phenomenon can be seen in homeobox genes, which encode the regulatory proteins controlling basic developmental processes. The homeobox was first described about ten years ago in Drosophila melanogaster as a highly conservative gene region with characteristic structure. Later, homologous consequences were observed in many eukaryotic genes found in organisms ranging from yeast to humans. Biochemical, biophysical, and genetic studies have proved irrefutably that the homeobox controls a DNA-binding domain that allows proteins containing this domain to exert regulatory functions. The decisive role played by homeobox genes during development has stimulated great interest among researchers in this superfamily of genes. At present, more than 300 homeobox genes have been identified in animals of various taxa and their number is constantly growing. This review summarizes the published data about the structure and functions of homeobox genes and their role in development and regeneration.

An allozyme variation of loci-encoding lactate dehydrogenase was compared in different vertebrate classes. A lower level of heterozygosity in warm-blooded as compared to cold-blooded vertebrates was shown. The highest heterozygosity was revealed in anurous and caudate amphibians; the lowest, in birds and mammals. Fishes and reptiles exhibited an intermediate level of heterozygosity. In higher groups of vertebrates, differences in the electrophoretic mobilities between alleles decreased. A key aspect of this is a hiatus of these differences in fishes and mammals. On the basis of this analysis, the following conclusions may be drawn: (1) the rate of amino-acid substitutions in homologous proteins is unequal in distant phylogenetic lineages; (2) the level of heterozygosity is related to the average amount of electrophoretic allelic differences at the class level; (3) significant differences in variation pattern in the phylogenetic lineage of vertebrates is probably associated with the evolutionary features of the genomic organization of groups at different evolutionary levels.

Differential expression of homeobox genes of the Six family, homologues of Drosophila gene sine oculis, was revealed during limb regeneration of ribbed newt Pleurodeles waltlii and common frog Rana temporaria. Expression of these genes correlated to regeneration capacity in the studied animals. The primary structure of the most conservative region in the identified genes has been determined. We assessed the differences in the primary structure of Six family genes obtained from regenerating limbs of the newt and frog (86 and 94.9% similarity at the nucleotide and amino acid levels, respectively). Comparison of the obtained and published sequences of Six family genes has demonstrated that they are most similar to mouse gene Six1 (84.2 and 82.8% nucleotide sequence similarity in the frog and newt, respectively, and 99.2 and 94.1% amino acid sequence similarity in the frog and newt, respectively.

Cytogenetic investigations of amphibian and rodent populations from the contaminated by radionuclides regions of Belarus was conducted. A significant increase in the levels of cytogenetic anomalies in bone marrow cells and intestinal epithelium of amphibians and rodents, the levels of alveolar macrophages of rodents, and micronuclei in peripheral blood erythrocytes of amphibians was revealed. Presence of chromosome type aberrations indicates that radiation as a mutagenic factor was involved. No strict relationship between the levels of contamination of the area and levels of cytogenetic anomalies was found.

This review deals with the authors' own and literary data on the ultrastructural and cytochemical organization of insect and amphibian oocyte nuclei, with special attention being paid to the karyosphere and its capsule. The evidence provided is supplemented with data on isolated karyospheres in Rana temporaria oocytes. A conclusion is made that the karyosphere is a complex structure which contains all chromosomes in the limited space of the oocyte nucleus, and that these chromosomes are, as a rule, in the process of inactivation. It is inferred that the karyosphere capsule commonly appears in gigantic oocyte nuclei (more than 100 micron in diameter) containing extrachromosomal DNA. The analogy of capsule organization in different invertebrate and vertebrate species is discussed, in addition to the involvement of presumably homologous nuclear structures (e.g. derivatives of synaptonemal complexes and nuclear envelope) in capsule formation. It is assumed that the karyosphere capsule is a specially organized part of the nuclear matrix. The capsule provides nuclear compartmentalization and chromosome localization in the germinal vesicle. Studies of this sort open up new possibilities to further investigation of intranuclear morphogenesis.

The sequences of two regions flanking the 5'D1 domain of 28S rRNA of 13 vertebrate species were determined by direct rRNA sequencing through reverse transcriptase extension of DNA primers. Comparative treatment of these new data and previously reported rRNA sequences was undertaken with special reference to phylogenetic affinity of Amphibia by using some programs of Felsenstein's PHYLIP 3.3 package. The results obtained suggest that Amphibia is rather a biphyletic than a monophyletic group, and that birds and mammals are the closest relevant. These data were compared with those obtained on Vertebrata by using an analogous comparative analysis of 18S rRNA sequences.