| Abstrakt: |
Differential staining patterns on amphibian chromosomes are in some respects distinct from those on mammalian chromosomes; C-bands are best obtained, whereas G- and Q-bands are either unobtainable (on anuran chromosomes) or coincide with C-bands (chromosomes of urodeles). In amphibians, rRNA genes are located at secondary constrictions, but in urodeles they are also found at other chromosome sites, the positions of these sites being strictly heritable. DNA content in amphibian cells is tens and hundreds times higher than in mammals. DNA contents in anurans and urodeles differ within certain limits: from 2 to 25 pg/N and from 30 to over 160 pg/N respectively. Species characterized by slow morphogenesis have larger genomes. Genome growth is normally due to an increase in the amount of repetitive DNA (mostly intermediate repetitive sequences), the amount of unique sequences being almost constant (11 pg/genome in urodeles, and 1.5 pg/genome in anurans). In anurans in general no satellite DNA was found, whereas such fractions were found in many Urodela species. Nucleosome chromatin structure in amphibians is identical to that of other eukariotes. It is postulated that differences in chromosome banding between amphibians and mammals are due to differences in chromatin packing which in turn is related to the distinct organization of DNA repetitive sequences. It is likely that fish chromosomes have a similiar structure. A comparison of such properties as the chromosome banding patterns, variations in nuclear DNA content and some genome characteristics enable us to group fishes and amphibians together as regards chromosome structure, as distinct from amniotes - reptiles, birds and mammals. It is probable that in the ancient amphibians - ancestors of reptiles - chromatin packing underwent a radical transformation, following changes in the organization of DNA repetitive sequences. [ABSTRACT FROM AUTHOR] |
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