Structure and evolution of B chromosomes in amphibians.
B chromosomes are known from 26 species of salamanders and frogs, equivalent to about 2% of amphibian species that have been karyotyped. In most cases, the structure of amphibian B chromosomes has not been extensively investigated. The exceptions are the B chromosomes of Hochstetter's frog, Leiopelma hochstetteri, from New Zealand, and the Coastal Giant salamander, Dicamptodon tenebrosus, from North America. Dicamptodon tenebrosus carries from 0 to 10 non-heterochromatic, telocentric B chromosomes per individual, averaging 0 to 3.4 B chromosomes per individual in populations throughout its extensive range. The B chromosomes of L. hochstetteri occur in frequencies averaging from 0 to 11.4 per individual in different populations, with a known maximum of 15 B chromosomes. Amphibian B chromosomes vary in size, heterochromatin, occurrence and frequency. They are commensurate in size and structure with the rest of the A set of chromosomes of the same species in which they occur. The B chromosomes are at least partly composed of repetitive DNA sequences which exist in numerous copies throughout the autosomes, in conformity to an hypothesis of intraspecific B chromosome origins.
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Redpath Museum, McGill University, Montreal, Quebec, Canada. david.m.green@mcgill.ca
D000663:Amphibians / Q000235:genetics*; D000818:Animals; D002875:Chromosomes / Q000737:chemistry* / Q000235:genetics*; D019143:Evolution, Molecular