Analysis of repetitive DNA sequences in the Mexican axolotl Ambystoma mexicanum

The mexican axolotl Ambystoma mexicanum is a member of a family of salamanders with large nuclear genomes. The known geographical distribution and evolutionary history of the genus Ambvstoma makes it an ideal model system for the study of the evolution of repetitive DNA sequences in large genome animals. The genome of the axolotl has been examined by various methods, in particular by reassociation kinetic analysis and associated techniques on bulk DNA and by the molecular cloning of several members of the repetitive fraction of the genome. The haploid genome (37 pg) can be resolved into several distinct components. A foldback fraction (4.6%), a moderately repetitive fraction (43.3%), a slow repetitive fraction (20.4%) and a unique fraction (31.7%). Foldback regions appear to form randomly dispersed clusters. Members of the moderately repetitive fraction are predominantly interspersed amongst other members of the same fraction. At moderate fragment length only (57.5%) of the genome is arranged as moderately repetitive DNA interspersed with more slowly repeating sequences. Fragments of axolotl DNA have been cloned by recombinant DNA techniques. Clones containing axolotl DNA were selected. One clone (pAMW113l) has an apparent repetition frequency of 1480 in the axolotl genome and may be part of a larger fragment undergoing reamplification in the genome. A second clone (pAMW1184) is highly repeated with a short repeat length. Some family members appear to be scattered through the genome. Some members are transcribed on lampbrush chromosomes. A third clone (pAMW1199) has several features characteristic of bulk repetitive DNA. It has a complex organisation in the genome and may contain a number of shorter repeats present in the genome at different frequencies. These clones represent different aspects of the organisation of the axolotl genome. Together with the general genome analysis they provide a starting point for comparative evolutionary analyses of the genus Ambvstoma