Some of the largest B chromosomes so far discovered in vertebrates are present in the cyprinid fish Alburnus alburnus. Previous cytogenetic analyses revealed a diploid chromosome number of 2n = 50. In addition, in some individuals one or two unusually large B chromosomes are present. Two morphologically different types of B chromosomes were observed. The frequency of animals bearing a supernumerary chromosome was found to vary considerably between different populations. A more detailed analysis of the A and B chromosomes of A. alburnus by conventional banding techniques, as well as fluorescence in-situ hybridization (FISH) with the telomeric DNA repeats (GGGTTA)7/(TAACCC)7, 18S + 28S rDNA and 5S rDNA were performed in the present study. Furthermore, a B chromosome-specific DNA probe obtained by amplified length polymorphism (AFLP) was hybridized on metaphases of A. alburnus carrying supernumerary B chromosomes. The banding analyses showed that the B chromosomes are completely heterochromatic, consist of GC-rich DNA sequences, replicate their DNA in the very late S-phase of the cell cycle and are composed mainly of a specific retrotransposable DNA element. Finally, blood probes from A. alburnus were collected for DNA-flow cytometric measurements. It could be shown that the huge supernumerary chromosomes represent nearly 10% of the total genome size of A. alburnus.

The cyprinid fish Alburnus alburnus possesses one of the largest supernumerary chromosomes in all vertebrates. In the present study, amplified fragment length polymorphism analyses (AFLP) and fluorescence in-situ hybridization (FISH) were performed in order to characterize these extraordinary chromosomes in detail. Sequence analysis of the B chromosome-specific DNA revealed a strong homology to a Drosophila Gypsy/Ty3 retrotransposon and also to a medaka (Oryzias latipes) one. The sequence is highly abundant on the B chromosome but undetectable in the normal A chromosome complement. It is also absent from the B chromosome of the closely related species, Rutilus rutilus, suggesting a specific spreading of the mobile element during evolution of the giant supernumerary chromosome within A. alburnus. Meitotic chromosomes were in-situ hybridized with the B chromosome-specific probe, documenting that the additional chromosome behaves as an autopaired ring chromosome in diakineses. Our results suggest that the supernumerary chromosome of A. alburnus is not derived from the normal chromosome complement but has evolved independently.

frog) 205 Mixophyes fasciolatus, M. schevilli (Australian ground frog) 239 mouse 7, 51, 62, 95, 100,

The mitotic chromosomes of an Ecuadorian population of the marsupial frog Gastrotheca espeletia were analyzed by means of banding techniques and fluorescence in situ hybridization. This species is characterized by unusual supernumerary (B) chromosomes. The maximum number of B chromosomes is 9 and they occur in three different morphological types. Banding analyses show that the B chromosomes are completely heterochromatic, consist of AT base pair-rich repeated DNA sequences, replicate their DNA in very late S-phase of the cell cycle, and are probably derived from a centromeric or paracentromeric region of a standard (A) chromosome. Exceptionally, the B chromosomes carry 18S + 28S ribosomal RNA genes and the conserved vertebrate telomeric DNA sequence appears to be underrepresented. Flow cytometric measurements of the nuclear DNA content differentiate between individuals with different numbers of B chromosomes. Significantly more B chromosomes are present in female than in male animals.