The primitive New Zealand frog Leiopelma hochstetteri shows exceptional karyotypic variation. On the North Island, females carry univalent W chromosome and both sexes have 0 to 15 supernumerary chromosomes. Frogs from Great Barrier Island have a conventional 2n = 22 karyotype, with no sex chromosome differentiation in C-banded mitotic chromosomes. However, the lampbrush chromosomes of a Great Barrier Island female show evidence of heterogamety. This suggests that presumed ancestral female heterogamety has persisted on Great Barrier Island and given rise to a WZZ-female/ZZ-male sex chromosome system on the North Island. / A repeated sequence, Lh1, varies greatly within populations in copy number and distribution on genomic EcoRI fragments. In situ hybridization revealed the variable presence of large Lh1 arrays on supernumerary chromosomes and two autosomes. The extensive Lh1 variability implies that, like the supernumeraries, it is more selfish than functional in the L. hochstetteri genome.

The endemic New Zealand frog, Leiopelma hochstetteri, is characterized by variable numbers of mitotically-stable B chromosomes. In order to assess whether the B chromosomes had been derived from the autosome complement, B DNA was isolated and amplified by micromanipulation in conjunction with degenerate oligonucleotide-primed PCR. Southern hybridization patterns of B DNA probes to genomic DNA from males and females characterized by differing numbers of B's demonstrated that the B chromosomes were derived from the univalent W chromosome which is specific to females. The presence of homologous B specific sequences in B chromosomes from geographically-distinct populations show that only a single univalent W to B event had occurred. Furthermore, a plesiomorphic homology shows that the B chromosomes originated soon after the univalent W had been derived from the ancestral WZ/ZZ karyotype, which is still present in frogs from Great Barrier Island. Finally, sequence analysis of the probes reveals that B DNA is composed of repeat sequences, and has the ability to form stable hairpin structures in vivo. The molecular dynamics of these structures may reflect the inherent propensity to undergo rapid change in nucleotide sequence and chromosome structure.

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