Karyotype of Araucaria angustifolia and the decondensation/activation mode of its nucleolus organiser region

The chromosomes of the gymnosperm Araucaria angustifolia (Bertol.) Kuntze were analysed with the fluorochromes chromomycin A3 (CMA) and 4′,6-diamidino-2-phenylindole (DAPI) and with C-banding. This species contains a diploid complement made of 26 chromosomes, with 18 larger metacentric, four smaller metacentric and four submetacentric chromosomes. The only CMA+/DAPI– region observed corresponded to the nucleolus organiser region (NOR) localised at the proximal portion of a large metacentric chromosome pair. C-banding marked the NOR as well as a terminal region of another chromosome pair. In addition, small C-bands were occasionally seen interspersed in many chromosomes. The NOR appeared to condense at approximately the same rate as the rest of the chromosome from prophase throughout metaphase. In interphase nuclei, NOR decondensation and activation was characterised by the formation of CMA+ blocks that resembled a string of beads inside the nucleolus. The number and size of beads was inversely proportional to the size of the nucleolus, suggesting that transcriptional activation of the nucleolar cistrons starts simultaneously at several points of the NOR. The mode of NOR activation in A. angustifolia differs from that observed in most species, providing a unique opportunity to study activation and transcriptional control of rRNA genes.

Heteromorphic sex chromosomes in the Australian frog Crinia bilingua (Anura: Myobatrachidae)

The karyotype of Crinia bilingua was examined and analysed with standard staining, C-banding, and silver-staining. Heteromorphic sex chromosomes of the ZW ♂/ZZ ♀ type were observed. The larger W chromosome is submetacentric and the smaller Z chromosome is acrocentric. The centromere and proximal region of the short arm of the W chromosome consist of constitutive heterochromatin (C-band region), and beyond this is a small euchromatic terminal region. The centromere of the Z chromosome did not C-band. The long arms of the Z and W chromosomes are euchromatic and equal in length. The nucleolar organiser region occurs terminally on the long arm of both the Z and W chromosomes, and there is no cytological evidence for inactivity of the nucleolar organiser region on the W chromosome. These features indicate an early stage in the evolution of heteromorphic sex chromosomes.Key words: heteromorphic sex chromosomes, frog, Crinia bilingua.