Summary. The

Nucleolar organiser regions (NORs) were examined in over 90~ of the species of Australian ground frogs (familiy Myobatrachidae), including representatives from all twenty currently recognised genera and the three subfamilies. Throughout the family, location of the NOR within the karyotype showed considerable variation yet karyotype morphology showed uniformity. The precise mechanism(s) whereby variation in NOR location evolved while karyotype morphology was unchanged remains uncertain. Comparison of the two major subfamilies showed that the Limnodynastinae had a greater diversity of NOR location than the Myobatrachinae.The limnodynastine genus, Heleioporus, was the only one to show multiple NOR sites in several species.NOR location was particularly stable within most polytypic genera. Differences in NOR location within the remaining polytypic genera (Heleioporus, Limnodynastes, Neobatrachus, Philoria'and Ranidella) pointed to taxonomic discriminations that were generally consistent with recent proposals based on other criteria.

Karyotypic analysis of six species of the Australian leptodactylid frog genus Neobatrachus showed that N. pictus, N. centralis, N. pelobatoides and N. wilsmorei are diploid (2n = 24) while N. sudelli and N. sutor are tetraploid (4n = 48). Polyploidy has not been reported previously among Australian anurans. Idiograms of the six species indicate that they are similar to the other Australian leptodactylids so far discribed. DNA values of the tetraploids are approximately double the values for diploids. Tetraploid nuclear and cell sizes are greater compared with diploids but total body size shows no increase. At diakinesis in primary spermatocytes of tetraploids, mainly tetravalents together with a few bivalents are present. Silver straining of metaphase spreads clearly demonstrates the location of NORs at the secondary constrictions and their frequent association in the tetraploid N. sutor. Nucleolar number in interphase nuclei provides a reliable guide for distinguishing tetraploid from diploid frogs in the absence of chromosome analysis and can be determined for both living and preserved specimens. The possible origins and relationships of the tetraploid species are discussed.

The chromosome complements of four specimens of the anatomically primitive frog Leiopelma hochstetteri are described from cultured cells and squashes. The basic karyotype in all cases consists of 22 chromosomes, 12 of which are acrocentric. Supernumerary chromosomes are either absent or variable in number, but appear to be constant in the somatic cells of any one individual. The limited evidence available suggests that the supernumerary chromosomes do not pair during male meiosis.The karotype of L. archeyi is described for the first time. Only the smallest pair of the total complement of 18 chromosomes is acrocentric. Supernumeraries are absent.The distribution and probable relationships of the species of Leiopelma are discussed. The karyotypes of Leiopelma and the North American ascaphid frog Ascaphus truei are compared, with particular reference to the relationship of the supernumerary chromosomes of Leiopelma and the microchromosomes of Ascaphus.

Marsupial (metatherian) mammals resemble their eutherian ('placental') counterparts in having inacti- vation of one of the two X chromosomes in the soma and premeiotic germ cells of their females. The marsupial X-inactivation system differs from the eutherian system in two respects: firstly, inactivation occurs for the paternally derived allele, i.e. it is not random, and secondly it is often incomplete. Data are available for four X-linked loci, all controlling enzyme structure: glucose-6- phosphate dehydrogenase (G6PD), phosphoglycerate kinase 1 (PGKl), alpha-galactosidase (GLA) and hypoxanthine phosphoribosyl transferase (HPRT). Both the G6PD and PGKl loci exhibit incomplete X-chromosome inactivation. The pattern of partial expression differs from tissue to tissue and from species to species. One of the two X chromosomes exhibits late replication, even in cells where a paternally derived gene is partly active, showing that late replication and absence of transcription are not completely correlated. Sex chromatin bodies are not as easily found as in some eutherians. In marsupials they are most clearly demonstrable in species with small Y chromosomes. Investigations into X-inactivation in early development have just begun. Absence of inactivation at the G6PD locus in yolk sac of a kangaroo has been observed. All other tissues exhibited complete paternal X-inacti- vation for G6PD. In a dasyurid, GLA showed complete paternal X-inactivation in all embryonic and extra-embryonic tissues. The role, if any, of methylation of cytosine residues in CpG pairs in the maintenance of X-inactivation in marsupials is unclear. Preliminary evidence indicates that sex-specific differences in methylation of sex linked genes do exist in marsupials.