Chromosomal rearrangements in rock wallabies, Petrogale (Marsupialia: Macropodidae). VIII. An investigation of the nonrandom nature of karyotypic change

Abstract

Although the nonrandom nature of many chromosome breaks is well known, few studies have investigated the potential significance of this breakage specificity in chromosome evolution. The macropod genus Petrogale is an ideal group in which to investigate this phenomenon, since it comprises a large number of chromosomal forms, many of which appear to have differentiated relatively recently. By exposing Petrogale cells to mutagenic agents it should be possible to compare the distribution and abundance of induced breaks with those that are known to have occurred in vivo during the chromosomal differentiation of the genus. In this study, breaks were induced in mitotic chromosomes from P. assimilis and P. mareeba by exposing synchronized cultured fibroblasts to low doses of gamma radiation. The results were remarkably similar for both species and the distribution of breaks among the chromosomes appeared to be nonrandom. It was found that chromosomes 5, 6, and (possibly) 10 had a substantially higher rate of breakage than expected. These are also the chromosomes that occur disproportionately among the rearrangements identified in Petrogale. While the distribution of breaks along the chromosome appeared uniform or normal for most chromosomes, a putative "hot spot" was identified near the centromere in chromosome 5 of P. mareeba and in a homologous position near the telomere of chromosome 5 in P. assimilis. In a further experiment, a 1- to 2-h pulse of mitomycin C was used to induce centric fusions in cultured fibroblasts of P. penicillata (2n = 22); 2408 cells were examined and 112 fusions were identified. While it was found that all chromosomes participated in forming fusions, chromosome 10 was found to be most frequently involved, being present in 28.6% of the identifed fusions. This frequency is far greater than would be expected if fusions were to occur at random (10%). It is significant then that chromosome 10 has been involved in five of the eight centric fusions that have been identified in Petrogale and that it is also the chromosome that has been most frequently rearranged in Petrogale. These results suggest that features of the karyotype may influence the distribution and frequency of chromosome breaks and therefore the rate and nature of chromosome evolution.Key words: Petrogale, chromosome evolution, irradiation, induced rearrangements, mitomycin C.