No abstract

G-banding data are presented for a wide range of Australian marsupial species and one South American species, all of which have 2n= 14. The chromosome banding pattern in each of these species is very similar. Variations between species can be explained by structural changes internal to individual chromosomes. This evidence favors the hypothesis of a conserved complement common to both Australian and American marsupials and underlies the dominant role of chromosome fission in the evolution of this group.

This review includes a list of the chromosome numbers of marsupials and a summary of the main features of chromosome evolution in this group of mammals. Special topics discussed include sex chromosome mosaicism, the size of the marsupial X chromosome, X chromosomes and nucleolar organisers, complex sex chromosome systems, repeated DNA sequences and aspects of meiosis.

Thermeda australis (R.Br.) Stapf is a polyploid complex based on n = 10, and diploid, triploid, tetraploid, pentaploid, and hexaploid individuals have been found. Over 98 per cent. of more than 800 individuals examined were either diploid or tetraploid. Some 300 populations, from localities on the Australian mainland below the Tropic of Capricorn, were characterized by their chromosome number and a very clear pattern of distribution was found. Diploid populations occur exclusively on the Eastern Highlands and slopes in southern Victoria and in Tasmania; elsewhere tetraploid populations occur across to Western Australia. Triploid, pentaploid, and hexaploid plants are found as individuals in populations of another chromosome number. The cytological evidence shows a very close relationship to exist between the constituent genomes of the polyploids. The significance of the distribution of the chromosome races, the effects of polyploidy, and the implications of this pattern for further studies on the native flora are discussed.

Pha/a1'is roau/c8CeJ18 Des!'. is a completely Relf-incompatible, diploid, perennial gmss. incompatible pollen gmins germinate normally but the pollen tubes fail to penetrate far into tho 8Lyle. The ineompatibility interrelationships of parents and progenios were dotermined by olmen-ing the pollen tubes and pollen grains after pollination. This eould be clone successfully only in plants induced to Hower early by eXJlosure to 100tg day treatment. Incompatibility is controlled by two loci, each with a series of multiple alleles. The loci are probably not linked but the data would not. allow detection of loose linlmge. Pollen determination is gametophytic, and the genes aet indepoudelltly in the style. 'eho rolationship of this incompatihility system to previously described systems is eonsid(lred, and theoretically possible incompatihility systems based on genetical eOlltrol nt. two loci are suggested.