Polyploidy and Ribosomal-DNA Spacer Variability in Poa labillardieri

Abstract

With changing agricultural practices, poa tussock (Poa labillardieri Steud.) has become a serious weed of Australian tableland pastures. To determine whether there are any genetic differences within the species that might have a bearing on this problem, accessions of P. labillardieri from seven localities in south-eastern Australia were grown in a field nursery at Bathurst for comparison of their morphological attributes. Seeds from plants grown in this nursery were collected, and a further generation of plants was grown in a glasshouse for cytogenetic and molecular genetic studies. In adult plants of this generation, there were three karyotypic groups with chromosome numbers varying from 2n = 4x = 28, in nonglaucous tussocks of the Upper Hunter Valley, to 2n = 8x = 56, in glaucous tussocks from the Central and Southern Tablelands, to 2n = 12x = 84, in a Victorian coastal tussock. Because the number of chromosomes present in some seedlings from Cooma parents varied from 35 to 49 to 56, diploid or hexaploid plants must also have been present in the Bathurst nursery and contributed to the odd-ploidy seedling chromosome numbers.

Ribosomal spacer DNA fragments were investigated using the wheat probe pTa250.4 to label homologous DNA sequences in genomic DNA extracted from the leaves of individual plants of P. labillardieri. Although specific DNA fragments could not be linked to either localities or levels of ploidy, they did show evidence of cross-pollination between plants of the different accessions. These levels of cytogenetic and DNA sequence variability, and the evidence they provide of widespread outcrossing within plants of similar ploidy and of hybridisation between plants of differing ploidy, may well help to explain the success of this species as a weed and affect the control measures used to stop its spread.