Abstract:
Most eukaryotes have monocentric chromosomes with a single centromere but large groups, such as the Cyperaceae and Juncaceae amongst the plants, have holocentric chromosomes. Holocentric chromosomes have centromeric activity distributed in a diffuse fashion along their entire length. Thus, chromosome organization and evolution may differ greatly as a consequence of the two types of chromosome organization. The genus Schoenus (Cyperaceae) contains approximately IOO species. Most species are found in Australia with nine native to New Zealand and a small number in Malaysia, South Africa and temperate regions of the Northern Hemisphere. Phylogenetic trees based on sequence variation in the internal transcribed region of the 45S rDNA locus were constructed using several phylogenetic models to reveal possible evolutionary relationships among the New Zealand species and a sample of the Australian species. Two major clades could be distinguished. One contained the species with large numbers of very small chromosomes that was ancestral to the other that comprised the species with small numbers of large chromosomes. The results also suggest that Schoenus in New Zealand has arrived by Iong distance dispersal from Australia. The New Zealand species are Chromosomally very variable. Chromosome number ranged from 2/7=8 to 2«=c.9O and there is considerable variation in total chromosome length. Flow cytometry using propidium iodide stained nuclei was used to measure genome size. A 12.8-fold variation in 2C DNA content was found, the values ranged from 1.54 pg to 19.71 pg. There was no obvious relationship between C-value and chromosome number, but basal species in the phylogenetic tree had the smallest C-values. The results suggested an overall increase in the genome size with evolution in Schoenus. A large amount of variation was found in the number and position of CMA and DAPI positive fluorescent bands and in the number of rDNA sites, although the position of the rDNA sites was conserved. Meiosis was regular with bivalent formation in all species examined. Interspecific hybridization was attempted to analyze chromosome pairing and genomic relationships between the species in F1 hybrids. However, no hybrids were produced but the barriers to hybridization were investigated. This investigation has not supported an earlier report on natural-occurring interspecific hybrids between S. nitens and S. concinnus.