Evolution in sown populations of subterranean clover (Trifolium subterraneum L. ) in South Australia

Abstract

The seed banks of three pastures at Kingscote, Parndana and Waterloo, S.A., were sampled to determine the frequency of divergent genotypes in subterranean clover, and the direction of natural selection. The seeds were grown in nursery rows at Adelaide, and the resulting plants classified into one of the commercial cultivars, or as divergent genotypes. The divergent genotypes from one locality, Kingscote, were described in terms of 17 variables, and compared with strains collected from southern Australia and the Mediterranean basin. At each locality there was more genetic diversity than had been sown, both in terms of additional cultivars and the presence of divergent genotypes. The percentage of divergent genotypes appeared to be proportional to the age of the pasture, and was greatest at Kingscote, where it reached 67% of the population. The Kingscote genotypes were genetically related to Mt Barker and Dwalganup, only five out of 283 genotypes having leaf markings that differed from both cultivars. The 17 variables fell between the values recorded for Mt Barker and Dwalganup, and there was evidence of directional selection. For example, formononetin content was less, and genistein more than would be expected in the absence of natural selection. Mean flowering time was about equal to that of Seaton Park, and closer to Mt Barker than to Dwalganup. Flowering time was related to elevation above the lowest point in the paddock, the latest genotypes tending to inhabit sites at the foots of slopes, and the earliest genotypes sites on the tops. Seed and burr weight were slightly larger than expected. Hybridization was the most important, but not the sole source of genetic divergence at Kingscote, whereas contamination was the main source at Waterloo. It is likely that only a minority of the divergent genotypes was generated by mutation. The Australian subterranean clovers were as diverse as those from the Mediterranean. Those from Kingscote were also diverse, but, on the whole, had longer peduncles, shorter internodes and were taller than strains from other parts of Australia and from the Mediterranean basin.