Ancestors of white clover (Trifolium repens L.), as revealed by isozyme polymorphisms

Isozymes were used to study the putative ancestors of white clover (Trifolium repens L.). Ten enzymes were examined, and 18 loci were resolved via starch-gel electrophoresis for accessions representing T. repens, Trifolium isthmocarpum, Trifolium nigrescens, Trifolium occidentale and Trifolium uniflorum, in addition to two more distantly related species, Trifolium alpinum and Trifolium purseglovei. Nei's genetic identities indicate that T. uniflorum and T. nigrescens are the closest relatives of T. repens. The isozyme data thus support a hypothesis that the two genomes of the tetraploid T. repens could have been derived from hybridization between T. nigrescens and T. uniflorum. This conclusion is further supported by shared alleles between T. repens, T. nigrescens and T. uniflorum. However, the origin of T. repens is somewhat obscured by the presence of shared alleles between T. repens and both T. occidentale and T. isthmocarpum, suggesting that introgression of genes from the latter two species into T. repens may also have taken place. High values of genetic identity are shared between T. occidentale with T. nigrescens and T. uniflorum, also indicative of introgression. Alternatively the presence of shared alleles among the five species may reflect their recent common ancestry.