The evolutionary history of Geum (Rosaceae, Colurieae) : a study of ancient allopolyploid speciation

University dissertation from Stockholm : Botaniska institutionen

Abstract: Allopolyploid speciation plays an important role in flowering plant evolution. Its contribution to diversity has been suggested to be significant in Geum. In this thesis the evolutionary history of Geum and its closest relatives was investigated by phylogenetic analysis of sequence data from the trnL-trnF region of the chloroplast, the ITS region of nuclear ribosomal DNA, and the nuclear low-copy gene GBSSI. The data were analysed using parsimony, Bayesian inference, and maximum likelihood. The phylogenies were used to test hypotheses about allopolyploid speciation, to investigate the evolution of some fruit characters and the implications for the taxonomy of the group. The GBSSI phylogenies indicated that allopolyploid speciation has occurred in the history of Geum. However, earlier hypotheses based on cytogenetic data about the allopolyploid ancestry of certain species were rejected. One tetraploid species and a morphologically diverse group of hexaploids, likely also including species with higher ploidy levels, were suggested to be of allopolyploid origin. A hypothesis of reticulate organismal relationships, including two instances where new lineages have been formed by allopolyploidy, was proposed based on the gene trees. A molecular dating analysis indicated that these reticulations occurred several million years ago. None of the previously suggested circumscriptions of Geum, Acomastylis, or Sieversia sensu lato were found to be monophyletic. Nested within Geum sensu lato were Waldsteinia, Coluria, and Taihangia and therefore Geum was recircumscribed to include these taxa. Colurieae, the former Dryadeae (in part), was recircumscribed and given a phylogenetic definition. According to this classification, Colurieae includes Fallugia, Sieversia sensu stricto, and Geum. Morphological evolution within the putative allopolyploid group appears to be homoplasious. Initial optimisation of the presence of a "joint", an abscission layer where the style, or part of it, is shed at maturity, on the gene trees indicated that it evolved four times from persistent, elongating styles. However, scanning electron microscopy showed that the development of the various types of fruits with jointed styles is similar, indicating that the joint and the deciduous segment of the style, of different lineages are homologous. The joint was thus hypothesised to have originated in the ancestral lineage of Waldsteinia and Coluria and to have been passed on via hybridisation to the polyploids. It is, however, only expressed in some lineages among the polyploids.

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