Abstract
Results are presented of research on a family of flowering plants, the Annonaceae, species of which are found in tropical rainforest across the world.
The project focussed on one group of species, the genus Cremastosperma, which is found in the South and Central American tropics. A taxonomic revision of Cremastosperma
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was made. Most of the species of Cremastosperma are found along the foothills of the Andes. Twenty-nine species were recognised, 13 of which were new to science. They are often narrow endemics, found only in areas such as the valleys of the Magdalena River in Colombia (e.g. C. magdalenae), and Marañon in Peru (e.g. C. bullatum). A resolved phylogeny can be used to infer the ancestral distributions of a clade, and possible factors behind its radiation. DNA sequences were compared in order to reconstruct the phylogeny of 19 of the 29 species of Cremastosperma. A number of clades were revealed. These included species distributed either west or east of the Andes mountain chain, providing further evidence to suggest the importance of the Andean orogeny as a vicariance event in the history of the evolution of Cremastosperma.
Many other groups of plants share this ‘Andean-centred’ distribution, and the rising of the Andean mountain chain has thus been hypothesised to have caused the origin of a large proportion of the high biodiversity of the New World tropics. DNA sequences were used to reconstruct the relationships between species of four Andean-centred genera of Annonaceae, Cremastosperma, Klarobelia, Malmea and Mosannona, and to estimate how long ago those species shared common ancestors. The conclusion was drawn that the Andean-centred distribution patterns as observed are not the arbitrary result of the definition of poly-or paraphyletic groups. The timing of species diversifications in these groups was shown to coincide with the rise of the northern Andes, within the last 20-30 million years.
Further research compared the timing of diversifications in Cremastosperma and Mosannona with those in two more widely distributed and species rich genera, Duguetia and Guatteria. The effects of taxon and character sampling on date estimates in these genera were assessed and compared. Higher sampling of crown group taxa of the species-rich genus Guatteria resulted in significantly older age estimations, representing a potentially serious bias in a widely used molecular dating method (nonparametric rate-smoothing). Molecular dating techniques should be assessed for sensitivity to levels of taxon sampling under differing conditions.
Finally, an ancient paralogue of the widely used chloroplast marker trnL-F was discovered, the origin of which was inferred to have taken place in a common ancestor of the Annonaceae. The paralogue discovered evolves at a faster rate and appears to have complementary phylogenetic signal. It may be a useful phylogenetic marker. Although the exon appears to be intact, some intron sequences show signs of disruption of the secondary structure which is otherwise conserved across land plants. Function may thus have been lost. Future research should attempt to determine both the whereabouts and origin of both copies of trnL-F in the Annonaceae genome.
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