Cremastosperma (and other evolutionary digressions)

This thesis presents results of molecular systematic and biogeographic studies of groups in the flowering plant family Annonaceae, and a taxonomic revision of the genus Cremastosperma, which occurs in the Neotropics. In Chapter 2 preliminary results are presented comparing the timing of diversifications in four predominantly Neotropical genera of Annonaceae; Cremastosperma, Duguetia, Guatteria, and Mosannona. Explanations were sought for the disparity in numbers of species in these four genera. Phylogeny reconstruction was used to assess their monophyly and molecular dating techniques (applying the nonparametric rate-smoothing method) used to arrive at preliminary estimates of the relative ages of their most recent common ancestors (MRCAs). 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 estimation for its MRCA. This represents a potentially serious bias in a widely used molecular dating method, which in this case made further comparison of species richness in the four genera meaningless. Molecular dating techniques should be assessed for sensitivity to levels of taxon sampling under differing conditions. In Chapter 3 a South American-centred clade was identified (the SAC clade). The SAC clade comprises all the short-branch clade (SBC) genera distributed in South America and mostly only to a limited extent into Central America, but not those endemic to Asia and Central America. In the absence of a fully resolved phylogeny of the SAC clade, this was interpreted to suggest a common origin in South America. The age estimations (produced using three different rate smoothing methods) supported the hypothesis that the SAC clade originated in South America by dispersal across the Boreotropics. Gentry’s hypothesis of a more ancient origin by Gondwanan vicariance was rejected, as was the possibility of later dispersal from Africa. Monophyly was confirmed in Cremastosperma, Malmea, and Mosannona. The monotypic genus Pseudephedranthus was found to be nested within Klarobelia, the species of which otherwise formed a monophyletic group. 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 ages estimated for the MRCAs of each clade were not significantly different from each other. Although the strength of this test was limited by imprecision in the molecular dating results, these ages appeared to fall within the time frame of the orogeny of the Northern Andes. To test these hypotheses further, species level phylogenies of Cremastosperma, Klarobelia, Mosannona and Malmea are needed. Additional data should also be sought to test the age calibration of the Annonaceae and wider Magnoliales phylogeny. This approach could shed further light on the dynamic processes of the recent invasion of Central America, and the origin of high species diversity in tropical America. In Chapter 4 an ancient paralogue of the widely used chloroplast marker trnL-F was discovered. The divergence of the paralogous copies was inferred to have taken place in a common ancestor of the Annonaceae. Primers were developed to preferentially amplify and sequence the different paralogues, and the resulting sequences were compared in order to assess their functional homology and phylogenetic utility. PseudtrnL-F (the paralogue not normally sequenced) evolves at a faster rate than trnL-F and appears to have complementary phylogenetic signal. It may represent a useful phylogenetic marker in itself. Although the exon appears to be intact, some pseudtrnL-F, intron sequences show signs of disruption of the secondary structure which is otherwise conserved across land plants. They may thus be non-functional. The higher evolutionary rate of pseudtrnL-F could reflect reduced selective pressure due to loss of function. However, it might also be the result of transfer to the nucleus where the rate of change is generally higher. This could have implications for the interpretation of its phylogenetic signal. Future research should attempt to determine both the whereabouts and origin of trnL-F and pseudtrnL-F in the Annonaceae genome. In Chapter 5 a taxonomic revision and phylogeny of the genus Cremastosperma was presented. Twenty-nine species are recognised. Thirteen have been described as new during this project. C. bullatum Pirie, C. cenepense Pirie & Zapata, and C. yamayakatense Pirie from the basin were described following a field expedition in 2003 to Peru. A new species from northern Venezuela (C. venezuelanum Pirie) was described; one from Amazonian Ecuador (C. napoense Pirie), seven from Colombia and western Ecuador (C. antioquense Pirie, C. awaense Pirie, C. chococola Pirie, C. dolichocarpum Pirie, C. longipes Pirie, C. magdalenae Pirie, and C. stenophyllum Pirie) and one from Panama (C. westrae Pirie). A further two putative species were described informally awaiting availability of further collections. Two species plus one variety were brought into synonymy: C. killipii R.E.Fr. under C. longicuspe R.E.Fr. and both C. juruense R.E.Fr. and C. monospermum (Rusby) R.E.Fr. var. brachypodum R.E.Fr. under C. monospermum. A resolved phylogeny can be used to infer the ancestral distributions of a clade, and possible factors behind its radiation. Multiple chloroplast DNA markers and pseudtrnL-F were applied to reconstruct the phylogeny of 19 of the 29 species of Cremastosperma. A number of clades were revealed. These included species limited to particular geographic areas. The divergence of the Venezuelan and Guianan lineages occurred prior to those of the clades found either in the tropical Andes, or in the Chocó/Darién/western Ecuador region or Central America (i.e. either west or east of the Andes mountain chain). The tropical Andes species all fell into one of two clades. Relationships between these two clades, a further clade including the Central American species Cremastosperma panamense Maas and C. westrae, and two isolated lineages corresponding to accessions from Costa Rica (C. spec. A) and the Magdalena valley of Colombia (C. magdalenae), remain unresolved. These results provide further evidence to suggest the importance of the Andean orogeny as a vicariance event in the history of the evolution of Cremastosperma. Further conclusions await a more resolved phylogeny of the genus.

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